Unifying Features of Archegoniates

Archegoniates are a group of non-vascular and vascular plants that reproduce via archegonia, which are specialized female reproductive structures that produce and house egg cells. This group includes:

• Mosses (Bryophyta)
• Liverworts (Marchantiophyta)
• Ferns (Polypodiophyta)
• Most gymnosperms (e.g., conifers, cycads)

Archegoniates are considered among the most ancient plant groups, with fossil records dating back to the Devonian period (around 416 million years ago).

Structure of the Archegonium:

• It is a flask-shaped structure.
• It consists of a neck and a swollen base called a venter.
• The venter contains the egg.
• The neck protrudes above the surface of the gametophyte.
• It is composed of four-tier of cells that form an opening called the neck canal.
• The neck canal cells form a passage for the entry of sperms.
• The neck canal cells on maturity, sometimes lyse to provide an easy entry for the sperm.

Structure of archegonium

Archegoniates, a diverse group of plants, share several key characteristics that highlight their evolutionary adaptations and biological significance.

Ancestral Origin and Shared Features

• Ancestral Origin: Likely originated from a monophyletic group of ancient aquatic green algae.
• Reproductive Structures: Possess distinct sexual organs – female archegonium and male antheridium.
• Photosynthetic Pigments: Contain chloroplasts with chlorophyll a, chlorophyll b, and carotene.
• Life Cycle: Exhibit a multicellular gametophytic and sporophytic generations with a heteromorphic alternation of generations.
• Embryo Protection: Provide protection to their embryos.

Adaptations for Terrestrial Life

• Motile Gametes: Male gametes are flagellated and motile in bryophytes and pteridophytes. Female gametes (eggs) are non-motile.
• Dependence on Water for Fertilization: Bryophytes and pteridophytes rely on fluid water for fertilization, while gymnosperms utilize a pollen tube.
Soil Exploration, Anchorage and Nutrient Uptake: Differentiated rhizoids and roots provided strong anchorage and efficient absorption of nutrients and water, supporting growth and development on land.
• Evolution of vasculature: Development of efficient vascular systems for transport of water and nutrients.
Land Adaptations: Developed mechanisms to regulate internal water and nutrient balance, reducing dependence on external environmental conditions.
Specialized Spore Dispersal: Developed diverse spore dispersal mechanisms, enhancing genetic variation and facilitating colonization of new terrestrial habitats.
Desiccation-Resistant Spores: Spores became increasingly resistant to water loss, particularly in seed plants, ensuring survival in dry environments.

Structural and Functional Innovations

• Photosynthetic Efficiency: Increased green surface areas enhanced chlorophyll availability and increased photosynthetic efficiency.
• Transpiration Mechanism: Developed transpiration to regulate internal temperatures.
• Vascular System: An efficient vascular system evolved to transport water and nutrients throughout the plant body.
• Waxy Cuticle and Stomata: Minimized water loss and regulated gaseous exchange.
• Structural Support: Differentiated tissues with thickened cell walls supported an erect growth habit.
• Spore Dispersal: Evolved efficient spore dispersal mechanisms to spread and colonize diverse habitats.

Origin of Alternation of Generations

In a life cycle, when the two generations: sexual (haploid) and asexual (diploid) generations alternate with each other, by the production of gametes and spores respectively, it is called alternation of generations. However, the origin of alternation of generations is unclear.

Some workers opine that the alternation of the sporophytic and gametophytic generations came into effect with the successful establishment of land habit and when transformation of the ancestral algae to the ancestral at archegoniates occurred. The other opinion states that it was already a well-established phenomenon in the algae, before the transformation of the archegoniates occurred.

There are two opposing theories for this phenomenon, both of which are supported by the data in the fossil records. These two theories are the antithetic theory and the homologous theory. In both the cases, the gametophytic phase is considered to have evolved earlier. So, the mode of origin of the sporophyte is the main issue of consideration.

The late-nineteenth/early-twentieth century debated over homologous versus antithetic alternation of generations. Supporters of both theories, at first, used Coleochaete as a model for the origin of land-plant life cycles. The early debate focused on the morphological interpretation of the sporophyte and on whether vascular cryptogams had bryophyte-like ancestors. The terms of the debate shifted after the discovery that the alternation of morphological generations was accompanied by an alternation of chromosome number (Haig, 2008).

Supporters of homologous alternation now promoted a model in which land plants had been derived from an algal ancestor with an isomorphic alternation of haploid and diploid generations whereas supporters of antithetic alternation favored a model in which land plants were derived from a haploid algal ancestor with zygotic meiosis. Modern evidence that embryophytes are derived from charophycean green algae is more compatible with an updated version of the antithetic theory (Haig, 2008).

Antithetic theory:

This theory is also known as interpolation or intercalation theory. 

Proposer: Celakovsky, 1874 

Supporters: Bower, Chamberlain and others 

Theory: This theory assumes that the gametophyte, i.e. the sexual generation is the original or historically prior generation while the sporophyte or spore producing generation is an entirely new phase, derived from the progressive elaboration of the zygote of some algal ancestors and has been intercalated or interpolated into the life cycle in course of evolution, between the successive events of fertilization (syngamy) and meiosis (reduction division) and is thus different in structure from the gametophyte from its very inception.

Chamberlain (1935) states, 'The alternation of generation viewed as an alternation of n and 2n phases of the life history strictly antithetic.'

Evidence and proposals: 

• The sporophyte of the land plants originated from the green algae which already had alternation of generations and were of diplohaplontic type.
• Delay in the zygotic meiosis could have late to the development of multicellular sporophyte (Bower, 1908).
• The sporophyte of the land plants evolved as a new component of the life cycle and from algae which lacked alternation of generations.

Arguments: 

• The free-living sporophyte becoming parasitic on the gametophyte is confusing.

Intercalation of prolonged diploid phase led to the evolution of the sporophyte (Antithetic theory)

Homologous theory:

This theory is also known as modification or transformation theory.

Proposer: Pringsheim (1878)

Supporters: Scott, Goebel, Zimmermann, Fritsch, Stebbing, Events, Church and others

Theory: This theory holds that the sporophytic and gametophytic generations are fundamentally similar in nature and the sporophyte is a direct modification of the gametophyte and is not a new structural type. Thus, this theory supports the homologous alternation of generation which are phylogenetically similar but differs in the presence or absence of the sex organs. 

The advocate of this theory pointed to the fact that in algae the gametophytic plant reproduces by both the methods of reproduction and bears the asexual spores as well as sexual gametes. In course of evolution, these two functions became separated in two distinct individuals producing spores and gametes and were designated as the sporophyte and gametophyte respectively.

Evidences and proposals: 

• Presence of isomorphic alternation of generations between the spore producing and gamete producing individuals in algae. Example: Ulva
• Initially, both the sporophyte and the gametophyte where independent and free living. Gradually the sporophyte became parasitic on the gametophyte and got reduced.
• The phenomenon of apogamy and apospory also supports this theory.

Arguments: 

• Simultaneous and successful migration of both the gametophytes and sporophyte from water to land is impossible. 
• The two generations in algae and the land plants are analogous but not homologous.

Conclusion

Molecular phylogenetic data unambiguously relate charophycean green algae to the ancestry of monophyletic embryophytes and identify bryophytes as early divergent land plants. Comparison of reproduction in charophyceans and bryophytes suggests that the following stages occurred during evolutionary origin of embryophytic alternation of generations: 

• origin of oogamy
• retention of eggs and zygotes on the parental thallus
• origin of matrotrophy (regulated transfer of nutritional and morphogenetic solutes from parental cells to the next generation)
• origin of a multicellular sporophyte generation
• origin of non-fagellate, walled spores

The charophycean green algae, the closest extant relatives of embryophytes, possess a multicellular haploid body but lack a multicellular diploid body. Therefore, the sporophyte has been interpolated into a basically haploid life cycle. One could interpret this conclusion as a vindication of the antithetic theory championed by Bower (1908) and as a rejection of the homologous theory. However, debate continues about whether the sporophyte originated from a dispersed zygote or from a zygote that was retained on a maternal gametophyte (Graham and Wilcox, 2000; Haig, 2008).

Further reading:

Graham LK, Wilcox LW. The origin of alternation of generations in land plants: a focus on matrotrophy and hexose transport. Philos Trans R Soc Lond B Biol Sci. 2000 Jun 29;355(1398):757-66; discussion 766-7. doi: 10.1098/rstb.2000.0614. PMID: 10905608; PMCID: PMC1692790.

Haig, D. Homologous Versus Antithetic Alternation of Generations and the Origin of Sporophytes. Bot. Rev 74, 395–418 (2008). https://doi.org/10.1007/s12229-008-9012-x

Vashista B.R., Sinha A.K., Kumar A -Botany for degree students – Bryophyta.

Parihar N. -.Bryophytes: An Introduction to Embryophyta. Vol I

Alternation of Generations in Bryophyte

 Alternation of generations is a common component of all land plants, both vascular and non-vascular. The life cycle of bryophytes consists of two distinct phases, one of which is the green thalloid or leafy plant body, and the other is the sporogonium. In a life cycle, when the two generations, sexual (haploid) and asexual (diploid) generations, alternate with each other by the production of gametes and spores respectively, it is called alternation of generation.

In bryophytes, the gametophytic or gamete-producing generation is distinct in its morphology and is haploid having a single set of chromosomes. It is green, thalloid (liverworts) or leafy (mosses) and bears sex organs, i.e. antheridium (male) and archegonium (female). They produce male and female gametes i.e. antherozoids and eggs respectively, that unite sexually to form diploid zygote. 

The zygote is the first cell of the sporophytic generation. The zygote then divides to form an embryo which further differentiates to form the sporogonium. The sporophyte or spore-producing generation is morphologically different from the gametophyte and is diploid in nature. It is usually differentiated into foot, seta, and capsule. The diploid spore mother cells produced inside the capsule undergoes meiosis to form haploid spores which germinate to produce haploid gametophyte. 

In this way the life cycle is completed with an alternation of generations. This type of alternation of generations are termed as diplohaplontic life cycles. 

In bryophytes, the alternating individuals in the life cycle differs morphologically as well as physiologically. The gametophyte is independent while the sporophyte is attached and generally dependent fully or partially on the gametophyte. This kind of alternation of generation is known as heterologous or heteromorphic.

Life cycle of bryophyte showing distinct alternation of generations

Amphibious Nature of Bryophytes: Adaptations to Land Habit

Plants grow in two well-defined habitats: water (aquatic) and land (terrestrial.). Between these two habitats is a transitional zone, represented by the swamps and the areas where land and water meet. This zone is called the amphibious zone. Inhabiting this zone are the moses, liverworts, and hornworts which constitute a group of non-vascular land plants called the bryophytes.

Evidence supports the view that these early land plants descended from the algal ancestors which were green. 

This movement of plants from water to land required them to make several adaptations to survive this new and strange environment. These adaptive features are as follows:

Development of organs for anchorage and absorption of water: With the change of habitat the bryophytes unlike the algae are not bathed in water. So, absorption of water and nutrients through body surface is not possible anymore. This led to the development of rhizoides which help in absorption as well as anchorage of the plant to the substratum. Two types of rhizoids are found: smooth and pegged. In addition to this, scales are developed in bryophytes with increases absorptive surface. For example, simple rhizoids are present in anthocerotopsida. In Riccia, both simple as well as pegged rhizoids are present. Septate rhizoids, which increases absorptive function, are present in mosses.

Rhizoids of Riccia

Protection against desiccation: A major problem due to change of habitat from water to land is desiccation. So, to protect it from desiccation, thick, compact, multicellular epidermis is developed in the bryophytes. In some liverworts the free surface of the epidermal cells is covered with waxy substances like cuticle which reduces water loss.

Gaseous exchange: In the terrestrial habitat the plants have to take carbon dioxide and oxygen from the atmospheric air. So numerous minute pores called airpores are developed on the upper surface of the thallus. These airpores facilitate gaseous exchange between atmospheric air and the interior of the thallus. In addition to this, air canals (vertical slits between pseudo-mesophylls) also help in gaseous exchange. For example, airpores are present in Marchantia. In Riccia, air chambers are present.

T.S. of Marchantia thallus showing air pores, air chambers and rhizoids

Protection of reproductive structures from drying and mechanical injury: As terrestrial habitat is more prone to mechanical injury and drying up the reproductive structures, i.e. sex organs in bryophytes become multicellular and jacketed. This gives additional protection to the eggs and the sperms.

The fertilized egg is retained within the archegonium where it obtains food and water and remains protected from drying as it develops into an embryo.  For example, in Marchantia, the antheridium is protected by antheridial wall while the archegonium is protected by archegonial jacket. The zygote is retained within the venter.

Marchantia thallus bearing reproductive structures

Development of vascular bundle-like structures in higher bryophytes: Primitive vascular system in the form of a conductive strand is present due to land adaptation. Bryophytes have been group into three main groups on the basis of water absorption:

Ectohydric: They can absorb water or solutes by body surface only. They do not possess well developed conducting strands. Example: leafy liverworts

Endohydric: They absorb water by rhizoids which are connected to well-developed conducting strands. Example: some moses and thalloid liverworts

Mixohydric: They absorb water and solutes both by body surface as well as rhizoids. Example: Funaria

Necessity of water to complete life cycle: Bryophytes cannot carry on their reproductive activities without sufficient moisture. Without water sex organs cannot mature and do not dehisce. Water is essential for transport of sperm to archegonium. This explains why the bryophytes usually inhabit moist shaded situations. 

Since the bryophytes usually grow in amphibious situations and cannot complete their life cycle without presence of external water they are called amphibians of the plant world.

Salient Features of Bryophytes

1. Presence of a distinct and well-defined heteromorphic alternation of generation in the life history that is gametophyte and sporophyte generations which follow one another in regular succession. Gametophyte is independent while sporophyte is dependent on gametophyte.
2. In their vegetative structure they have become completely adapted to land habit. However, they still require water for fertilization and hints to complete their life cycle. They are called the amphibians of plant Kingdom.
3. The plant body lacks true root, stem, and leaf. In lower bryophytes the plant body is thalloid containing rhizoids (Example: Riccia). In higher bryophytes, however, the gametophytic plant body is differentiated into roots like (rhizoids), stem like (caulid), and leaf like (phyllid) structures (Example: moss).
4. The bryophytes (like thallophytes) lack vascular bundles (which is characteristic feature of higher plants). However, vascular bundle-like structures in form of a strength maybe present in higher bryophytes like Pogonatum.
5. Sexual reproduction is highly oogamous. The sex organs are jacketed and multicellular (in contrast to algae where sex organs are unicellular and non-jacketed).
6. Female sex organ in form of archegonium appears for the first time in the bryophytes.
7. Sperms are biflagellate and both the flagella are whiplash type.
8. The fertilized egg is retained within the venter. It neither becomes independent of the parent gametophyte, nor passes into resting period (unlike algae).
9. The zygote divides to form embryo. The first division of the zygote is transverse, and the apex of the embryo develops from the outer cell. Such an embryogeny is called exoscopic (Embryo is absent in thallophyta).
10. The venter enlarges with the developing embryo into a protective multicellular structure called calyptra.
11. The embryo develops into sporophyte. It is differentiated into foot, seta, and capsule. In some species Seta is absent (Example: Corsinia).
12. The sporogonium is concerned with the production of wind-disseminated, non-motile, cutinized spores which belong to the category of gonospores or meiospores. This meiospores are of one kind, i.e. they are homospores.

A Virtual Tour Through Plant Tissue Culture Laboratory

 

Lipid Biochemistry (Part II)

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Simple lipids: Fats and oils 

(Storage lipids)

These are triglycerides or triacylglycerols.

These are esters of 3 fatty acid molecules with trihydroxy alcohol, glycerol.

A fat is solid at room temperature while oil is liquid.

Triglycerides are most abundant among all lipids, constituting about 98% of total dietary lipids.

They are major storage components or depot fats in plant and animal cells but are not normally found in membranes.

They are non-polar, hydrophobic molecules since they contain no electrically charged or highly polar functional groups.

Triglycerides which contain the same kind of fatty acids in all three positions are called simple triacylglycerols and are named after the fatty acid they contain. 

Simple triacylglycerols of 16:0, 18:0, and 18:1, for example, are tristearin, tripalmitin, and triolein, respectively.

Triglycerides which contain two or more different fatty acids are called mixed triacylglycerols.

To name these compounds unambiguously, the name and position of each fatty acid must be specified.

Functions of fats and oils

In eukaryotic cells, triacylglycerols form a separate phase of microscopic, oily droplets in the aqueous cytosol, serving as depots of metabolic fuel.

Adipocytes or fat cells store large amounts of triacylglycerols as fat droplets.

Triacylglycerols are also stored as oils in the seeds of many types of plants, providing energy and biosynthetic precursors during seed germination.

Adipocytes and germinating seeds contain lipases that catalyze the hydrolysis of stored triacylglycerols, releasing fatty acids for export to sites where they are required as fuel.

Humans have adipocytes under the skin, in the abdominal cavity, etc.

Seals, walruses, penguins, and other warm-blooded polar animals are amply padded with triacylglycerols which provide insulation.

In hibernating animals, the huge fat reserves accumulated before hibernation serve the dual purposes of insulation and energy storage.

In sperm whales, a store of triacylglycerols and waxes (low density) allows the animals to match the buoyancy of their bodies to that of their surroundings during deep dives in cold water.

Simple lipids: waxes 

(Energy Stores and Water Repellents)

Waxes are esters of fatty acids with high molecular weight monohydroxy alcohols.

Fatty acids range between C14 and C36 while alcohols range from C16 to C36.

Melting varies from 60° - 100ºC.

The term ‘wax’ originated from old English ‘weax’ meaning ‘material of the honeycomb’.

Carnauba wax is the hardest known wax. It consists of fatty acids esterified with tetracosanol [CH₃(CH₂)₂₂CH₂OH] and tetratriacontanol [CH₃(CH₂)₃₂CH₂OH].

Waxes are unusually inert due to their saturated nature of the hydrocarbon chain. However, they can be slowly split with hot alcoholic KOH.

Functions of waxes

In planktons, the chief storage form of metabolic fuel.

Certain skin glands of vertebrates secrete waxes to protect hair and skin and keep it pliable, lubricated, and waterproof.

Some aquatic birds, secrete waxes from their preen glands to keep their feathers water-repellent.

Leaves of many tropical plants are coated with a thick layer of waxes, which prevents excessive evaporation of water and protects against parasites.

Biological waxes find a variety of applications in the pharmaceutical, cosmetic, and other industries. 

Lanolin (from lamb’s wool), beeswax, carnauba wax (from a Brazilian palm tree), and wax extracted from spermaceti oil (from whales) are widely used in the manufacture of lotions, ointments, and polishes.

Compound lipids: Phospholipids 

(Structural lipids)

Phospholipids are the most abundant membrane lipids.

They serve primarily as structural components of membranes and are never stored in large quantities.

Phospholipids contain phosphorus in the form of phosphoric acid groups. They differ from triglycerides in possessing usually one hydrophilic polar ‘head’ group and usually two hydrophobic nonpolar ‘tail’ groups. They are often called polar lipids. Thus, phospholipids are amphipathic.

In phospholipids, two of the –OH groups in glycerol are linked to fatty acids while the third –OH group is linked to phosphoric acid. The phosphate is further linked to one of a variety of small polar head groups (alcohols).

Phospholipids can be classified into:

Phosphoglycerides

Phosphoinositides

Phosphosphingosides

Phosphoglycerides

Major phospholipids found in membranes.

It consists of two fatty acid molecules or ‘tails’ esterified with the first and second –OH groups of glycerol.

The third –OH group of glycerol forms an ester bond with phosphoric acid. An additional substituent group is esterified with the phosphoric acid. This is referred to as ‘head group’ as it is present at one end of the long phosphoglyceride molecule.

Of the two fatty acid molecules, the one on C1 is saturated (C16-C18) while the one on C2 is unsaturated (C18-C20).

C2 of glycerol is asymmetric in nature.

All phosphoglycerides contain a negative charge on the phosphoric acid at pH 7. In addition, the head group may also have one or more electrical charges at pH 7.

Phosphoglycerides are of 3 types:

Lecithins (Phosphatidyl cholines)

Cephalins (Phosphatidyl ethanolamine and phosphatidyl serine)

Plasmalogens (Phosphoglyceracetals)

Lecithins (Phosphatidyl cholines)

Found in various oil seeds like soybeans and yeasts. In animals, glandular and nervous tissues are rich in lecithins.

Required for normal transport and utilization of other lipids in the liver. In its absence, accumulation of lipids occurs in the liver to as much as 30% (against 3-4% in normal) giving rise to a condition called ‘fatty liver’. This may lead to fibrotic changes.

Lecithins contain two fatty acids esterified with any two –OH groups of glycerol while the third –OH group is esterified with a phosphoric acid group. The phosphoric acid group is again linked to a nitrogen base, choline.

Hydrolysis of lecithins

On complete hydrolysis, lecithins yield a mixture of choline, phosphoric acid, glycerol, and two moles of fatty acids.

But partial hydrolysis of lecithins by lecithinase (active components of snake venom) causes removal of one fatty acid to yield lysolecithins.

When subjected to the bloodstream (as a result of snakebite), lysolecithins cause rapid rupture of RBC (hemolysis).

Cephalins

These are closely associated with lecithins in animal tissues. They are also structurally similar to lecithins except that the choline is replaced by either ethanolamine or serine.

Accordingly, two types of cephalins are recognized:

Phosphatidyl ethanolamine

Phosphatidyl serine

Since the primary amino group is a weaker base than the quaternary ammonium group of choline, the cephalins are more acidic than lecithins. Cephalins are also comparatively less soluble in alcohol than lecithins.

Snake venoms containing lecithinase can also split off fatty acids from cephalins leaving hemolytic lysocephalins.  

Plasmalogens (phosphoglyceracetals)

Plasmalogens constitute about 10% of phospholipids in the brain and muscles. About half of heart phospholipids are plasmalogens.

These ether phospholipids are common in membranes of halophilic bacteria because they are resistant to phospholipases.

Structurally they are similar to lecithins and cephalins but have one of the fatty acid chains replaced by an unsaturated ether.

Since nitrogen base can be choline, ethanolamine, or serine, plasmalogens can be of three types:

Phosphatidal choline

Phosphatidal ethanolamine

Phosphatidal serine

Phosphoinositides (phosphatidyl inositols)

Phosphoinositides have been found to occur in phospholipids of brain tissues and soybeans. They play an important role in the transport process as well as a signaling intermediate in cells.

They have a cyclic hexahydroxy alcohol called inositol which replaces the base. The inositol is present as the stereoisomer, myo-inositol.

Number of phosphate groups may be one, two, or three. Accordingly, mono-, di- and triphosphoinositides are found.

Phosphosphingosides (sphingomyelins)

Commonly found in nerve tissues, especially in the myelin sheaths. Absent in plants and microorganisms.

In a syndrome called Niemann-Pick disease, the sphingomyelins are stored in the brain in large quantities.

They lack glycerol and instead contain sphingosine or a closely related dihydrosphingosine.

They are electrically charged molecules and contain phosphocholine as a polar head group.

Functions of phospholipids

In association with proteins, phospholipids form the structural components of membranes and regulate membrane permeability.

Phospholipids in the mitochondria maintain the conformation of electron transport chain components and thus cellular respiration.

They participate in the absorption of fats from the intestine.

They are essential for the synthesis of different lipoproteins and thus participate in the transport of lipids.

They prevent the accumulation of fats in the liver (lipotropic factors).

They participate in the transport of cholesterol and thus help in the removal of cholesterol from the body.

They act as surfactants (respiratory distress syndrome).

Cephalin participates in blood clotting.

Phosphatidyl inositol is the source of the second messenger that is involved in the action of some hormones.