The small RNA biogenesis in rice is regulated by MAP kinase-mediated OsCDKD phosphorylation.

CDKs are the master regulator of cell division and their activity is controlled by the regulatory subunit cyclins and phosphorylation by the CAKs. However, the role of MAP kinases in regulating plant cell cycle or CDKs have not been explored. Here, we report that the MAP kinases OsMPK3, OsMPK4, and OsMPK6 physically interact and phosphorylate OsCDKD and its regulatory subunit OsCYCH in rice. MAP kinases phosphorylate CDKD at Ser-168 and Thr-235 residues in OsCDKD. The MAP kinase-mediated phosphorylation of OsCDKD is required for its activation to control the small RNA biogenesis. The phosphodead version of OsCDKD fails to activate the C-terminal domain of RNA Polymerase II, thereby negatively impacting small RNA transcription. Further, the overexpression lines of wild-type (WT) OsCDKD and phosphomimic OsCDKD show increased root growth, plant height, tiller number, panicle number, and seed number in comparison to WT, phosphodead OsCDKD-OE, and kinase-dead OsCDKD-OE plants. In a nutshell, our study establishes a novel regulation of OsCDKD by MAPK-mediated phosphorylation in rice. The phosphorylation of OsCDKD by MAPKs imparts a positive effect on rice growth and development by regulating miRNAs transcription.

MPK4-mediated phosphorylation of PHYTOCHROME INTERACTING FACTOR4 controls thermosensing by regulating histone variant H2A.Z deposition.

Plants can perceive a slight upsurge in ambient temperature and respond by undergoing morphological changes, such as elongated hypocotyls and early flowering. The dynamic functioning of PHYTOCHROME INTERACTING FACTOR4 (PIF4) in thermomorphogenesis is well established, although the complete regulatory pathway involved in thermosensing remains elusive. We establish that an increase in temperature from 22˚C to 28˚C induces upregulation and activation of MITOGEN-ACTIVATED PROTEIN KINASE 4 (MPK4) in Arabidopsis (Arabidopsis thaliana), subsequently leading to the phosphorylation of PIF4. Phosphorylated PIF4 represses the expression of ACTIN-RELATED PROTEIN 6 (ARP6), which is required for mediating the deposition of histone variant H2A.Z at its target loci. Furthermore, we demonstrate that variations in ARP6 expression in PIF4 phosphor-null and phosphor-mimetic seedlings affect hypocotyl growth at 22˚C and 28˚C by modulating the regulation of ARP6-mediated H2A.Z deposition at the loci of genes involved in elongating hypocotyl cells. Interestingly, the expression of MPK4 is also controlled by H2A.Z deposition in a temperature-dependent manner. Taken together, these findings highlight the regulatory mechanism of thermosensing by which MPK4-mediated phosphorylation of PIF4 affects ARP6-mediated H2A.Z deposition at the genes involved in hypocotyl cell elongation.

Deciphering the regulation of transporters and mitogen-activated protein kinase in arsenic and iron exposed rice.

This study investigates the influence of arsenic (As) and iron (Fe) on the molecular aspects of rice plants. The mRNA-abundance of As (OsLsi, OsPHT, OsNRAMP1, OsABCC1) and Fe (OsIRT, OsNRAMP1, OsYSL, OsFRDL1, OsVIT2, OsSAMS1, OsNAS, OsNAAT1, OsDMAS1, OsTOM1, OsFER) related genes has been observed in 12-d old As and Fe impacted rice varieties. Analyses of phytosiderophores synthesis and Fe-uptake genes affirm the existence of specialized Fe-uptake strategies in rice with varieties PB-1 and Varsha favouring strategy I and II, respectively. Expression of OsNAS3, OsVIT2, OsFER and OsABCC1 indicated PB-1's tolerance towards Fe and As. Analysis of mitogen-activated protein kinase cascade members (OsMKK3, OsMKK4, OsMKK6, OsMPK3, OsMPK4, OsMPK7, and OsMPK14) revealed their importance in the fine adjustment of As/Fe in the rice system. A conditional network map was generated based on the gene expression pattern that unfolded the differential dynamics of both rice varieties. The mating based split ubiquitin system determined the interaction of OsIRT1 with OsMPK3, and OsLsi1 with both OsMPK3 and OsMPK4. In-silico tools also confirmed the binding affinities of OsARM1 with OsLsi1, OsMPK3 and OsMPK4, and of OsIDEF1/OsIRO2 with OsIRT1 and OsMPK3, supporting our hypothesis that OsARM1, OsIDEF1, OsIRO2 were active in the connections discovered by mbSUS.

Rice Mitogen-Activated Protein Kinase regulates serotonin accumulation and interacts with cell cycle regulators under prolonged UV-B exposure.

Stress conditions such as UV-B exposure activates MAPKs in Arabidopsis and rice. UV-B radiation is hazardous to plant as it causes photosystem disruption, DNA damage and ROS generation. Here we report its effect on biological pathways by studying the global changes in transcript profile in rice seedling exposed to UV-B radiation for 1 h and 16 h. Short UV-B exposure (1 h) led to moderate changes, while a drastic change in transcript landscape was observed after long term UV-B exposure (16 h) in rice seedlings. Prolonged UV-B exposure negatively impacts the expression of cell cycle regulating genes and several other metabolic pathways in developing seedlings. MAP kinase signaling cascade gets activated upon UV-B exposure similar to reports in Arabidopsis indicating conservation of its function in both dicot and monocot. Expression analysis in inducible overexpression transgenic lines of MPK3 and MPK6 shows higher transcript abundance of phytoalexin biosynthesis gene like Oryzalexin D synthase and Momilactone A synthase, along with serotonin biosynthesis genes. An accumulation of serotonin was observed upon UV-B exposure and its abundance positively correlates with the MPK3 and MPK6 transcript level in the respective over-expression lines. Interestingly, multiple cell cycle inhibitor proteins including WEE1 and SMR1 interact with MPK3 and MPK6 thus, implying a major role of this pathway in cell cycle regulation under stress condition. Overall overexpression of MPK3 and MPK6 found to be detrimental for rice as overexpression lines shows higher cell death and compromised tolerance to UV-B.

MAP kinases may mediate regulation of the cell cycle in rice by E2F2 phosphorylation.

E2F is the key transcription factor that determines the proliferative status of cells by regulating the G1/S phase of the cell cycle. In this study, we show that in rice (Oryza sativa), OsE2F2 is a phosphorylation target of MAP kinases. The MAP kinases OsMPK3, OsMPK4, and OsMPK6 interact with and phosphorylate OsE2F2. Next, we determined the serine and threonine residues that could play a role in the phosphorylation of OsE2F2. Subsequently, our study suggests a possible link between MAP kinase-mediated OsE2F2 phosphorylation and its impact on DNA proliferation in the roots of rice seedlings. Finally, we found positive feedback regulation of OsMPK4 by OsE2F2. Therefore, our study hints at the potential impact of MAP kinase signaling on the cell cycle of rice plants.

Plant cell cycle regulators: Mitogen-activated protein kinase, a new regulating switch?

Cell cycle is essential for the maintenance of genetic material and continuity of a species. Its regulation involves a complex interplay between multiple proteins with diverse molecular functions such as the kinases, transcription factors, proteases and phosphatases. Every step of this cycle requires a certain combination of these protein regulators which paves the way for the next stage. It is now evident that plants have their own unique features in the context of cell cycle regulation. Cell cycle in plants is not only necessary for maintenance of its physio-morphological parameter but it also regulates traits important for mankind like grain or fruit size. This makes it even more important to understand how plants regulate its cell cycle amidst various a/biotic stresses it is subjected to during its lifetime. The association of MAPK signaling pathways with every major developmental and stress response pathways in plants raises the question of its potential role in cell cycle regulation. There are number of cell cycle regulating proteins with putative sites for MAPK phosphorylation. The MAPK signaling pathway may directly or in a parallel pathway regulate the plant cell cycle. Unraveling the role of MAPK in cell cycle will open up new arenas to explore.