Arabidopsis thaliana bZIP44: a transcription factor affecting seed germination and expression of the mannanase-encoding gene AtMAN7.

Endo-β-mannanases ( MAN; EC. 3.2.1.78) catalyze the cleavage of β1→4 bonds in mannan polymers and have been associated with the process of weakening the tissues surrounding the embryo during seed germination. In germinating Arabidopsis thaliana seeds, the most highly expressed MAN gene is At MAN7 and its transcripts are restricted to the micropylar endosperm and to the radicle tip just before radicle emergence. Mutants with a T- DNA insertion in At MAN7 have a slower germination than the wild type. To gain insight into the transcriptional regulation of the At MAN7 gene, a bioinformatic search for conserved non-coding cis-elements (phylogenetic shadowing) within the Brassicaceae MAN7 gene promoters has been done, and these conserved motifs have been used as bait to look for their interacting transcription factors ( TFs), using as a prey an arrayed yeast library from A. thaliana. The basic-leucine zipper TF Atb ZIP44, but not the closely related Atb ZIP11, has thus been identified and its transcriptional activation upon At MAN7 has been validated at the molecular level. In the knock-out lines of Atb ZIP44, not only is the expression of the At MAN7 gene drastically reduced, but these mutants have a significantly slower germination than the wild type, being affected in the two phases of the germination process, both in the rupture of the seed coat and in the breakage of the micropylar endosperm cell walls. In the over-expression lines the opposite phenotype is observed. [ABSTRACT FROM AUTHOR]