The Arabidopsis AtbZIP9 protein fused to the VP16 transcriptional activation domain alters leaf and vascular development

The Arabidopsis group C bZIP transcriptional regulatory factors includes four members that are homologous to the maize Opaque-2 regulatory locus. These four Arabidopsis bZIP were organized into three groups of orthologues each of which possibly representing an ancestral angiosperm function. To better define the evolution of group C functions we initiated the characterization of AtbZIP9, a single Arabidopsis gene corresponding to one of the three group C ancestral functions and for which little functional information is available. Promoter fusion with GUS revealed that AtbZIP9 expression is restricted to the phloem of all organs analyzed and in situ hybridization confirmed this conclusion. AtbZIP9 mRNA accumulation was also shown to be repressed by glucose and induced by abscissic acid and cytokinin. Knockout T-DNA mutant or transgenic lines overexpressing AtbZIP9 mRNA were undistinguishable from the wild type indicating that post-transcriptional regulation and/or genetic redundancy act on AtbZIP9. To overcome the redundancy aspect, we produced transgenic plants expressing a fusion between AtbZIP9 cDNA and the VP16 transcriptional activator domain. These plants displayed leaf developmental defects and accumulation of phenolic compounds in the mesophyl. These alterations may be the consequence of changes in the phloem developmental process.