Developmental Regulation and the Influence of Plant Sinks on Vegetative Storage Protein Gene Expression in Soybean Leaves

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Abstract
Soybeans (Glycine max L.) accumulate a storage glycoprotein which is abundant in vegetative tissues, but is only a minor component of seeds. Changes in vegetative storage protein gene expression in leaves of control and depodded plants were monitored throughout plant development. Western and Northern blot hybridization analysis of protein and mRNA levels, respectively, showed that expression of these genes was highly regulated during development. Expression correlated with periods when expected demand for mobilized leaf reserves by other plant sinks was low. Vegetative storage protein mRNA comprised about 0.5% of the total mRNA in immature leaves and declined at least 20-fold by flowering. Depodding or blockage of leaf petiole phloem transport increased these mRNAs to about 16% of the total mRNA. Transcript levels also increased dramatically after seed maturation, just before leaf senescence. Protein levels followed a similar pattern and were inversely related to the number of seed pods allowed to develop on the plants. The results support the role for these proteins as temporary storage molecules which can be rapidly synthesized or degraded according to the need for nutrients by other plant tissues.