Arabidopsis AtMPV17, a homolog of mice MPV17, enhances osmotic stress tolerance.

Mutation in the human MPV17 gene or the functional yeast orthologue SYM1 result in mitochondrial DNA depletion. MPV17 homologs are also found in plants including Arabidopsis, but the function of these genes remain unclear. Arabidopsis genome contains 10 MPV17 homologs. Among these, the AtMPV17 protein was localized in mitochondria as MPV17 and SYM1. The yeast sym1 knock out mutant cannot grow on ethanol-containing medium at 37 °C. AtMPV17 complements the ethanol growth defection of sym1 yeast MPV17 ortholog cells at 37 °C, suggesting that AtMPV17 is a functional ortholog of SYM1. AtMPV17 knock out mutant, atmpv17 show similar growth and seed development to those of the wild-type plant on normal growth condition. However, atmpv17 mutant is more sensitive to ABA and mannitol during germination and seedling growth than wild type plants. Growth retardation of the atmpv17 knock out mutant on medium containing ABA and mannitol is complemented by AtMPV17 overexpression. These results suggest that the AtMPV17 contributes to osmotic stress tolerance in plants.

Arabidopsis Raf-Like Kinase Raf10 Is a Regulatory Component of Core ABA Signaling.

Abscisic acid (ABA) is a phytohormone essential for seed development and seedling growth under unfavorable environmental conditions. The signaling pathway leading to ABA response has been established, but relatively little is known about the functional regulation of the constituent signaling components. Here, we present several lines of evidence that Arabidopsis Raf-like kinase Raf10 modulates the core ABA signaling downstream of signal perception step. In particular, Raf10 phosphorylates subclass III SnRK2s (SnRK2.2, SnRK2.3, and SnRK2.6), which are key positive regulators, and our study focused on SnRK2.3 indicates that Raf10 enhances its kinase activity and may facilitate its release from negative regulators. Raf10 also phosphorylates transcription factors (ABI5, ABF2, and ABI3) critical for ABAregulted gene expression. Furthermore, Raf10 was found to be essential for the in vivo functions of SnRK2s and ABI5. Collectively, our data demonstrate that Raf10 is a novel regulatory component of core ABA signaling.

OsMAPKKK63 is involved in salt stress response and seed dormancy control.

Approximately 75 MAP kinase kinase kinases (MAPKKKs) have been identified in the rice genome. However, only a few of them have been functionally characterized. In this paper, we report the function of a rice MAPKKK, OsMAPKKK63. OsMAPKKK63 was found to be induced by several abiotic stresses, including high salinity, chilling and drought. Our data indicate that OsMAPKKK63 possesses in vitro kinase activity and that it interacts with rice MAP kinase kinase OsMKK1 and OsMKK6. The two rice MKKs are known mediator of the salt stress response, implying that OsMAPKKK63 may be involved in the high salinity response. Our analysis of an OsMAPKKK63 knockout mutant indeed demonstrated that it is necessary for normal response to high salt. On the other hand, OsMAPKKK63 OX lines exhibited viviparous phenotype in both rice and Arabidopsis. The result suggests that OsMAPKKK63 may also be involved in seed dormancy control.

PtDRG1, a Desiccation Response Gene from Pyropia tenera (Rhodophyta), Exhibits Chaperone Function and Enhances Abiotic Stress Tolerance.

Pyropia are commercially valuable marine red algae that grow in the intertidal zone. They are extremely tolerant to desiccation stress. We have previously identified and reported desiccation response genes (DRGs) based on transcriptome analysis of P. tenera. Among them, PtDRG1 encodes a polypeptide of 22.6 kDa that is located in the chloroplast. PtDRG1 does not share sequence homology with any known gene deposited in public database. Transcription of PtDRG1 gene was upregulated by osmotic stress induced by mannitol or H2O2 as well as desiccation stress, but not by heat. When PtDRG1 was overexpressed in Escherichia coli or Chlamydomonas, transformed cells grew much better than control cells under high temperature as well as osmotic stress induced by mannitol and NaCl. In addition, PtDRG1 significantly reduced thermal aggregation of substrate protein under heat stress condition. These results demonstrate that PtDRG1 has a chaperone function and plays a role in tolerance mechanism for abiotic stress. This study shows that red algae have unknown stress proteins such as PtDRG1 that contributes to stress tolerance.

Arabidopsis MAP3K16 and Other Salt-Inducible MAP3Ks Regulate ABA Response Redundantly.

In the Arabidopsis genome, approximately 80 MAP3Ks (mitogen-activated protein kinase kinase kinases) have been identified. However, only a few of them have been characterized, and the functions of most MAP3Ks are largely unknown. In this paper, we report the function of MAP3K16 and several other MAP3Ks, MAP3K14/15/17/18, whose expression is salt-inducible. We prepared MAP3K16 overexpression (OX) lines and analyzed their phenotypes. The result showed that the transgenic plants were ABA-insensitive during seed germination and cotyledon greening stage but their root growth was ABA-hypersensitive. The OX lines were more susceptible to water-deficit condition at later growth stage in soil. A MAP3K16 knockout (KO) line, on the other hand, exhibited opposite phenotypes. In similar transgenic analyses, we found that MAP3K14/15/17/18 OX and KO lines displayed similar phenotypes to those of MA3K16, suggesting the functional redundancy among them. MAP3K16 possesses in vitro kinase activity, and we carried out two-hybrid analyses to identify MAP3K16 substrates. Our results indicate that MAP3K16 interacts with MKK3 and the negative regulator of ABA response, ABR1, in yeast. Furthermore, MAP3K16 recombinant protein could phosphorylate MKK3 and ABR1, suggesting that they might be MAP3K16 substrates. Collectively, our results demonstrate that MAP3K16 and MAP3K14/15/17/18 are involved in ABA response, playing negative or positive roles depending on developmental stage and that MAP3K16 may function via MKK3 and ABR1.