Abstract
Proline is one of common osmolytes in osmotic-stressed plants. We previously reported that an ACTCAT sequence acts as a cis-acting element involved in hypoosmolarity- and proline-responsive expression of an Arabidopsis proline dehydrogenase (ProDH) gene.
Search of the database for cis-acting elements revealed that the ACTCAT sequence is similar to the GCN4 motif recognized by bZIPs. We examined whether Arabidopsis bZIPs function as transcription factors for the ACTCAT sequence. Transient assay revealed that the four bZIPs, named an ATB2 subgroup, activate the expression of a reporter gene driven by the ACTCAT sequence. The transactivation activity of the ATB2 subgroup was enhanced by hypoosmolarity. The recombinant ATB2 subgroup proteins specifically bound to the ACTCAT sequence. Northern blotting indicated that expression of two genes of the ATB2 subgroup, AtbZIP2/GBF5 and AtbZIP53, is induced by hypoosmolarity. These results suggest that the ATB2 subgroup functions as a transcriptional activator for hypoosmolarity-inducible expression of ProDH.
