The following points highlight the growth regulators released by pathogens during colonization. The growth regulators are: 1. Auxins (IAA) 2. Gibberellins 3. Cytokinins 4. Etylene (CH2 = CH2) 5. Abscisic Acid.
1. Auxins (IAA):
Indole-3 acetic acid (IAA) is a naturally occurring auxin in plants generally in very low concentrations. Studies of many plants infected by fungal, bacterial, viral, mycoplasmal, and nematode pathogens have shown increased concentration of IAA in them. Diseases like club root of cabbage (Plasmodiophora brassicae), smut of corn (Ustilago maydis), bacterial wilt of solanaceous plants (Pseudomonas solanacearum), crown gall disease of plants (Agrobacterium tumefaeiens), knot disease of olive (Pseudomonas savastanoi), and root-knot disease (Meloidogyne sp.; nematode) represent some good examples in which the pathogens not only induce increased levels of IAA in their respective hosts but are themselves capable of producing IAA inside plants.
On the other hand, however, there are some plants where the pathogens seem to lower the IAA concentration during the course of disease development. For instance, Lycopersicon esculentum, Phaseolus vulgaris and Beta vulgaris infected plants with curly top virus show lower levels of IAA when compared to healthy ones.
2. Gibberellins:
Gibberellins, first isolated from the fungus Giberella fujikuroi causing “foolish seedling disease” of rice in Japan, represent a group of chemically similar substances. However, the best known gibberellin is gibberellic acid.
Gibberellins have been studied more extensively in relation to viral and mycoplasmal diseases. Although no difference has been reported so far in the gibberellin content of diseased plants, their spraying with gibberellins overcomes some of the symptoms caused by pathogens.
In most of these treatments the pathogen itself does not seem to be affected, and the symptoms reappear on the plants after gibberellin-applications are stopped. Some diseases that have shown gibberellin- spraying effects are stunting of corn plants (corn stunt virus), auxiliary bud suppression on cherry (sour cherry yellows virus (SCYV), and tobacco leaf curl (tobacco leaf curl virus).
3. Cytokinins:
Cytokinins regulate cell growth and differentiation. In addition, they inhibit the breakdown of proteins and nucleic acids thereby causing inhibition of senescence. The first cytokinin identified was kinetin by Skoog and Miller in the early 1950s. Since then several cytokinins, for instance, zeatin and isopentenyl adenosine (IPA) have been isolated from plants.
The role of cytokinins in relation to plant disease development has just begun to be studied. However, cytokinin activity has been reported increased in gall diseases such as club root galls, crown galls, and smut and rust galls. Similarly, an increase in cytokinin content has been noticed in tumors caused by Ustilago zeae, wart disease of potato (Synchytrium endobioticum), and peach leaf curl disease (Taphrina deformans).
4. Etylene (CH2 = CH2):
Ethylene, which is naturally produced by plants and is biologically highly active growth regulator and exerts a variety of effects on plants (including chlorosis, leaf abscission, epinasty, adventitious root stimulation, and fruit ripening), is also produced by several plant pathogenic fungi and bacteria.
5. Abscisic Acid:
Abscisic acid is an important growth inhibitor produced by plants and by at least some plant pathogens. Several diseases are known in which infected plants become stunted, for example, Verticillum wilt of tomato, tobacco mosaic and cucumber mosaic, and bacterial wilt of tobacco. In all these cases, diseased plants contain higher than normal concentrations of abscisic acid. It is, therefore, considered that the abscisic acid is one of the factors aiding to stunting of diseased plants.
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