ZmMKK4, a novel group C mitogen-activated protein kinase kinase in maize (Zea mays), confers salt and cold tolerance in transgenic Arabidopsis

A novel salt-induced gene from sheepgrass, LcSAIN2, enhances salt tolerance in transgenic Arabidopsis

Salt stress affects plant growth and development, and limits the productivity of crops. Sheepgrass can grow well under various environmental and soil conditions and is a good wild resource in Triticeae. Using 454 high throughout sequencing technique, a large number of salt stress responsive genes have been picked out from sheepgrass. In this study, a novel salt-induced gene and its promoter were cloned and the gene was designated as LcSAIN2 (Leymus chinensissalt-induced 2). Bioinformatics analysis predicted that LcSAIN2 has one transmembrane helix and is localized in nucleus. Experiments of subcellular localization in tobacco leaf cells also indicated that it was mainly localized in nucleus. Several stress responsive elements were found in the promoter region of the LcSAIN2 gene. The expression analysis confirmed that LcSAIN2 was induced by salinity, PEG, ABA, and cold stresses, especially by high salinity. Overexpression of LcSAIN2 in Arabidopsis enhanced salt tolerance of transgenic plants by accumulating osmolytes, such as soluble sugars and free proline, and improving the expression levels of some stress-responsive transcription factors and key genes. Our results suggest that LcSAIN2 might play an important positive modulation role in salt stress tolerance and be a candidate gene utilized for enhancing stress tolerance in wheat and other crops.

Genome-wide identification and analysis of expression profiles of maize mitogen-activated protein kinase kinase kinase

Mitogen-activated protein kinase (MAPK) cascades are highly conserved signal transduction model in animals, yeast and plants. Plant MAPK cascades have been implicated in development and stress responses. Although MAPKKKs have been investigated in several plant species including Arabidopsis and rice, no systematic analysis has been conducted in maize. In this study, we performed a bioinformatics analysis of the entire maize genome and identified 74 MAPKKK genes. Phylogenetic analyses of MAPKKKs from maize, rice and Arabidopsis have classified them into three subgroups, which included Raf, ZIK and MEKK. Evolutionary relationships within subfamilies were also supported by exon-intron organizations and the conserved protein motifs. Further expression analysis of the MAPKKKs in microarray databases revealed that MAPKKKs were involved in important signaling pathways in maize different organs and developmental stages. Our genomics analysis of maize MAPKKK genes provides important information for evolutionary and functional characterization of this family in maize.

ZmMKK3, a novel maize group B mitogen-activated protein kinase kinase gene, mediates osmotic stress and ABA signal responses

Mitogen-activated protein kinase (MAPK) cascades are important intracellular signaling modules and function as a convergent point for crosstalk during abiotic stress signaling. In this article, we isolated a novel group B MAPKK gene, ZmMKK3, from Zea mays. ZmMKK3 protein might be localized in both the cytoplasm and the nucleus. RNA blot analysis indicated that the ZmMKK3 transcription was up-regulated by abscisic acid (ABA), hydrogen peroxide (H(2)O(2)) and PEG, and that H(2)O(2) mediated PEG-induced expression of ZmMKK3. Constitutive expression of ZmMKK3 in Nicotiana tabacum reduced H(2)O(2) accumulation under osmotic stress by affecting antioxidant defense systems and alleviated reactive oxygen species-mediated injury under oxidative stress. Transgenic tobacco exhibited attenuated ABA sensitivity by means of an increased germination rate and main root growth. Taken together, these results indicate that ZmMKK3 is a positive regulator of osmotic tolerance and ABA signaling in plants.

Overexpression of a multiple stress-responsive gene, ZmMPK4, enhances tolerance to low temperature in transgenic tobacco

Mitogen-activated protein kinase (MAPK) cascades play important roles in mediating biotic and abiotic stress responses. In this study, we found that ZmMPK4 protein was predominantly localized in the nucleus. Semi-quantitative RT-PCR analysis revealed that the ZmMPK4 transcription in maize leaves was up-regulated by low temperature, high temperature and exogenous signaling molecules such as hydrogen peroxide, methyl jasmonate and ethephon. Hydrogen peroxide acted as second messenger to mediate 4°C-induced up-regulation of ZmMPK4 mRNA. Transgenic tobacco of overexpressing ZmMPK4 accumulated less reactive oxygen species (ROS), more peroxidase and catalase activities, more proline and soluble sugar contents, and more stress-responsive genes expression, leading to enhancing low temperature stress tolerance compared to the control plants. Taken together, these results strongly suggest that ZmMPK4 positively regulates low temperature stress tolerance in plants.

ZmHSP16.9, a cytosolic class I small heat shock protein in maize (Zea mays), confers heat tolerance in transgenic tobacco

Various organisms produce HSPs in response to high temperature and other stresses. The function of heat shock proteins, including small heat shock protein (sHSP), in stress tolerance is not fully explored. To improve our understanding of sHSPs, we isolated ZmHSP16.9 from maize. Sequence alignments and phylogenetic analysis reveal this to be a cytosolic class I sHSP. ZmHSP16.9 expressed in root, leaf and stem tissues under 40 °C treatment, and was up-regulated by heat stress and exogenous H₂O₂. Overexpression of ZmHSP16.9 in transgenic tobacco conferred tolerance to heat and oxidative stresses by increased seed germination rate, root length, and antioxidant enzyme activities compared with WT plants. These results support the positive role of ZmHSP16.9 in response to heat stress in plant.

KEY MESSAGE

The overexpression of ZmHSP16.9 enhanced tolerance to heat and oxidative stress in transgenic tobacco.

Cotton GhMKK5 affects disease resistance, induces HR-like cell death, and reduces the tolerance to salt and drought stress in transgenic Nicotiana benthamiana

Mitogen-activated protein kinase (MAPK) cascades are involved in various processes from plant growth and development to biotic and abiotic stress responses. MAPK kinases (MAPKKs), which link MAPKs and MAPKK kinases (MAPKKKs), play crucial roles in MAPK cascades to mediate a variety of stress responses in plants. However, few MAPKKs have been functionally characterized in cotton (Gossypium hirsutum). In this study, a novel gene, GhMKK5, from cotton belonging to the group C MAPKKs was isolated and characterized. The expression of GhMKK5 can be induced by pathogen infection, abiotic stresses, and multiple defence-related signal molecules. The overexpression of GhMKK5 in Nicotiana benthamiana enhanced the plants' resistance to the bacterial pathogen Ralstonia solanacearum by elevating the expression of pathogen resistance (PR) genes, including PR1a, PR2, PR4, PR5, and NPR1, but increased the plants' sensitivity to the oomycete pathogen Phytophthora parasitica var. nicotianae Tucker. Importantly, GhMKK5-overexpressing plants displayed markedly elevated expression of reactive oxygen species-related and cell death marker genes, such as NtRbohA and NtCDM, and resulted in hypersensitive response (HR)-like cell death characterized by the accumulation of H(2)O(2). Furthermore, it was demonstrated that GhMKK5 overexpression in plants reduced their tolerance to salt and drought stresses, as determined by statistical analysis of seed germination, root length, leaf water loss, and survival rate. Drought obviously accelerated the cell death phenomenon in GhMKK5-overexpressing plants. These results suggest that GhMKK5 may play an important role in pathogen infection and the regulation of the salt and drought stress responses in plants.

Biochemical identification of the OsMKK6-OsMPK3 signalling pathway for chilling stress tolerance in rice

MAPK (mitogen-activated protein kinase) pathways have been implicated in stress signalling in plants. In the present study, we performed yeast two-hybrid screening to identify partner MAPKs for OsMKK (Oryza sativa MAPK kinase) 6, a rice MAPK kinase, and revealed specific interactions of OsMKK6 with OsMPK3 and OsMPK6. OsMPK3 and OsMPK6 each co-immunoprecipitated OsMKK6, and both were directly phosphorylated by OsMKK6 in vitro. An MBP (myelin basic protein) kinase assay of the immunoprecipitation complex indicated that OsMPK3 and OsMPK6 were activated in response to a moderately low temperature (12°C), but not a severely low temperature (4°C) in rice seedlings. A constitutively active form of OsMKK6, OsMKK6DD, showed elevated phosphorylation activity against OsMPK3 and OsMPK6 in vitro. OsMPK3, but not OsMPK6, was constitutively activated in transgenic plants overexpressing OsMKK6DD, indicating that OsMPK3 is an in vivo target of OsMKK6. Enhanced chilling tolerance was observed in the transgenic plants overexpressing OsMKK6DD. Taken together, our data suggest that OsMKK6 and OsMPK3 constitute a moderately low-temperature signalling pathway and regulate cold stress tolerance in rice.

Type 4 metallothionein genes are involved in regulating Zn ion accumulation in late embryo and in controlling early seedling growth in Arabidopsis

Type 4 metallothionein (MT) genes are recognized for their specific expression in higher plant seeds, but their functions are still unclear. In this study, the functions of two Arabidopsis metallothionein genes, AtMT4a and AtMT4b, are investigated in seed development, germination and early seedling growth. Transcriptional analysis showed that these two genes are specifically expressed in late embryos. Subcellular localization displayed that both AtMT4a and AtMT4b are widespread distributed in cytoplasm, nucleus and membrane. Co-silencing RNAi of AtMT4a and AtMT4b reduced seed weight and influenced the early seedling growth after germination, whereas overexpression of these two genes caused the opposite results. Detailed analysis showed clearly the correlation of AtMT4a and AtMT4b to the accumulation of some important metal ions in late embryos, especially to Zn ion storing in seeds, which then serves as part of early Zn ion resources for post-germinated seedling growth. Furthermore, phytohormone abscisic acid (ABA) and gibberellic acid (GA) may play roles in regulating the expression and function of AtMT4a and AtMT4b during seed development; and this may influence Zn accumulation in seeds and Zn ion nutrient supplementation in the early seedling growth after germination.

Drought, salt, and temperature stress-induced metabolic rearrangements and regulatory networks

Plants regularly face adverse growth conditions, such as drought, salinity, chilling, freezing, and high temperatures. These stresses can delay growth and development, reduce productivity, and, in extreme cases, cause plant death. Plant stress responses are dynamic and involve complex cross-talk between different regulatory levels, including adjustment of metabolism and gene expression for physiological and morphological adaptation. In this review, information about metabolic regulation in response to drought, extreme temperature, and salinity stress is summarized and the signalling events involved in mediating stress-induced metabolic changes are presented.

ZmMKK4 regulates osmotic stress through reactive oxygen species scavenging in transgenic tobacco

Mitogen-activated protein kinase kinase (MAPKKs) are important components of MAPK cascades, which are universal signal transduction modules and play important role in regulating both plant development and biotic or abiotic stress responses. In this study, we identified the group C MAPKK gene, ZmMKK4, in maize (Zea mays L.). Overexpression of ZmMKK4 in tobacco enhanced tolerance to osmotic stress by increased proline content and antioxidant enzyme (POD) activities compared with wild-type plants. RT-PCR revealed that one peroxidase (POX) gene, NtPOX1, was higher in ZmMKK4-overexpressing plants than in the wild-type plants. In addition, the accumulation of reactive oxygen species (ROS) in ZmMKK4-overexpressing plants is much less than that of wild-type plants. These results suggest that ZmMKK4 may be involved in ROS signaling. Taken together, these results indicate that ZmMKK4 is a positive regulator of osmotic stress by regulating scavenging of ROS in plants.