Plant viruses enter into host cell at points of injuries upon leaf surfaces or inoculated through Arthropod vectors. Viruses enter into our body by insect biting (e.g. dengue fever is caused by Aedes mosquito which also causes yellow fever) genital contact (e.g. AIDS), blood transfusion, food and drinks, skin abrasions, animals bites (rabies), Hypodemic needles (AIDS, Hepatitis B etc.).
Bacteria cause various dangerous diseases like tuberculosis, pneumonia, typhoid fever, cholera, dysentery in man. In plants bacteria causes potato wilt, fire blight of apple and pear, crown gall, canker of citrus, Black rot of cabbage etc.
“Infection is invasion and multiplication of microorganisms in body tissues, especially that causing local cellular injury due to competitive metabolism, toxins, intracellular replication, or anitigen-antibody response”.
Immunity is the condition of being immune i.e., being highly resistant to a disease because of the formation of humoral antibodies cells, or both, or as a result of some other mechanisms, as interferon activities in viral infections. (Interferon’s are proteins made by virus infected cells of vertebrates, which prevents infection by a second kind of virus or it can also be defined as species specific, but virus- unspecific proteins, one of the main defense mechanisms of humans and animals against the numerous viral pathogens).
Immunity is security against a particular disease, nonsusceptibility to the invasive or pathogenic effects of foreign microorganisms or to the toxic effect of anitgenic substances. Immunity can also be said as capacity to distinguish foreign material from self and to neutralize, eliminate or metabolize that which is foreign by the physiologic mechanisms of the immune response.
In case of plants, when they are affected by microorganisms or are wounded or are affected by some kind of stress, there are certain mechanisms to overcome those conditions. Under salinity stress, water stress or heavy metal toxicity stress, there is enhanced production of certain quaternary ammonium compounds like glycine betaine and levels of some amino acid like proline increases to sustain life of plant.
Also the level of some enzyme’s activity increases to fight the condition, for example under water and salinity stress conditions activities of catalase, superoxide dismutase, peroxidase and other oxidases increases to scavange the peroxide radicals produced under stressful conditions while activities of metabolic enzymes like malate dehydrogenase and glutamate dehydrogenase in plants decreases under stressful conditions.
Some plant harmones also fights against diseases or wounds. Plant hormones abscisic acid (ABA) is known to affect a number of physiological processes, such as dormancy, flowering, of plant cells to water and resistance against various stress conditions, for example, in maize ABA induces heat shock protein (hsp 70), which saves plant or makes plant able to fight increased heat. DNA sequences encoding many of these stress proteins and those regulating their expression have been isolated and are presently being characterized.
One such gene induced by ABA in response to water stress, encodes a glycine rich protein in maize embryo. Further characterization of the genes should provide a significant insight into role of ABA in stress tolerance in plants.
Resistance to most plant diseases is not mediated by single gene and hence is not detectable in culture. However, sometimes the toxic effect of phytotoxin on tissues, cell or protoplast cultures is equivalent to its effect on whole plant (Earle, 1978).
Furthermore, if the phytotoxin is the only reason for pathogenecity, direct selection for disease resistance in vitro using the phytotoxin may come to reality. It was Carlson (1973) who extended experimental support to this hypothesis, he reported that plants regenerated from tobacco protoplasts selected for resistance to methionine sulfoximine (MSO) showed enhanced resistance to Pseudomonas tabaci.
Though methionine sulfoximine has not been analogous to the phytotoxin of P. tabaci, this observation opened the door of selecting phytotoxin-resistant cell lines. Similarly, plants produce various proteins (enzymes) for being resistant to herbicides, resistance to amino acids and their analogues e.g.,5-methyltryptophan resistant potato cell line has been used to demonstrate the presence of two possible isoenzymes of anthranilate synthase (Carlson & Widholm, 1978) the first regulatory enzyme unique to tryptophan biosynthesis.
Further 5 MT resistant cell lines were isolated by Jacobsen et. al. (1975). The cells accumulated free tryptophan and phenylalanine and tyrosine demonstrating the possibility of manupulating the free amino-acid pools of potato cells. Salt tolerant & low temperature tolerant cell lines of Nicotiaha tabacum and N. sylvestris have been isolated.
Under certain conditions secondary metabolites likes high amount synthesis of serpentine and ajmalicine by selected cell lines of Catharanthus roseus have been reported. Anthraquinone is commercially obtained from the cortical cells and bark cells of the roots of Morinda citrifolia.
This alkaloid is now being obtained from cell line suspension cultures. If calculated in percentage dry weight, the cell line cultures result in about eight times higher alkaloid content than the whole plant yield. Suspension cultures of Coleus blumei have been reported to accumulate rosmarinic acid upto 15% of the dry weight of the cells which stands five times higher than the alkaloid content in the intact plant.
Important Plant Diseases :
(i) Caused by Viruses:
(a) Yellow mosaic of Mung
(b) Curlytop of Sugar beet
(c) Malformation of Mango
(d) Leaf Curl of Tomato
(e) Phyllody of Sesam and Sunhemp
(f) Bunchytop of Banana
(g) Stenosis of Cotton
(h) Mosaic, Leaf curl of Papaya
(i) Tristeza, Viral Necrosis of citrus
(j) Verigated Mosaic of Apple
(k) Mosaic of Tobacco
(l) Tobacco Necrosis
(m) Yellow Vein Mosaic of Bhindi
(n) Potato leaf rolls