RESUMEN
BACKGROUND: Developing monocots that accumulate more vegetative tissue protein is one strategy for improving nitrogen-sequestration and nutritive value of forage and silage crops. In soybeans (a dicotyledonous legume), the vspA and B genes encode subunits of a dimeric vegetative storage protein that plays an important role in nitrogen storage in vegetative tissues. Similar genes are found in monocots; however, they do not accumulate in leaves as storage proteins, and the ability of monocot leaves to support accumulation of an ectopically expressed soybean VSP is in question. To test this, transgenic maize (Zea Mays L. Hi-II hybrid) lines were created expressing soybean vspB from a maize ubiquitin Ubi-1 promoter. RESULTS: From 81 bombardments, 101 plants were regenerated, and plants from five independent lines produced vspB transcripts and VSPbeta polypeptides. In leaves from seven-week-old plants (prior to flowering), VSPbeta accumulated to 0.5% of the soluble leaf protein in primary transgenic plants (R0), but to only 0.03% in R1 plants. During seed-filling (silage-stage) in R1 plants, the VSPbeta protein was no longer detected in leaves and stems despite continued presence of the vspB RNA. The RNA transcripts for this peptide either became less efficiently translated, or the VSPbeta protein became unstable during seed-fill. CONCLUSION: Developmental differences in the accumulation of soybean VSPbeta when transgenically expressed in maize show that despite no changes in the vspB transcript level, VSPbeta protein that is readily detected in leaves of preflowering plants, becomes undetectable as seeds begin to develop.