Effects of ABA on the synthesis of total and chloroplast proteins in pumpkin cotyledons and on the accumulation of chloroplast gene transcripts

Effects of ABA on the synthesis of total and chloroplast proteins and on the accumulation of transcripts of chloroplast genes rbcL, psbA, psaA, and atpA were studied in detached green pumpkin (Cucurbita pepo L.) cotyledons. ABA suppressed chlorophyll formation to a higher extent than it suppressed the growth of either mature cotyledons with the well-developed photosynthetic apparatus or young, greening cotyledons. ABA (10(-4) M) inhibited the synthesis of some polypeptides and induced others, as was shown by one- and two-dimensional PAGE followed by fluorography. After 24-h incubation, ABA completely inhibited the synthesis of the large and small subunits of Rubisco, though the amount of mRNA encoded by the rbcL gene was reduced only by 25%. This evidence indicates that at least the large subunit of Rubisco was controlled at the post-transcriptional level. The pattern of ABA action on transcript accumulation depended on treatment duration. During the first 15 to 48 h, ABA suppressed the accumulation of all transcripts by 25-40%. When cotyledons were incubated on water and ABA solution for 120 to 168 h, the amounts of mRNAs encoded by the rbcL, atpA, and psaA genes were decreased, and ABA slowed their decay. The transcript encoded by the psbA gene remained at a high level throughout 168 h in both water- and ABA-treated cotyledons. Stabilizing effects of long-term ABA treatment on the content of chloroplast transcripts match the ABA-induced retardation of senescence in cotyledons incubated on the hormone solution for one or two weeks. The latter effect may be related to the production of proteins protecting cell macromolecules from degradation within senescing cotyledons.