Constitutive expression of the beta-phaseolin gene in different tissues of transgenic alfalfa does not ensure phaseolin accumulation in non-seed tissue

Abstract

Phaseolin is a glycoprotein that constitutes the major storage protein in bean seeds. The phaseolin gene promoters function in a seed-specific manner. In an attempt to understand if events following transcription of the gene also contribute to the seed-specific accumulation of the phaseolin protein, we studied the effect of substituting the constitutive CaMV-35S promoter for the beta-phaseolin gene promoter on expression of the phaseolin gene in different plant organs. A chimeric gene consisting of the 35S promoter, the coding sequence of the beta-phaseolin gene (all five introns and six exons) and the 3'-flanking region of the beta-phaseolin gene, was introduced into alfalfa via Agrobacterium tumefaciens. While all organs examined shared high levels of phaseolin transcripts, the only organ that showed significant accumulation of the phaseolin protein were the mature seeds. Co-migration of the major immunoreactive polypeptides from the non-seed organs with the authentic beta-phaseolin polypeptides on SDS-PAGE indicates that the protein in non-seed organs undergoes correct post-translational processing and modification, but are more unstable in a non-seed environment.