No abstract

The maize abscisic acid responsive protein Rab17 is a highly phosphorylated late embryogenesis abundant protein involved in plant responses to stress. In this study, we provide evidence of the importance of Rab17 phosphorylation by protein kinase CK2 in growth-related processes under stress conditions. We show the specific interaction of Rab17 with the CK2 regulatory subunits CK2␤-1 and CK2␤-3, and that these interactions do not depend on the phosphorylation state of Rab17. Live-cell fluorescence imaging of both CK2 and Rab17 indicates that the intracellular dynamics of Rab17 are regulated by CK2 phosphorylation. We found both CK2␤ subunits and Rab17 distributed over the cytoplasm and nucleus. By contrast, catalytic CK2␣ subunits and a Rab17 mutant protein (mRab17) that is not a substrate for CK2 phosphorylation remain accumulated in the nucleoli. A dual-color image shows that the CK2 holoenzyme accumulates mainly in the nucleus. The importance of Rab17 phosphorylation in vivo was assessed in transgenic plants. The overexpression of Rab17, but not mRab17, arrests the process of seed germination under osmotic stress conditions. Thus, the role of Rab17 in growth processes is mediated through its phosphorylation by protein kinase CK2. T he plant hormone abscisic acid (ABA) plays a major role in adaptation to osmotic stress and induces a number of genes that encode proteins generally assumed to be involved in protecting the cell and promoting recovery from stress. Proteins responsive to ABA accumulate during seed maturation; they naturally disappear during seed germination and can be induced to reappear by ABA treatment or osmotic stress in vegetative tissues (1, 2).Late embryogenesis abundant proteins (Lea) from group 2, responsive to ABA (Rab), or dehydrins are among the most common plant proteins involved in adaptation to water or osmotic stress. Several hypothetical roles have been proposed for Rab͞dehydrin proteins based on different experimental evidence, including binding to phosphate or sulfate ions (3), nuclear localization signal (NLS) peptides (4), calcium (5), and lipid vesicles containing acidic phospholipids (6), among others. All are aimed toward a protective role as chaperones to stabilize molecules or structures under stress conditions (4,7,8).Many proteins included in this Lea family contain the S domain (8), consisting of a tract of serines with several phosphorylation sites (9). Maize Rab17 protein is one of the most heavily phosphorylated proteins in mature embryos and is found both in nucleus and cytoplasm (4). The S domain of Rab17 is followed by a protein kinase CK2 phosphorylation consensus site (9). We previously established that Rab17 is phosphorylated by CK2 in serine residues of the S domain (10). Phosphorylation͞ dephosphorylation is an important mechanism that may regulate the function of Rab17 in the cell; however, the actual physiological function for Rab͞dehydrin proteins is still unknown, and a precise understanding of the function of Rab17 and the importance of its phosphorylation...

Lignin is an essential polymer in vascular plants that plays key structural roles in vessels and fibers. Lignification is induced by external inputs such as wounding, but the molecular mechanisms that link this stress to lignification remain largely unknown. In this work, we provide evidence that three maize (Zea mays) lignin repressors, MYB11, MYB31, and MYB42, participate in wound-induced lignification by interacting with ZML2, a protein belonging to the TIFY family. We determined that the three R2R3-MYB factors and ZML2 bind in vivo to AC-rich and GAT(A/C) cis-elements, respectively, present in a set of lignin genes. In particular, we show that MYB11 and ZML2 bind simultaneously to the AC-rich and GAT(A/C) cis-elements present in the promoter of the caffeic acid O-methyl transferase (comt) gene. We show that, like the R2R3-MYB factors, ZML2 also acts as a transcriptional repressor. We found that upon wounding and methyl jasmonate treatments, MYB11 and ZML2 proteins are degraded and comt transcription is induced. Based on these results, we propose a molecular regulatory mechanism involving a MYB/ZML complex in which wound-induced lignification can be achieved by the derepression of a set of lignin genes.