STRESS RESPONSES IN ALFALFA (MEDICAGO-SATIVA L) .19. TRANSCRIPTIONAL ACTIVATION OF OXIDATIVE PENTOSE-PHOSPHATE PATHWAY GENES AT THE ONSET OF THE ISOFLAVONOID PHYTOALEXIN RESPONSE

We have isolated cDNA clones encoding the pentose phosphate pathway en zymes 6-phosphogluconate dehydrogenase (6PGDH, EC 1.1.1.44) and glucos e 6-phosphate dehydrogenase (G6PDH, EC 1.1.1.49) from alfalfa (Medicag o sativa L.). These exhibit extensive nucleotide and amino acid sequen ce similarity to the corresponding genes from bacteria, Drosophila and mammals. Transcripts encoding both enzymes are expressed at high leve ls in roots and nodules. Exposure of alfalfa suspension cells to an el icitor from yeast cell walls results in co-ordinated increases in tran scription rates for both genes, followed by increased steady state tra nscript levels but only slightly increased extractable enzyme activiti es, at the onset of accumulation of isoflavonoid phytoalexins. Levels of NADPH and NADP remain relatively constant in alfalfa cells followin g elicitation. The rapid transcriptional activation of 6PGDH and G6PDH does not therefore appear to be a response to altered pyridine nucleo tide redox state. These genes appear to respond to early events in eli citor-mediated signalling rather than to subsequent elicitor-induced c hanges in secondary metabolism. Hydrogen peroxide, a potential signal for elicitation of anti-oxidative genes in biologically stressed plant cells, did not induce 6PGDH or G6PDH transcripts or enzymatic activit y.