Expression of Key Genes of the Isoflavonoid Pathway in Hypocotyls and Cotyledons During Soybean Seed Maturation
Résumé
Soybean [Glycine max (L.) Merr.] seed accumulates large amounts of isoflavones, which are known for their phytoestrogenic activities. Three isoflavones (genistein, daidzein, and glycitein) are found in the seed. They are 4 to 10 times more concentrated in hypocotyls than in cotyledons. Isoflavone composition markedly diverges between these two seed parts and their accumulation begins in cotyledons at 40 d after flowering (DAF) once completed in hypocotyls. The relation between the genes of the isoflavonoid pathway and the isoflavone content has been investigated through (i) the variability of isoflavone synthase (IFS) genomic sequences in six cultivars with contrasted isoflavone content and (ii) the expression of key genes in two cultivars at 25, 40, and 60 DAF in hypocotyls and cotyledons. IFS1 and IFS2 polymorphism was not related to the isoflavone content. The expression of chalcone synthases (CHS7 and CHS8) was related to isoflavone accumulation kinetics in the cotyledons but not with the total content at maturity. CHS9 was the most expressed CHS isoform and the only one correlated with the hypocotyl isoflavone accumulation kinetics. The chalcone reductase (CHR) and CHI expression profiles were significantly different between the two seed parts. These results indicate that the regulation of the isoflavone pathway is controlled differently in cotyledons and hypocotyls during seed maturation. This discrepancy should be taken into account in the studies on genetic and environmental effects on isoflavone contents.