Accumulation and degradation of Em proteins in Arabidopsis thaliana: Evidence for post-transcriptional controls

Arabidopsis AtEm1 and AtEm6 proteins were overexpressed in E. coli and used to raise antibodies which were used to analyse Em expression at the protein level. One of the sera is specific for AtEm1 protein whereas the second reacts with both AtEm1 and AtEm6 proteins. These antibodies were used to analyse expression of Em genes at the protein level and to complete previous studies at the mRNA level. During seed maturation, AtEm1 protein accumulates earlier than AtEm6, in parallel with the corresponding mRNA but with a 3 d delay. During germination, AtEm1 protein undergoes two successive cleavages before being degraded. Both proteins are much more stable than the corresponding mRNA. Soaking of dormant seeds indicates that imbibition is sufficient to induce Em protein degradation and that germination per se is not required. AtEm1 and AtEm6 mRNA can be precociously induced by ABA in immature siliques, but protein accumulation could not be observed. A similar observation was made with leaves of transgenic plants ectopically expressing ABI3. These results establish clearly that Em protein accumulation is also tightly controlled by post-transcriptional mechanisms in addition to transcriptional ones.