1.9.14
REDUCTION OF LIGHT-INDUCED ANTHOCYANIN ACCUMULATION IN INOCULATED SORGHUM MESOCOTYLS: IMPLICATIONS FOR A COMPENSATORY ROLE IN THE DEFENSE RESPONSE

SC LO1 and RL NICHOLSON1

1Department of Botany and Plant Pathology, Purdue University, West Lafayette, IN 47907-1155, USA

Background and objectives
The synthesis of 3-deoxyanthocyanidin phytoalexins represents a significant defense mechanism for sorghum in response to attempted pathogen infection [1]. The 3-deoxyanthocyanidins differ from normal anthocyanidins as they are not hydroxylated at the number 3 carbon of the flavonoid oxygen heterocycle. Both types of compounds are derived from the phenylpropanoid and flavonoid pathways and their biosynthesis represents two partially overlapping, competing pathways in sorghum. It has been suggested that repression of some cellular functions is likely to occur to ensure a metabolic balance during the host's responses to the stress of disease [2]. Consistent with this, we have observed that anthocyanin accumulation in sorghum is altered following inoculation with the non-pathogenic fungus Cochliobolus heterostrophus. This observation suggested that infected plants were producing the 3-deoxyanthocyanidin phytoalexins at the expense of the light-induced accumulation of anthocyanins.

Results and conclusions
The plant accumulates the anthocyanin cyanidin 3-dimalonyl glucoside in etiolated mesocotyls in response to light. Inoculation with C. heterostrophus drastically reduced the light-induced accumulation of anthocyanin by repressing the transcription of the anthocyanin biosynthesis genes encoding flavanone 3-hydroxylase, dihydroflavone 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 post-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.

References
1. Snyder BA, Nicholson RL, 1990. Science 248, 1637-1639.
2. Kombrink E, Hahlbrock K, 1990. Planta 181, 216-219.