Stress responses in alfalfa (Medicago sativa L.) XIX. Transcriptional activation of oxidative pentose phosphate pathway genes at the onset of the isoflavonoid phytoalexin response

Abstract

We have isolated cDNA clones encoding the pentose phosphate pathway enzymes 6-phosphogluconate dehydrogenase (6PGDH, EC 1.1.1.44) and glucose 6-phosphate dehydrogenase (G6PDH, EC 1.1.1.49) from alfalfa (Medicago sativa L.). These exhibit extensive nucleotide and amino acid sequence similarity to the corresponding genes from bacteria, Drosophila and mammals. Transcripts encoding both enzymes are expressed at high levels in roots and nodules. Exposure of alfalfa suspension cells to an elicitor from yeast cell walls results in co-ordinated increases in transcription rates for both genes, followed by increased steady state transcript levels but only slightly increased extractable enzyme activities, at the onset of accumulation of isoflavonoid phytoalexins. Levels of NADPH and NADP remain relatively constant in alfalfa cells following elicitation. The rapid transcriptional activation of 6PGDH and G6PDH does not therefore appear to be a response to altered pyridine nucleotide redox state. These genes appear to respond to early events in elicitor-mediated signalling rather than to subsequent elicitor-induced changes 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 activity.

Iron-Superoxide Dismutase Expression in Transgenic Alfalfa Increases Winter Survival without a Detectable Increase in Photosynthetic Oxidative Stress Tolerance

“…When plant cells are treated with elicitors, H 2 O 2 rises, ATP is depleted, and the NADH/NAD ratio transiently drops (Robertson et al, 1995). A similar experiment noted that in the long term (hours rather than minutes), the redox potential (NADPH/NADP) did not change but genes in the pentose-phosphate pathway were activated, which suggests a greater supply of NADPH (Fahrendorf et al, 1995). Cellular redox is an important contributing factor in the acclimation process.…”

Stress responses in alfalfa (Medicago sativa L.). XX. Transcriptional activation of phenylpropanoid pathway genes in elicitor-induced cell suspension cultures

“…CHOMT activity has been shown to increase in response to elicitation, although the levels of its product (a methoxychalcone nod gene inducer) that accumulate are much lower than the levels of the phytoalexin medicarpin [25]. Induction of 6PGDH appears necessary to maintain sufficient pentose phosphate pathway activity to supply the N A D P H required as co-substrate for 6 of the enzymatic steps leading to medicarpin formation [12]. The transcriptional activation of the gene encoding the lignin biosynthetic enzyme COMT is less easy to explain, as the activity of this enzyme did not significantly increase in the elicited cultures (W. Ni and R.A. Dixon, unpublished observations).…”

“…Rectangles of the same size were drawn around each slot and the entire area of the slot was quantitated. 1;3-fl-D glucanase as an example of a defense response gene distinct from those of secondary metabolism, and 6-phosphogluconate dehydrogenase as an elicitor-inducible gene of primary metabolism (the oxidative pentose phosphate pathway which supplies N A D P H for medicarpin synthesis) [12]. Nucleoside diphosphate kinase was selected as a gene which encodes a potential signal transduction component which may be wound-induced in plant cells [ 17], and for which we have recently obtained cDNAs from an alfalfa cell suspension library (T. Fahrendorf and R.A. Dixon, unpublished results).…”

“…Transcription of CHS, CHOMT, COMT, nucleoside diphosphate kinase and glucanase remained elevated (at least 50 ~o of maximum rate) up to 24 h after elicitation, whereas transcription of PAL, C4H, CHI, C H R and IFR was more transient. In replicate experiments with different cell batches, the absolute timings of transcriptional activation of the various genes varied somewhat, but the relative timings remained the same (data not shown; see [ 12] for a detailed analysis of the variability of transcription kinetics of pentose phosphate pathway enzymes in elicited alfalfa cultures).…”

Updates on Glucose 6-Phosphate Dehydrogenase (G6PD): From Prokaryotes to Human

“…In plants like alfalfa, the root nodules containing fungi have been observed to induce G6PD transcription and activity [118]. Light also tend to induces plastidic G6PD expression in light deprived potato shoots since plastidic G6PD can be activated by a low level of NADPH to NADP + ratio [119].…”