Metabolism of Indoleacetic Acid in Rust Diseases. I. Factors Influencing Rates of Decarboxylation

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Abstract
A daily comparison was made of indoleacetic acid (IAA) degradation in the dark by healthy and rust-affected 1st leaf of wheat. Normal leaves showed a progressive increase in capacity to decarboxylate exogenous IAA when supplied at a concentration usually active in growth assays. With infected leaves, 3 phases could be distinguished. During the first 2-3 days after inoculation, there was a significant increase in IAA decarboxylation. Shortly before development of symptoms and continuing into the sporulation stage of fungus growth, the initially high rates of decarboxylation declined to the same level as the healthy controls. Only after sporulation was well under way did the rates of diseased tissue fall appreciably below the control rates. It is concluded that decarboxylation reactions, presumed to occur through an IAA oxidase, cannot account for increases in IAA during the early critical events of mycelial development of the stem-rust fungus in susceptible wheat. Although IAA accumulation during sporulation may arise from inhibition of the oxidase, analysis of tissue and exogenous concentrations of residual radioactivity suggest other alternatives for the data on decarboxylation rates during sporulation, such as failure of IAA to be taken up by diseased tissues. Studies of blue, red, and far-red illumination indicate that decarboxylation rates may be regulated by co-factors or inhibitors controlled by a system similar to that described for photoperiod phenomena. Blue and far-red treatments, when effective, stimulated decarboxylation. Red irradiation usually was inhibitory, but occasionally in older tissue, stimulated rates after 2-3 hours'' exposure. Although the conditions influencing IAA decarboxylation have not been explored adequately, the data emphasize a need for caution in evaluating the role of IAA oxidase in the development of fungal infections.