(a) What is mycoplasma? Describe its types and importance in plant pathogenesis. 2+8=10 (b) Describe the ultrastructure of…

(a) Mycoplasma: Types and Plant Pathogenesis

Mycoplasmas are wall-less, highly pleomorphic, filterable prokaryotic microorganisms belonging to the class Mollicutes, bounded solely by a trilaminar unit membrane (sterol-containing in the sterol-requiring genera). Taxonomically and physiologically, they are categorized into families such as Acholeplasmataceae, which do not require sterols for growth and display non-helical forms, and Spiroplasmataceae, which are sterol-requiring and exhibit helical morphology and rotational motility. Phloem-inhabiting plant forms are classified into Phytoplasmas (non-helical, non-culturable) and Spiroplasmas (helical, culturable).

In plant pathogenesis, these organisms act as obligate parasites restricted to the sieve tube elements of phloem tissue, transmitted biologically by insect vectors such as leafhoppers and psyllids. They induce systemic physiological disorders, floral virescence, phyllody, stunting, and witches'-broom proliferation. In India, phytoplasmas cause economically destructive diseases such as Little Leaf of Brinjal (Solanum melongena) and Sandal Spike Disease (Santalum album), severely disrupting normal shoot architecture and biomass productivity.

**(b) Ultrastructure of Endospore of Bacillus sp.**

The endospore of Bacillus sp. is an extraordinarily resistant, dormant, multi-layered survival structure formed endogenously under nutrient stress. From the exterior to the interior, its ultrastructure reveals:

The exosporium is the outermost, loose, balloon-like membranous layer composed of proteins, polysaccharides, and lipids. Immediately beneath lies the spore coat, a dense, multi-layered proteinaceous shell rich in cysteine and hydrophobic amino acids, providing impermeable protection against lytic enzymes and chemicals. Beneath the coat is the cortex, a thick layer of loosely cross-linked peptidoglycan containing muramic-delta-lactam.

The inner core is bounded by the core wall (germ cell wall), which matures into the vegetative cell wall during germination, and the inner spore membrane, an intact lipid bilayer maintaining extreme impermeability. The innermost region is the core (protoplast), containing the condensed chromosome, ribosomes, and metabolic machinery. The core exhibits extreme dehydration and contains a high concentration of a dipicolinic acid and calcium complex (Ca-DPA), alongside Small Acid-Soluble Spore Proteins (SASPs) that bind and protect DNA from heat, desiccation, and ultraviolet radiation.

**(c) Structure of Strobilus of *Equisetum***

The strobilus of Equisetum is a compact, cone-like reproductive structure terminating the main fertile axis or lateral branches. It consists of a stout, central longitudinal axis surrounded by closely packed whorls of specialized reproductive appendages termed sporangiophores, with an anulus forming a protective ring at the base.

Each sporangiophore consists of a short cylindrical stalk attached at a right angle to the central axis, terminating in an expanded, polygonal, peltate shield-like disc. On the inner, adaxial surface of this peltate disc, 5 to 10 elongated, cylindrical, sac-like sporangia hang downwards parallel to the central stalk. Each sporangium encloses numerous identical, spherical, green haploid spores. Every spore is encircled by four ribbon-like, spoon-shaped, hygroscopic elaters derived from the outermost perispore (epispore) wall layer and attached at a single point, which uncoil during dry conditions to facilitate spore dispersal.

(d) Fine Structure of Chloroplast in Chlorophyceae

The chloroplast of Chlorophyceae is a specialized, double-membraned photosynthetic organelle showing diverse macroscopic forms, ranging from the cup-shaped plastid of Chlamydomonas to the spiral ribbon of Spirogyra.

Ultrastructurally, it is bounded by a chloroplast envelope consisting of an outer membrane, an intermembrane space, and a selectively permeable inner membrane. Enclosed within is the protein-rich gel matrix termed the stroma, which contains circular plastid DNA (cpDNA), 70S ribosomes, plastoglobules, and enzymes of the Calvin-Benson cycle.

The internal photosynthetic membrane system comprises thylakoids arranged in bands of 2 to 6 appressed membranes forming grana-like stacks, interconnected by unstacked stromal lamellae. Embedded within the stroma are one or more conspicuous pyrenoids, which serve as primary micro-compartments for ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO). Each pyrenoid is enveloped by a distinct sheath of crescentic starch plates, where synthesized starch grains accumulate.

(e) Comparative Account of Icosahedral and Helical Symmetry in Viruses

Viral capsids are constructed through the self-assembly of protein subunits (capsomeres) following two fundamental geometric designs: icosahedral and helical symmetry.

Icosahedral symmetry produces a quasi-spherical, closed isometric shell. The capsid forms a regular polyhedron with exactly 20 equilateral triangular faces, 12 vertices, and 30 edges, operating on a 5-3-2 rotational symmetry (five-fold axes through the vertices, three-fold axes through the face centers, and two-fold axes through the edges). The viral genome is densely packaged within the hollow central interior cavity without directly determining the capsid's outer geometry. Prominent examples include Adenovirus and Cauliflower Mosaic Virus (CaMV).

Helical symmetry produces an elongated, cylindrical, rod-shaped or filamentous structure. The capsomeres are organized in a continuous spiral ribbon around a central rotational axis, forming a hollow central tube. The viral nucleic acid is intimately embedded within an interior protein groove, such that the length of the capsid is strictly governed by the length of the encapsulated genome, and the width is determined by the size and packaging of capsomeres. A classical example is Tobacco Mosaic Virus (TMV).