The Arabidopsis Mediator CDK8 module genes CCT (MED12) and GCT (MED13) are global regulators of developmental phase transitions.

Temporal coordination of developmental programs is necessary for normal ontogeny, but the mechanism by which this is accomplished is still poorly understood. We have previously shown that two components of the Mediator CDK8 module encoded by CENTER CITY (CCT; Arabidopsis MED12) and GRAND CENTRAL (GCT; Arabidopsis MED13) are required for timing of pattern formation during embryogenesis. A morphological, molecular and genomic analysis of the post-embryonic phenotype of gct and cct mutants demonstrated that these genes also promote at least three subsequent developmental transitions: germination, vegetative phase change, and flowering. Genetic and molecular analyses indicate that GCT and CCT operate in parallel to gibberellic acid, a phytohormone known to regulate these same three transitions. We demonstrate that the delay in vegetative phase change in gct and cct is largely due to overexpression of miR156, and that the delay in flowering is due in part to upregulation of FLC. Thus, GCT and CCT coordinate vegetative and floral transitions by repressing the repressors miR156 and FLC. Our results suggest that MED12 and MED13 act as global regulators of developmental timing by fine-tuning the expression of temporal regulatory genes.

Mutations in two non-canonical Arabidopsis SWI2/SNF2 chromatin remodeling ATPases cause embryogenesis and stem cell maintenance defects.

SWI2/SNF2 chromatin remodeling ATPases play important roles in plant and metazoan development. Whereas metazoans generally encode one or two SWI2/SNF2 ATPase genes, Arabidopsis encodes four such chromatin regulators: the well-studied BRAHMA and SPLAYED ATPases, as well as two closely related non-canonical SWI2/SNF2 ATPases, CHR12 and CHR23. No developmental role has as yet been described for CHR12 and CHR23. Here, we show that although strong single chr12 or chr23 mutants are morphologically indistinguishable from the wild type, chr12 chr23 double mutants cause embryonic lethality. The double mutant embryos fail to initiate root and shoot meristems, and display few and aberrant cell divisions. Weak double mutant embryos give rise to viable seedlings with dramatic defects in the maintenance of both the shoot and the root stem cell populations. Paradoxically, the stem cell defects are correlated with increased expression of the stem cell markers WUSCHEL and WOX5. During subsequent development, the meristem defects are partially overcome to allow for the formation of very small, bushy adult plants. Based on the observed morphological defects, we named the two chromatin remodelers MINUSCULE 1 and 2. Possible links between minu1 minu2 defects and defects in hormone signaling and replication-coupled chromatin assembly are discussed.

Salt stress increases the expression of p5cs gene and induces proline accumulation in cactus pear.

Proline (Pro) is one of the most accumulated osmolytes in salinity and water deficit conditions in plants. In the present study, we measured the Pro content, the activity and the expression level of delta 1-pyrroline-5-carboxylate synthetase (P5CS: gamma-glutamyl kinase, EC 2.7.2.11 and glutamate-5-semialdehyde dehydrogenase, EC 1.2.1.41), a key regulatory enzyme involved in the biosynthesis of Pro, in cactus pear (Opuntia streptacantha) subjected to 6, 9 and 11 days of salt stress. Treatment with NaCl of O. streptacantha young plants resulted in a decrease in the cladode thickness and root length, and in a significant and gradual accumulation of Pro in young cladodes, in a time- and concentration-dependent manner. P5CS activity, studied as gamma-glutamyl kinase, was reduced at all times as a consequence of salt treatment, except at the sixth day at 75 and 150mM of NaCl, where a slight increase was observed. We isolated an open reading frame (ORF) fragment of p5cs gene. The deduced amino acid sequence of the P5CS protein exhibited 90.4% of identity with the P5CS protein from Mesembryanthemum crystallinum. RT-PCR analysis revealed that the Osp5cs gene of O. streptacantha was induced by salt stress at 9 and 11 days of treatment. Furthermore, ABA-induced Osp5cs gene expression was observed in cladodes of cactus pear young plants. We observed an evident correlation between the transcript up-regulation and the Pro accumulation under salt stress; however, these results do not parallel with the changes in P5CS enzymatic activity. This Pro accumulation might function as an osmolyte for the intracellular osmotic adjustment and might be playing a critical role in protecting photosynthetic activity in O. streptacantha plants under salt stress.