The ABI4 transcription factor is post-transcriptionally regulated by abiotic stress and hormones

The Abscisic Acid Insensitive 4 (ABI4) gene encodes an AP2-family transcription factor that plays a role in ABA signaling. ABI4 is also involved in seed development and germination, response to abiotic stresses such as drought and salinity and lateral root formation. ABI4 is relatively highly expressed during seed maturation and in early stages of seed germination. Its transcription levels dramatically reduced at later developmental stages. It is activated by glucose, ABA, salt and repressed by the growth hormone auxin. It was also shown that the protein levels of ABI4 are very low, suggesting that its protein levels may be also regulated posttranscriptional. We used transgenic Arabidopsis plants expressing eGFP-tagged ABI4 driven by the strong viral CaMV 35S promoter, to study posttranscriptional regulation of this highly modulated transcription factor. We found that steady-state levels of ABI4 were extremely low in the roots of seedling grown in optimal conditions. These levels were markedly enhanced upon exposure of the seedlings to abiotic stress and ABA. In addition, ABI4 is rapidly degraded by the 26S proteasome. Although the expression of the eGFP-tagged ABI4 was driven by the constitutive 35S CaMV promoter, that is highly active in most plant cells, the fluorescent ABI4 –tagged protein was observed primarily in the stele. This study suggests that both the level and the cell type accumulation of ABI4 protein is tightly regulated. Abiotic factors and plant hormones have similar effects on its transcript and protein levels. These double-check control of ABI4 reflects on its central role in plant development and cellular roles.