Fundamentals of Plant Pathology

BACTERIAL CELL STRUCTURE

Bacterial Cell Structure

• The bacterial cell is surrounded by a cell wall composed of peptidoglycan consisting of chain of alternating N-acetyl muramic acid and N-acetyl glucosamine units cross linked by tetrapeptide and pentaglycine units.
• The cell wall allows the inward passage of nutrients and the outward passage of waste matter and digestive enzymes.
• All the material inside the cell wall constitutes the protoplast.
• The protoplast consists of a cytoplasmic or protoplast membrane, which determines the degree of selective permeability of various substances into and out of the cell.
• The cytoplasmic membrane of bacteria resembles those of eukaryotes, but also contains respiratory and other enzymes located in the bacteria.
• The cytoplasm, which is a complex mixture of proteins, lipids, carbohydrates, many other organic compounds, minerals and water.
• The nuclear material consists of large circular chromosome, composed of DNA.
• The chromosomal DNA makes up the main body of genetic material of the bacterium and appears as a spherical, ellipsoidal, dumb-bell or Y-shaped body in the cytoplasm, but without any membrane.
• Such nuclear material does not show meiosis and mitosis.
• Some species also have additionally single or multiple copies of smaller circular genetic material called plasmids.
• Plasmids can move from one bacterium to another and even from the bacterium to plants as in crown gall disease. This special property is being utilized with much success in genetic engineering for transformation of some desired genes from one plant to another by using it as vector.

Flagella

• In bacteria, flagella are the organs of locomotion.
• They are very delicate and fragile and cultures are to be handled carefully for their staining.
• The flagella vary from 10-12 nm in width which is smaller than wavelength of light, therefore, cannot be seen by ordinary staining.
• Mordants like potassium sulphate and mercuric chloride are generally precipitated on flagella making the width more for making them visible under light microscope.

Parts of a Flagellum

• Filament: It is the outermost region of flagellum, and is helical, composed of flagellin with a molecular weight of 30000-40000 and is synthesized in the cell, which moves to the hollow core of the flagellum to the tip. Flagellin is a protein with 14 amino acids and is characterised by higher content of aromatic amino acids and absence of cysteine in many cases.
• Hook: Filament is attached to hook which is wider than the flagellum. This is 45 nm wide and made up of different types of protein. The hook of gram positive bacterium is longer than that of gram negative bacteria.
• Basal body: The third part called basal body consists of small central rod which is inserted into a system of rings. The gram positive and gram negative bacteria are different in the number of rings. The inner pair of rings (S and M) are embedded in cell membrane and are formed in both gram positive and gram negative bacteria. L and P rings are formed only in gram negative bacteria. S and M rings are important for movement of flagella.

Pili

• In some bacteria, small hair like structures are also present which are called pili.
• These are shorter than the flagella and are thicker (3-15 nm in diameter).
• The term fimbriae is sometimes also used for pili, but the term pili is reserved for those which are involved in conjugation.
• They are made up of protein sub-units pilin of molecular weight of 70000.
• It consists of a helically coiled fibre with a central hole of 2 nm in diameter.
• Fimbriae may be involved in attachment, whenever there is infection. Both flagella and pili originate from cell membrane and extend outward through the cell wall.

Reproduction
Bacteria multiply at a phenomenal rate by the process of fission or binary fission.

• As the cytoplasm and cell wall undergo division into two, the nuclear material is organized into a circular chromosome like structure which ultimately duplicates itself and gets distributed equally into 2 newly formed cells.
• Similarly, plasmids also duplicate and come into 2 daughter cells.
• The duplication occurs rapidly, once every 20 minutes.
• As a result a bacterium like Eschersia coli, starting from one bacterium may produce 1 million bacteria in 10 hours.
• However, this number is not reached because of gradual limitations of nutrients and toxic metabolites. Still what is achieved normally is phenomenonal.
• Such prolificacy in multiplication must be of great advantage both in survival of bacterial pathogen, and also for successive plant infections.

Recombination
The genetical recombination in bacteria has been noticed by the following sexual-like processes:

• Conjugation: Conjugation occurs when two compatible bacteria come into contact and part of the chromosomal or non-chromosomal genetic material of one is transferred to the other and incorporated into the genome of later through conjugal zygote formation and breakage and reunion. It was first observed by Lederberg and Tatum (1956) in E. coli.
• Transformation: It occurs when the bacterium is genetically transformed by absorption of genetic material of another compatible bacterium, secreated by or released in a culture during the rupture, and its incorporation into the genome of the former. It was first observed by Griffith (1928) in Enterococcus pneumoniae.
• Transduction: When genetic material from one bacterium is carried by its phage (virus) to another bacterium that it visits next and the later is genetically transformed. It was first discovered by Zinder and Lederberg (1952) in Salmonella.

Mycoplasma/PPLO’s
Mycoplasma, earlier known as ‘Pleuro Pneumonia Like Organisms’ (PPLO’s) were discovered to be associated with the disease bovine pleuro pneumonia and were described in one of the orders mycoplasmatales under Eubacteria.

• Mycoplasma represent a group of organisms that lack cell wall and contain a very small genome.
• Phylogenetically, they are closely related to clostridia, the gram positive bacteria.
• As per the requirement for their growth, they can be divided into those which require sterol (mycoplasma and spiroplasma); and those which do not require sterols (acholeplasma and thermoplasma).
• The mycoplasma cells are small, pleomorphic (of different shapes) and divide by budding. The colonies of mycoplasma on agar exhibit a characteristic ‘fried egg’ appearance because of the formation of dense central core surrounded by lighter circular spreading area.
• The growth of mycoplasma is not inhibited by penicillin or other antibiotics that inhibit cell wall synthesis. But they are sensitive to tetracycline.
• The Spiroplasma genus is important plant pathogenically and has cork screw shaped cells. They are motile and exhibit undulating or rotating movement. Spiroplasma citri has been associated with citrus plants, where it causes citrus stubborn disease and corn plants which causes corn stunt.

Phytoplasma
They were earlier called MLO’s and were found to be associated with several yellows and witches’ broom diseases after their discovery by Doi et al. in 1967.

• They are different from mycoplasma in the sense that they can not be cultured on synthetic media.
• The change in terminology from MLO’s to phytoplasma occurred since the studies of DNA homology in the highly conserved genes encoding ribosomal RNA and ribosomal protein.
• It showed that the phytoplasma comprise a coherent group distinct from other prokaryotes. Their closest relatives are in the genus Acholeplasma.
• As they have not been cultured on artificial medium in vitro and characterized apart from their host, they are referred to Candidatus status.
• They are associated with about 200 plant diseases including aster yellows, apple proliferation, peanut witches’ broom, peach-x-disease, rice yellow dwarf and elm yellows.
• They are phloem inhabiting organisms and are graft transmissible in nature, and can also be transmitted by leaf hoppers.

Bacteria as Plant Pathogens

• Bacteria are known to grow in a wide range of habitat.
• All the plant pathogenic bacteria are mesophilic (they can grow at a temperature of 20-35ºC); and remain in the host plants as plant parasites and only partly in plant residues or as saprophytes in soil.
• They enter the plants either through natural openings such as stomata, lenticels or hydathodes or through wounds.
• The presence of free water is essential for bacterial infection. Once inside the plant tissues, they multiply only if there is water or at very high humidity.
• They multiply in the intercellular spaces and produce pectolytic and other cell wall degrading enzymes, thereby creating more space to move inside the host tissue.
• They kill the host cells by the action of extracellularly released enzymes and toxins and subsequently invade the dead cells. Most of the bacterial pathogens are necrotrophs.
• Some are apparently biotrophs.
• Some species colonize the xylem vessels and because of their physical presence or the slime ultimately cause the plugging of the water conducting tissues and cause wilt symptoms.
• Plant pathogenic bacteria produce various types of symptoms in plants as are caused by fungal pathogens. They cause soft rot of vegetables and fruits, wilts, cankers, scabs and also over-growths.