Spatial and temporal expression of Cab mRNAs in cotyledons of the developing soybean seedling

Changes in the temporal and spatial distribution of the mRNAs for the chlorophyll-a/b-binding-protein gene (Cab) in cotyledons from developing soybean (Glycine max (L.) Merr.) seedlings were studied. Cab mRNAs could be detected in the polysomal polyadenylated poly(A)(+) mRNA population of cotyledons within 3 d after start of imbibition, prior to their emergence from soil, and declined prior to the onset of cotyledonary senescence. The Cab mRNA levels were compared to the levels of rbcS (ribulose-1,5-bisphosphate carboxylase small subunit) mRNAs in cotyledons and distinct differences in their expression programs were noted. Quantitative analyses with S1 nuclease were used to monitor the accumulation of the mRNAs of individual members of the Cab gene family. Cab 3, Cab 4, and Cab 5 mRNAs were differentially regulated in the cotyledons during post-germinative development. Cab 4 was the most abundant Cab gene mRNA representing approx. 4.3% of the cotyledonary polysomal poly(A)(+) mRNA population. In-situ hybridizations using methacrylate-imbedded tissue and (3)H-antisense- and -sense-strand RNA probes were used to determine the qualitative and quantitative distribution of Cab RNAs in cotyledonary cells. Cab RNAs were most abundant in the palisade cells. These results indicate an interaction of both developmental and environmental cues in modulating the expression of the Cab gene family in soybean cotyledons.