Osmotic stress induces genes, enzymes and accumulation of galactinol, raffinose and stachyose in seedlings of pea (Pisum sativum L.)

The objective of the present study was to recognize the molecular background of the accumulation of raffinose family oligosaccharides (RFOs) in pea (Pisum sativum L.) seedlings under osmotic stress conditions. The exposure of 5-day-old pea seedlings to osmotic stress for 48 h created by immersing roots in PEG8000 solution (-1.5 MPa) induced synthesis of galactinol and RFOs (raffinose and stachyose) in the epicotyl and root tissues, but not in cotyledons. After 24 h of recovery, galactinol completely disappeared, raffinose decreased fourfold and stachyose decreased twofold in roots, but increased in epicotyls. The temporary accumulation of RFOs resulted from a dramatic increase in the enzymatic activity and changes in expression of galactinol synthase (PsGolS), raffinose synthase (PsRS) and stachyose synthase (PsSTS) genes. PsGolS was induced by osmotic stress in both epicotyls as well as in roots. PsRS and PsSTS were induced only in epicotyls, but repressed or remained unaffected in roots, respectively. During recovery, the expression and activity of PsGolS, PsRS and PsSTS dramatically decreased. The expression of PsGolS gene, that level of mRNA transcript significantly decreased during recovery and whose promoter region was identified to contain some stress-related regulating elements, seems to play a crucial role in the biosynthesis of RFOs under osmotic stress. Possible signals that may trigger the induction of expression of PsGolS, PsRS and PsSTS genes and accumulation of RFOs in pea seedlings are discussed.