Analysis of the response of Medicago truncatula calcium oxalate mutants to abiotic stress.

摘要

Calcium oxalate crystals are found in most plant species. In Medicago truncatula wild-type A17, crystals accumulate in leaves along the secondary veins and the only role attributed to them so far is defense against chewing insects. Calcium oxalate deficient (cod) mutants were isolated in M. truncatula; the cod mutants include cod5, which completely lack crystals in the leaves and cod6 that accumulates fewer and smaller crystals compared to A17. We analyzed gene expression in the cod mutants and A17 using GeneChipRTM Medicago Genome Arrays and found important differences in transcriptome between the three genotypes. In particular, we found a gene annotated as glyoxalase I (GLXI) referred by its tentative consensus number, TC122307, which is up-regulated in cod6 and down-regulated in cod5 as compared to constitutive levels in A17. Similarly, another GLXI-like gene, TC123769, is up-regulated in cod6 and transcripts are down-regulated in cod5. In previous studies, overexpression of GLXI has been linked to salt tolerance using transgenic approaches. Therefore, we evaluated salt tolerance in the cod mutants to determine if they respond differently to salt stress and if this response could be attributed to differential expression of the GLXI -like genes in M. truncatula. The cod6 mutant was significantly more tolerant to sodium chloride compared to A17 and cod5. Transcript levels of TC122307 and TC123769 do not increase in leaves following sodium chloride treatment. We also measured GLXI activity in the cod mutants and A17 and found no differences between genotypes. Moreover, tolerance to methylglyoxal, the substrate of GLXI, was evaluated and we found that the three genotypes were equally sensitive to toxic levels of methylglyoxal. Therefore, we conclude that TC122307 and TC123769 do not encode a functional GLXI. Another gene, TC122323, which is more likely to encode GLXI based on sequence analysis, is constitutively expressed in the three genotypes which would explain the similar glyoxalase I activity levels in the cod mutants and the equal sensitivities to methylglyoxal. Overall, TC122307 and TC123769 are not likely to encode GLXI, but potentially contribute to salt tolerance in cod6 and encode members of the vicinal oxygen chelate enzyme superfamily. Therefore, the genes encoding these proteins are excellent candidates for further study as tools to enhance salt tolerance in crop plants.