How do plants avoid desiccation

Our findings suggest that the evolution of desiccation tolerance is complex and likely quantitative. Next steps: We identified a series of gene duplications and rewired pathways that likely control desiccation tolerance.

The next step is to functionally validate these interactions using the existing transformation system in Lindernia. Components of desiccation-related pathways will be useful for engineering improved drought tolerance into crop plants. The Plant Cell: In a Nutshell. Feature image by Xiaomin Song. Member Services Contact Us. Plant Science Research Weekly Signup. This unpaired electron is readily donated, and as a result, most free radicals are highly reactive.

Dehydration enhances the formation of ROS, and therefore biochemical systems that effectively prevent their formation or scavenge them when formed are essential for the survival of green algae Kranner and Birtic, ROS are the most likely source of damage to nucleic acids, proteins, and lipids. Particularly, the hydroxyl radical is extremely reactive, and it easily hydroxylates, for example, the purine and pyrimidine bases in DNA, thus enhancing mutation rates Halliwell, ROS damage to proteins is mediated by configuration changes, mostly by oxidizing the free thiol residues of cysteine.

Conspicuous shifts in amino-acid composition have been observed during Streptophyte transition to land Jobson and Qui, These authors found that within the Streptophyta there is an increased utilization of charged amino acids, which was considered an important biochemical strategy to maintain protein hydration during desiccation because the proteome struggles to retain adequate cytoplasmic solute concentrations. Protection from desiccation is provided by charged amino acids Asp, Glu, Arg, and Lys , particularly with hydrophilic EKR residues, which are polar side chains deemed important for protein thermostabilization Haney et al.

The frequency of these EKR residues in the plastid proteome was significantly higher in bryophytes than in algae Jobson and Qui, Among streptophytic green algae, the aeroterrestrial genus Chlorokybus exhibited higher concentrations of positively charged amino acids HKR; Glu, Arg, Lys compared to their closely related freshwater genus Mesostigma Jobson and Qui, Besides the changes in amino acid composition, to our best knowledge, only one study conducted a proteomic analysis upon desiccation in the Trebouxiophycean green alga Asterochloris Gasulla et al.

Desiccation increased the abundance of only 11—13 proteins, which was reported to be independent of the drying rate Gasulla et al. The altered proteins are involved in glycolysis, cellular protection, cytoskeleton, cell cycle and degradation.

Interestingly, although ultrastructural changes were observed see below , no major changes occurred in the proteome Gasulla et al. In these plants, the lipid composition underwent major changes, including the removal of monogalactosyldiacylglycerol from the thylakoids Gasulla et al. While compositional changes of lipids in microalgae Chlorella vulgaris under nitrogen depletion have been addressed on a transcriptomic level Guarnieri et al.

Dehydration of green algae leads primarily to a shrinkage process. Cytological alterations may occur to different extents and can be seen after staining the mitochondria Figures 4C , D or the F-actin cytoskeleton, through the confocal laser scanning microscope Figures 4E , F ; Holzinger et al.

Comparisons of hydrated and experimentally desiccated cells of the streptophytic green alga Klebsormidium crenulatum observed by light- A,B and confocal laser scanning microscopy C—F. Only a few publications have dealt with ultrastructure as a response to desiccation in green algae e. This is largely due to methodological difficulties in attempting to observe the effects of desiccation at the ultrastructural level.

Desiccated stages can easily be investigated in a scanning electron microscope, but only the damage to the cell walls is visible, in comparison to hydrated control cells Figures 5A , B. In contrast, chemical fixation of naturally desiccated samples for transmission electron microscopy TEM usually results in rather poor preservation of the ultrastructure e.

Most of the samples investigated were collected from the field and represented permanent stages adapted to withstand unfavorable conditions e. After chemical fixation, mostly thick cell walls were visible, but many details of the ultrastructure could not be recognized.

In contrast, upon experimental desiccation of cultured Klebsormidium cells, high-pressure freeze-fixation gave better results and allowed the depiction of ultrastructural details in desiccated cells Figures 5C , D ; Holzinger et al.

Overall the cytoplasm appeared extremely dense, with the major organelles, the nucleus and the chloroplast clearly visible. Within the chloroplasts, the number of plastoglobules was increased after desiccation, demonstrating the capability of reorganization during the desiccation process. However, increased numbers of plastoglobules were also observed after various stresses, including light stress e.

Comparisons of the ultrastructure of hydrated and experimentally desiccated cells of the streptophytic green alga Klebsormidium. Plastoglobules are lipoprotein subcompartments of the chloroplast, which were shown to be permanently coupled to thylakoid membranes Austin et al. They contain biosynthetic and metabolic enzymes Austin et al.

Substantial destruction of the actin filament system was another consequence of desiccation Figures 4E , F , as investigated by phalloidin staining and confocal laser scanning microscopy Holzinger et al. The changes in the ultrastructural appearance of the cytoplasm were very similar to those seen after experimental dehydration by hypertonic sorbitol solution Kaplan et al.

The key differences after desiccation were the strongly undulating cross-cell walls, resulting from shrinkage Holzinger et al. The cell wall is relatively poor in cellulose but contains callose instead, which explains its outstanding flexibility upon desiccation Holzinger et al. Flexible cell walls might be a key structure contributing to the desiccation tolerance of Klebsormidium.

While the ultrastructure of the cytoplasm appeared similar after osmotic dehydration, the cell walls remained intact and a large periplastic space became visible in Klebsormidium Kaplan et al.

Similar observations have been made in Closterium Domozych et al. Another approach was followed by Gasulla et al. While slow dehydration resulted in an increasing number of lipid bodies together with a reduction in size, the quantity of starch deposits located within the chloroplasts and electron-dense deposits in the chloroplasts increased Gasulla et al. In the slowly dried and rehydrated cells, the plasma membrane still remained slightly retracted from the cell wall.

In contrast, rapidly dried cells of Asterochloris following rehydration clearly exhibited a degenerate ultrastructure. The cytoplasm was highly vacuolated and filled with lipid bodies, the cytoplasm and the chloroplasts still appeared shrunken, thylakoids were swollen or fused, and numerous starch deposits were visible Gasulla et al. Rapidly dried Asterochloris cells exhibited extensive plasmolysis and cytolysis.

However, even with this damage, the cells survived the dehydration treatment. The possible flexibility of the cell walls was not investigated in the rehydrated cells. Although the primary production of aeroterrestrial green algae has not been studied, it is reasonable to assume that dehydration will have a strong negative effect, because dehydration leads to a decreased in photosynthetic activity.

Because most lichens and their photobionts grow relatively slowly, their primary productivity is fairly low in most ecosystems. Nevertheless, a yearlong field study of a lichen in the Negev Desert showed that this cryptogamic organism was metabolically active on most days of the year.

Along with lichens, bryophytes and microfungi, mainly green algae and cyanobacteria form a joint matrix by gluing together soil particles and themselves, thereby forming a productive microbial biomass in many arid regions Belnap and Lange, The resulting biological soil crusts have important, multi-functional ecological roles in primary production, nitrogen cycling, mineralization, water retention, stabilization of soils, and dust trapping Evans and Johansen, ; Reynolds et al.

These essential ecological functions for arid and semi-arid regions are under threat, because a recent study of global terrestrial primary production indicated that the past decade — has been the warmest since instrumental measurements began, which led to reduction in regional terrestrial primary production due to large-scale droughts and a drying trend in the Southern Hemisphere Zhao and Running, These climatic changes will of course also affect aeroterrestrial green algae, whether free-living or in association with biological soil crusts or as photobionts with lichens.

Most tidal-influenced rocky shores show a conspicuous zonation of seaweeds, which are segregated into horizontal bands across the vertical rock surface Benson, The highest is the so-called supralittoral zone, which is mainly exposed to the atmosphere and only influenced by spray or splashing waves during storm events at high tides.

In this zone, typical green macroalgae of the genera Prasiola or Rosenvingiella occur under almost fully terrestrial conditions, and thus are exposed to drying for long intervals Rindi et al. Prasiola species in polar regions often grow in association with penguin or seagull colonies, even several meters above sea level, because they are extremely nitrophilous and prefer habitats rich in mixed excreta and feces of birds Rindi et al.

Just below the supralittoral zone is the so-called upper intertidal zone, which is covered with seawater only during high tides. Here, many filamentous or foliose green macroalgae such as members of the opportunistic genera Ulothrix , Ulva , Urospora , etc.

These algae are also influenced by drying at low tides, but more on a diel scale. The intertidal zone represents an interface between the terrestrial and marine environment, which may vary with season of the year and geography.

Typical for such ecotones are edge effects, where certain species spend most or all of their time in this transitional habitat. Dehydration is the main environmental factor in the supralittoral and high intertidal zones, and the green macroalgae living in these zones are exposed regularly to air, yet still survive.

These plants lose considerable amounts of water when exposed to the atmosphere and sun, but for example, Urospora species can survive more than 20 days of air exposure. However, the intertidal zone and the zonation of seaweeds are controlled not by drying alone, but by a combination of abiotic and biotic factors.

In addition, microhabitats such as crevasses, underneath boulders, or beneath the canopy of overlying macroalgae can offer protection against dehydration. One strategy of aeroterrestrial and aquatic green algae against desiccation is to avoid dehydration by self-protection. Under natural conditions, aeroterrestrial filamentous green algae such as Klebsormidium can form multi-layered mat-like structures on top of or interwoven with the upper millimeters of soil, which contribute to a high degree of self-shading and reduced loss of water from individual filaments within such a population.

Mat formation has also been observed in Arctic Zygnema sp. Holzinger et al. Gray et al. These data clearly indicate that single green-algal cells, which are closely associated with other algal cells in an aggregate e.

Additional factors that contribute to avoidance or at least retardation of water loss include a low algal surface-to-volume ratio and morphological features such as thick cell walls and mucilage layers e. Extracellular polysaccharides EPS are critical in desiccation tolerance of cyanobacteria and have beneficial effects on desiccation tolerance in the green alga Chlorella sp.

Knowles and Castenholz, Under exposed conditions, macroalgal canopies of Ulva sp. Factors leading to spore formation and germination have been reviewed extensively Coleman, ; Agrawal and Singh, ; Agrawal, , Desiccation events have a major impact on the transition from the vegetative to different forms of permanent stages such as akinetes, zygospores, oospores, or cysts e.

The formation of akinetes has been observed frequently in field-collected Zygnema sp. These akinetes are older cells that have accumulated large amounts of lipids and phenolic substances in the cytoplasm, while the vacuoles are drastically reduced Holzinger et al. Similar observations were made in the closely related Zygogonium ericetorum collected in high-alpine habitats Holzinger et al.

Recently, the phenolic compounds were further characterized, and these also likely contribute to UV tolerance of these cells Aigner et al. Genkel and Pronina reported that Zygnema stellinum became greenish-brown, entering a resting state akinetes , which allowed them to desiccate during summer, before they formed parthenospores for overwintering.

Some cell walls of zygospores of Zygnema , described by Stancheva et al. Phenolic compounds in brown algae include phlorotannins, which contribute considerably to protecting sensitive life stages from irradiation e.

However, when considering the time needed for germination of spores, tolerating desiccation in the vegetative state is substantially faster, and organisms capable of this strategy have a clear advantage. For example, in glaciers, where liquid water may be only intermittently available due to freezing, only vegetative states of the Zygnematophyceae Mesotaenium and Ancylonema have been found Remias et al. As discussed above, the physiological mechanism that is most sensitive to dehydration is photosynthesis, and hence efficient control of light absorption and energy distribution in the photosynthetic apparatus during dehydration seems to be most important to reduce or prevent photoinhibition.

Photoinhibition can be distinguished as dynamic and chronic forms, the latter representing irreversible photodamage. In contrast, dynamic photoinhibition is a reversible, controlled down-regulation of photosynthesis in the light under different stress conditions Hanelt, The main mechanism of dynamic photoinhibition is non-photochemical quenching of excitation energy absorbed by PSII, through harmless dissipation as heat. Dynamic photoinhibition has also been confirmed for several species of desert and aquatic green algae Lunch et al.

These authors found that although photoprotective mechanisms in green algae are similar in principle, they exhibit lineage-specific modifications. De-epoxidation of xanthophyll-cycle pigments paralleled light-induced changes in non-photochemical quenching for species of Klebsormidiophyceae and Trebouxiophyceae, but not Zygnematophyceae, indicating that the pigments involved can contribute to photoprotection, although to different degrees in different lineages Lunch et al.

Recently, Kosugi et al. The evolution of photoprotective mechanisms upon land colonization was recently studied by Gerotto and Morosinotto in more recently evolved streptophycean green algae.

The need for photochemical quenching appears to be regulated by different proteins, on the one hand by a light-harvesting complex-like stress-related protein LHCSR , whereas a photosystem II subunit S protein PSBS was detected in Zygnematales, Charales, and Coleochaetales. Moreover, there is a major difference between the glycolate pathway between the Chlorophyta and Streptophyta, as investigated in Mougeotia scalaris Charophyta and Eremosphaera viridis Chlorophyta by Stabenau and Winkler Eremosphaera viridis does not possess peroxisomes as found in angiosperm leaves, and all reactions glycolate oxidation and ATP generation are performed exclusively in the mitochondria.

In addition to the protection of molecular components of the photosynthetic apparatus against dehydration, the thylakoids may also be partially or completely protected by the presence of low-molecular-weight carbohydrates during water stress Santarius, Responses similar to those that occur during water stress may be seen during cold acclimation in Klebsormidium Elster et al. Large amounts of sucrose have been detected in Klebsormidium flaccidum because of cold acclimation Nagao et al.

Membrane stabilization depends on the concentration of sugars and their molecular mass, so that the trisaccharide raffinose is more effective than the disaccharide sucrose and the monosaccharide glucose Santarius, Since green algae can synthesize and accumulate an array of chemically different low-molecular-weight carbohydrates, especially under conditions of low water potential Karsten et al.

In addition to the effects of desiccation on photosynthesis and the protection or de novo biosynthesis of the photosynthetic apparatus, various additional protective biochemical mechanisms have been suggested.

Non-reducing low-molecular-weight carbohydrates such as sucrose and trehalose protect not only thylakoids, but also other membranes and proteins from dehydration damage Santarius, Particularly trehalose is strongly involved in desiccation tolerance in many biological systems Yancey, During dehydration, this disaccharide may bind to biomolecules and membranes by replacing water and thus maintaining their basic structure. However, in yeast, trehalose was found to be neither necessary nor sufficient for desiccation tolerance Ratnakumar and Tunnacliffe, Moreover, this widely distributed sugar as well as other soluble carbohydrates were detected only in trace amounts in Klebsormidium Kaplan et al.

The hydration of proteins by water molecules is important in maintaining their three-dimensional structure and consequently their function. While morphological structures may prevent or delay water loss, the biosynthesis and accumulation of high concentrations of organic osmolytes such as polyols, betaines, proline etc.

Polyols perform multiple functions in metabolism; in addition to their roles as organic osmolytes and compatible solutes, they can also act as antioxidants, heat protectants stabilization of proteins , and rapidly available respiratory substrates energy supply for a maintenance metabolism under stress and for repair processes; Yancey, ; Karsten et al.

Typical aeroterrestrial green-algal taxa such as Apatococcus , Chloroidium , Coccomyxa , Prasiola , Stichococcus , and Trentepohlia synthesize and accumulate high concentrations of a range of polyols such as glycerol, erythritol, ribitol, arabitol, mannitol, sorbitol, and volemitol Feige and Kremer, ; Gustavs et al.

In contrast, marine green macroalgae such as Acrosiphonia , Cladophora , Ulothrix , Ulva , and Urospora lack polyols, and instead synthesize and accumulate other organic compounds such as sucrose, proline, glycine betaine, or dimethylsulfoniopropionate DMSP; Kirst, ; Karsten, Although they differ in their chemical structure, polyols and the other organic solutes in green algae have several features in common: they are highly soluble, have no net charges at physiological pH, and are non-inhibitory at high concentrations Kirst, ; Karsten et al.

The interactions of these compounds with intracellular macromolecules are not completely understood, and several mechanisms have been suggested. Bisson and Kirst discussed the different models proposed to explain protection of enzyme systems: I binding of the solute to the protein, II colligative action of the solute, III buffering of potentially damaging changes in solution properties, IV inhibition of conformational changes resulting in the formation of inter- or intramolecular disulfide bridges, and V preferential exclusion of the solute from the protein surface.

These models can be grouped into two basic types: 1 those that hypothesize the existence of direct solute-protein interactions, and 2 those that postulate that protein stability is mediated by solute-induced changes in water structure Roberts, ; Yancey, However, there is little experimental evidence in green algae for any of these models.

In addition, some pigments such as the secondary carotenoid astaxanthin, which is formed and accumulated in high concentrations by various green algae such as Haematococcus Borowitzka et al. A potent antioxidant system seems to be one of the underlying mechanisms of desiccation tolerance.

The late embryogenesis abundant LEA proteins are biomolecules in plants that protect other proteins from aggregating during dehydration, probably due to conformational changes in transcription factors or integral membrane proteins Goyal et al. The possible occurrence of LEA proteins was demonstrated in the green alga Chlorella vulgaris , by the nucleotide sequence of a cDNA clone of the h ardening- i nduced Chlorella hiC gene Honjoh et al.

It can be expected that these proteins also protect against desiccation. DNA is the only biomolecule in cells that is steadily maintained and repaired, while all other biomolecules such as proteins are degraded in case of damage, followed by de novo biosynthesis.

DNA repair involves a set of processes by which a cell identifies and corrects damage to the DNA molecules, which is vital to the integrity of the respective genome, and thus to the normal functioning of the genome and the organism.

A typical DNA repair enzyme is blue-light controlled photolyase Beel et al. Concerning repair mechanisms, one must be aware that polynucleotides have astonishing stability, as was demonstrated for cyanobacteria as summarized, e.

This organism grows in the Limestone Alps and therefore is frequently exposed to desiccation. Similar molecular protective strategies are expected to occur in eukaryotic green algae. As mentioned above, for optimum function of the photosynthetic apparatus, the D1 protein of PSII plays a key role. The natural turnover of this protein is rather rapid, e. When breakdown dominates biosynthesis, the PSII is inactivated, as often observed in green algae during dehydration.

The damaged D1 protein is cleaved by a specific endoprotease, and is degraded by a metalloprotease for removal from PSII. After PSII has been reassembled, with the incorporation of the newly synthesized D1 protein, the complex is fully functioning again Allakhverdiev and Murata, Mulo et al. In Chlamydomonas reinhardtii the expression of the psbA gene is strongly regulated by mRNA processing, particularly at the level of translation initiation.

Replacement of damaged D1 protein requires several auxiliary proteins, and many of these chaperones are conserved in both, prokaryotes and eukaryotes Mulo et al. In the present review, we summarize the current knowledge of desiccation effects in green algae. Since the review of desiccation tolerance of green algae by Holzinger , extensive information on this topic has been acquired through physiological and ultrastructural investigations.

It is clear that several distinct phylogenetic lineages of green algae are capable of desiccation tolerance in their natural environments. These algae are able to survive desiccation conditions through different strategies: I avoidance by intrinsic mechanisms to retain water, e.

This last mechanism, which is extremely important for the distribution of green algae, has been investigated experimentally in several lichen-forming algae from the Ulvophyceae, and in the Klebsormidiophyceae and Zygnematophyceae from the streptophytic lineage.

It is remarkable that desiccation tolerance has evolved several times, but is completely lost in some morphologically advanced Streptophyta e. However, recent studies on the Zygnematophyceae have suggested this represents the sister group to land plants e.

While desiccation-tolerance mechanisms were likely advantageous for the transition of algae from the aquatic to the terrestrial lifestyle, these mechanisms have not been established permanently in land plants. This failure to establish can also be viewed in the context of the costs of vegetative desiccation tolerance, as metabolic rates in desiccation-tolerant organisms are low compared to the metabolisms of desiccation-sensitive plants Oliver et al.

For plants to succeed permanently on land, the development of homoiohydric mechanisms of regulating the water status by water transport e. Presently, only a very small proportion of angiosperms the resurrection plants are desiccation-tolerant in their vegetative organs e.

Because of the lack of available genome information, it is still difficult to address the molecular mechanisms involved in the desiccation tolerance of green algae. In addition, only a few studies have used modern approaches such as metabolomics or proteomics to examine these organisms.

Therefore, the determination of the genomes of more types of aeroterrestrial green algae is urgently needed to reach a fundamental understanding of desiccation stress responses. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. Abele, D. The radical life-giver. Nature , Affenzeller, M.

Salt stress induced cell death in the unicellular green alga Micrasterias denticulata. Agrawal, S. Factors affecting spore germination in algae — review. Folia Microbiol. Factors controlling induction of reproduction in green algae — review: the text. Vegetative survival, akinete formation and germination in three blue-green algae and one green alga in relation to light intensity, temperature, heat shock and UV exposure.

Aigner, S. Unusual phenolic compounds contribute to the ecophysiological performance in the purple-colored green alga Zygogonium ericetorum Zygnematophyceae, Streptophyta from a high-alpine habitat.

CrossRef Full Text. Allakhverdiev, S. Heat stress: an overview of molecular responses in photosynthesis. Environmental stress inhibits the synthesis de novo of proteins involved in the photodamage-repair cycle of Photosystem II in Synechocystis sp. PCC Acta , 23— Alpert, P. Constraints of tolerance: why are desiccation tolerant organisms so small or rare? Austin, J. II, Frost, E. Plastoglobules are lipoprotein subcompartments of the chloroplast that are permanently coupled to thylakoid membranes and contain biosynthetic enzymes.

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Beel, B. News about cryptochrome photoreceptors in algae. Plant Signal. Bell, R. Cryptoendolithic algae of hot semiarid land and deserts. Belnap, J. Berlin: Springer. Benson, K. The study of vertical zonation on rocky intertidal shores — a historic perspective. Bertsch, A. Planta 70, 46— Bewley, J. Physiological aspects of desiccation tolerance.

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Effects of nutrients on growth and cell type. Brito, J. Unravelling biodiversity, evolution and threats to conservation in the Sahara-Sahel. Brown, A. Water relations of sugar-tolerant yeasts: the role of intracellular polyols. Buscot and A. Varma Heidelberg: Springer , — Beck, and D. Bartels Heidelberg: Springer , 45— Buitink, J. Black and H. Cardon, Z. The green algal underground: evolutionary secrets of desert cells.

Bioscience 58, — Castillo-Monroy, A. Biological soil crusts modulate nitrogen availability in semi-arid ecosystem: insights from a Mediterranean grassland. Plant Soil , 21— Coleman, A. Fryxell Cambridge: University Press , 1— Inside the multicellular sporangia, the diploid sporocytes, or mother cells, produce haploid spores by meiosis, where the 2n chromosome number is reduced to 1n note that many plant sporophytes are polyploid: for example, durum wheat is tetraploid, bread wheat is hexaploid, and some ferns are ploid.

The spores are later released by the sporangia and disperse in the environment. Sporangia : Spore-producing sacs called sporangia grow at the ends of long, thin stalks in this photo of the moss Esporangios bryum.

Two different spore-forming methods are used in land plants, resulting in the separation of sexes at different points in the lifecycle. Seedless, non- vascular plants produce only one kind of spore and are called homosporous. The gametophyte phase 1n is dominant in these plants.

After germinating from a spore, the resulting gametophyte produces both male and female gametangia, usually on the same individual. In contrast, heterosporous plants produce two morphologically different types of spores. The male spores are called microspores, because of their smaller size, and develop into the male gametophyte; the comparatively larger megaspores develop into the female gametophyte.

Heterospory is observed in a few seedless vascular plants and in all seed plants. Lifecycle of heterosporous plants : Heterosporous plants produce two morphologically different types of spores: microspores, which develop into the male gametophyte, and megaspores, which develop into the female gametophyte.

When the haploid spore germinates in a hospitable environment, it generates a multicellular gametophyte by mitosis. The gametophyte supports the zygote formed from the fusion of gametes and the resulting young sporophyte vegetative form.

The cycle then begins anew. The spores of seedless plants are surrounded by thick cell walls containing a tough polymer known as sporopollenin. This complex substance is characterized by long chains of organic molecules related to fatty acids and carotenoids: hence the yellow color of most pollen.

Sporopollenin is unusually resistant to chemical and biological degradation. In seed plants, which use pollen to transfer the male sperm to the female egg, the toughness of sporopollenin explains the existence of well-preserved pollen fossils.

Sporopollenin was once thought to be an innovation of land plants; however, the green algae, Coleochaetes, also forms spores that contain sporopollenin. Gametangia singular, gametangium are organs observed on multicellular haploid gametophytes. In the gametangia, precursor cells give rise to gametes by mitosis. The male gametangium antheridium releases sperm. Many seedless plants produce sperm equipped with flagella that enable them to swim in a moist environment to the archegonia: the female gametangium.

The embryo develops inside the archegonium as the sporophyte. Gametangia are prominent in seedless plants, but are replaced by pollen grains in seed-producing plants. Plants developed a series of organs and structures to facilitate life on dry land independent from a constant source of water.

As plants adapted to dry land and became independent from the constant presence of water in damp habitats, new organs and structures made their appearance. Early land plants did not grow more than a few inches off the ground, competing for light on these low mats. By developing a shoot and growing taller, individual plants captured more light. Because air offers substantially less support than water, land plants incorporated more rigid molecules in their stems and later, tree trunks.

Shoots and roots of plants increase in length through rapid cell division in a tissue called the apical meristem, which is a small zone of cells found at the shoot tip or root tip. The apical meristem is made of undifferentiated cells that continue to proliferate throughout the life of the plant.

Meristematic cells give rise to all the specialized tissues of the organism. Elongation of the shoots and roots allows a plant to access additional space and resources: light, in the case of the shoot, and water and minerals, in the case of roots. A separate meristem, called the lateral meristem, produces cells that increase the diameter of tree trunks. Apical meristem : Addition of new cells in a root occurs at the apical meristem.

Subsequent enlargement of these cells causes the organ to grow and elongate. The root cap protects the fragile apical meristem as the root tip is pushed through the soil by cell elongation. In small plants such as single-celled algae, simple diffusion suffices to distribute water and nutrients throughout the organism. However, for plants to develop larger forms, the evolution of vascular tissue for the distribution of water and solutes was a prerequisite. The vascular system contains xylem and phloem tissues.

Xylem conducts water and minerals absorbed from the soil up to the shoot, while phloem transports food derived from photosynthesis throughout the entire plant. A root system evolved to take up water and minerals from the soil, while anchoring the increasingly taller shoot in the soil.

In land plants, a waxy, waterproof cover called a cuticle protects the leaves and stems from desiccation. However, the cuticle also prevents intake of carbon dioxide needed for the synthesis of carbohydrates through photosynthesis.

To overcome this, stomata, or pores, that open and close to regulate traffic of gases and water vapor, appeared in plants as they moved away from moist environments into drier habitats. Water filters ultraviolet-B UVB light, which is harmful to all organisms, especially those that must absorb light to survive.

This filtering does not occur for land plants. This presented an additional challenge to land colonization, which was met by the evolution of biosynthetic pathways for the synthesis of protective flavonoids and other compounds: pigments that absorb UV wavelengths of light and protect the aerial parts of plants from photodynamic damage.

Plants cannot avoid being eaten by animals. Instead, they synthesize a large range of poisonous secondary metabolites: complex organic molecules such as alkaloids, whose noxious smells and unpleasant taste deter animals. These toxic compounds can also cause severe diseases and even death, thus discouraging predation.

Humans have used many of these compounds for centuries as drugs, medications, or spices. In contrast, as plants co-evolved with animals, the development of sweet and nutritious metabolites lured animals into providing valuable assistance in dispersing pollen grains, fruit, or seeds.

Plants have been enlisting animals to be their helpers in this way for hundreds of millions of years. Land plants, or embryophytes, are classified by the presence or absence of vascular tissue and how they reproduce with or without seeds. The green algae, known as the charophytes, and land plants are grouped together into a subphylum called the Streptophytina and are, therefore, called Streptophytes. Land plants, which are called embryophytes, are classified into two major groups according to the absence or presence of vascular tissue.

Plants that lack vascular tissue, which is formed of specialized cells for the transport of water and nutrients, are referred to as non-vascular plants or bryophytes. Non-vascular embryophytes probably appeared early in land plant evolution and are all seedless. These plants include liverworts, mosses, and hornworts. Major divisions of land plants : Land plants are categorized by presence or absence of vascular tissue and their reproduction with or without the use of seeds.

In contrast, vascular plants developed a network of cells, called xylem and phloem, that conduct water and solutes throughout the plant. The first vascular plants appeared in the late Ordovician period of the Paleozoic Era approximately million years ago.

These early plants were probably most similar to modern day lycophytes, which include club mosses not to be confused with the mosses , and pterophytes, which include ferns, horsetails, and whisk ferns. Lycophytes and pterophytes are both referred to as seedless vascular plants because they do not produce any seeds.

The seed producing plants, or spermatophytes, form the largest group of all existing plants, dominating the landscape.