INVOLVEMENT OF CARBOHYDRATES AND CYTOKININS IN PATHOGENICITY OF HELMINTHOSPORIUM-CARBONUM

Significant stimulation of the number of appressoria, penetration and colonization by conidia of Helminthosporium carbonum occurred on decolorized maize leaves when exogenous carbohydrates and leaf leachates were added. Germination and germ tube length, however, did not exhibit appreciable differences on decolorized or non-decolorized maize leaves. Lower germination was recorded by leached conidia on decolorized leaves; while appressoria, penetration and colonization were absent. Addition of exogenous nutrients (sucrose > leaf leachates > yeast extract > glucose) enabled conidia to accomplish appressoria, penetration and colonization. Optimum levels for various nutrients observed were 2% (w/v) sucrose/glucose or 0.1% (w/v) yeast extract. Higher concentrations inhibited the infection stages of the pathogen. Depletion of host carbohydrates from green islands/infection sites adversely affect appressoria formation, penetration and colonization; and the loss of carbohydrates from the spore affects germination. Cytokinin-like activity at the infection sites/green islands increased with the period of incubation of the host as compared to the surrounding tissue or tissue under water drops. The culture filtrate extracts of H. carbonum recorded cytokinin-like activity which increased with growth of the fungus. TLC (thin layer chromatography) of cytokinin-like substances (tissue extract and culture filtrate) revealed major activity was confined to Rf zones 0.6 to 0.8 which co-chromatographed with zeatin and zeatin riboside. These substances increase at infection sites by virtue of which carbohydrates accumulate at these sites ensuring a continuous supply to the growing pathogen.