a) Auxins:

• Indole-3-acetic acid (IAA) is the naturally occurring auxin.
• It is continuously produced in young meristematic tissue and moves rapidly to older tissues.
• If auxin concentration is more, its concentration is reduced by the enzyme, IAA oxidase.

Functions: IAA regulates cell elongation and differentiation, also affects permeability of the membrane, increases respiration, and promotes synthesis of mRNA.

How disease is induced?

• Increased IAA results in hypertrophy and decreased IAA results in atrophy.
• Increased IAA may be due to inhibition of IAA oxidase.
• Ex: Ralstonia solanacearum (Pseudomonas solanacearum), the causal agent of wilt of Solanaceous plants, induces a 100 fold increase in IAA level in diseased plants.
• Increased IAA levels have been reported in plants infected with the following pathogens Phytophthora infestans (Late blight of potato), Ustilago maydis (Maize smut), Plasmodiophora brassicae (Club root of crucifers), Sclerospora graminicola (Downy mildew of sorghum), Agrobacterium tumefaciens (Crown gall of apple), and Meloidogyne (Root knot nematode).

b) Gibberellins:

• First isolated from Gibberella fujikuroi (Conidial stage: Fusarium moniliforme), the causal agent of bakanae or foolish seedling disease of rice.
• Infected seedlings show abnormal elongation due to excessive elongation of internodes.

Functions: Cell elongation, stem and root elongation, promote flowering and growth of fruits. It also induces IAA synthesis. IAA and GA act synergistically.

Ex: Sclerospora sacchari, the causal agent of downy mildew of sugarcane induces GA production.

c) Cytokinins:

• Kinetin was the first compound isolated from herring sperm DNA and does not occur naturally in plants.

Functions:

• It inhibits breakdown of proteins and aminoacids and thereby inhibit senescence and they have the capacity to direct the flow of aminoacids and other nutrients towards high cytokinin concentration.
• Cytokinin activity increases in club root, in crown galls and in rust infected bean leaves.