Reduction of Light-Induced Anthocyanin Accumulation in Inoculated Sorghum Mesocotyls¹
Implications for a Compensatory Role in the Defense Response

Sze-Chung Clive Lo and Ralph L. Nicholson^*

Department of Botany and Plant Pathology, Purdue University, West Lafayette, Indiana 47907-1155

Sorghum (Sorghum bicolor L. Moench) accumulates the anthocyanin cyanidin 3-dimalonyl glucoside in etiolated mesocotyls in response to light. Inoculation with the nonpathogenic fungus Cochliobolus heterostrophus drastically reduced the light-induced accumulation of anthocyanin by repressing the transcription of the anthocyanin biosynthesis genes encoding flavanone 3-hydroxylase, dihydroflavonol 4-reductase, and anthocyanidin synthase. In contrast to these repression effects, fungal inoculation resulted in the synthesis of the four known 3-deoxyanthocyanidin phytoalexins and a corresponding activation of genes encoding the key branch-point enzymes in the phenylpropanoid pathway, phenylalanine ammonia-lyase and chalcone synthase. In addition, a gene encoding the pathogenesis-related protein PR-10 was strongly induced in response to inoculation. The accumulation of phytoalexins leveled off by 48 h after inoculation and was accompanied by a more rapid increase in the rate of anthocyanin accumulation. The results suggest that the plant represses less essential metabolic activities such as anthocyanin synthesis as a means of compensating for the immediate biochemical and physiological needs for the defense response.

Plant Physiol. (1998) 116: 979-989
Copyright Clearance Center:   0032-0889/98/116/0979/11
© 1998 American Society of Plant Physiologists

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