BINDING-PROTEIN EXPRESSION IS SUBJECT TO TEMPORAL, DEVELOPMENTAL AND STRESS-INDUCED REGULATION IN TERMINALLY DIFFERENTIATED SOYBEAN ORGANS

Binding protein (BiP) is a widely distributed and highly conserved end oplasmic-reticulum luminal protein that has been implicated in cotrans lational folding of nascent polypeptides, and in the recognition and d isposal of misfolded polypeptides. Analysis of cDNA sequences and geno mic blots indicates that soybeans (Glycine max L. Merr.) possess a sma ll gene family encoding BiP. The deduced sequence of BiP is very simil ar to that of other plant BiPs. We have examined the expression of BiP in several different terminally differentiated soybean organs includi ng leaves, pods and seed cotyledons. Expression of BiP mRNA increases during leaf expansion while levels of BiP protein decrease. Leaf BiP m RNA is subject to temporal control, exhibiting a large difference in e xpression in a few hours between dusk and night. The expression of BiP mRNA varies in direct correlation with accumulation of seed storage p roteins. The hybridization suggests that maturing-seed BiP is likely t o be a different isoform from vegetative BiPs. Levels of BiP protein i n maturing seeds vary with BiP mRNA. High levels of BiP mRNA are detec ted after 3 d of seedling growth, Little change in either BiP mRNA or protein levels was detected in maturing soybean pods, although BiP-pro tein levels decrease in fully mature pods. Persistent wounding of leav es by whiteflies induces massive overexpression of BiP mRNA while only slightly increasing BiP-protein levels. In contrast single-event punc ture wounding only slightly induces additional BiP expression above th e temporal variations. These observations indicate that BiP is not con stitutively expressed in terminally differentiated plant organs. Expre ssion of BiP is highest during the developmental stages of leaves, pod s and seeds when their constituent cells are producing seed or vegetat ive storage proteins, and appears to be subject to complex regulation, including developmental, temporal and wounding.