EFFECTS OF ABSCISIC-ACID AND OSMOTICA ON HELIANTHININ GENE-EXPRESSION IN SUNFLOWER COTYLEDONS INVITRO

The regulation of sunflower seed storage protein synthesis by abscisic acid (ABA) and osmotic pressure was studied using a system of in vitro culture of developing cotyledons. In the presence of 10-5-10-4 M ABA or 0.5 and 0.7 M sorbitol, after 24 h of incubation, helianthinin (the major globulin protein in sunflower) is still detected as indicated by electrophoretic patterns and continues to accumulate as determined by [14C]leucine in vivo protein labelling. In controls, on the other hand, within 38 h, α and α′ helianthinin subunits are no longer detected. They exhibit an electrophoretic profile similar to that of a germinating seed 72 h after inhibition. This helianthinin degradation is preceded by a rapid decline in the amount of helianthinin-specific mRNA within 14 h of culture. Therefore, regulation by either ABA and/or sorbitol is at the transcript level. It appears quite clear that the effects of ABA on helianthinin gene expression are indirect, perhaps by inhibition of water uptake because high sorbitol concentrations maintain levels of mRNA and helianthinin synthesis without an increase in endogenous ABA content in cotyledons. In this respect, sunflower cotyledons appear to be unique in comparison with other plant tissues as shown by an unenhanced capacity to synthesize ABA as a function of increased osmotic pressure. © 1990.