Actin expression in germinating seeds of Phaseolus vulgaris L.

Actin was present at very low levels in the seeds of common bean (Phaseolus vulgaris L.) compared with those from other species, and was observed most ly in the embryo. A time-course of actin expression in germinating bean see ds revealed an induced expression of both the mRNA and protein. Initially, the actin mRNA in seeds was barely detectable by northern blot analysis. Ho wever, there was a substantial increase in the expression of the actin mRNA at 24, 48 and 72 h after imbibition, compared with an internal control con sisting of a late-embryogenesis-abundant (LEA) type IV gene from P. vulgari s. An increase in the amount of actin in total seed extracts that parallell ed that of the mRNA was detected by western blotting starting at 24 h after imbibition. This increase was more apparent when the embryo alone was anal yzed. Two-dimensional western blots initially revealed three actin isoforms with isoelectric points (pIs) of approximately 5.6, 5.7 and 5.8, the amoun ts of which increased within a 48-h period, when a new minor isoform of pi approximately 5.5 appeared; however, after 72 h, the pI-5.8 isoform had alm ost disappeared and the pI-5.5 isoform had disappeared completely, indicati ng that these two minor isoforms are expressed transiently. These results i ndicate that actin is at very low levels in the dry seed but undergoes an i ncreased and differential expression during imbibition, an event probably r equired to carry out all the necessary functions for germination.