Putative osmosensor--OsHK3b--a histidine kinase protein from rice shows high structural conservation with its ortholog AtHK1 from Arabidopsis

OsRR26, a type-B response regulator, modulates salinity tolerance in rice via phytohormone-mediated ROS accumulation in roots and influencing reproductive development

MAIN CONCLUSION

OsRR26 is a cytokinin-responsive response regulator that promotes phytohormone-mediated ROS accumulation in rice roots, regulates seedling growth, spikelet fertility, awn development, represses NADPH oxidases, and negatively affects salinity tolerance. Plant two-component systems (TCS) play a pivotal role in phytohormone signaling, stress responses, and circadian rhythm. However, a significant knowledge gap exists regarding TCS in rice. In this study, we utilized a functional genomics approach to elucidate the role of OsRR26, a type-B response regulator in rice. Our results demonstrate that OsRR26 is responsive to cytokinin, ABA, and salinity stress, serving as the ortholog of Arabidopsis ARR11. OsRR26 primarily localizes to the nucleus and plays a crucial role in seedling growth, spikelet fertility, and the suppression of awn development. Exogenous application of cytokinin led to distinct patterns of reactive oxygen species (ROS) accumulation in the roots of both WT and transgenic plants (OsRR26OE and OsRR26KD), indicating the potential involvement of OsRR26 in cytokinin-mediated ROS signaling in roots. The application of exogenous ABA resulted in varied cellular compartmentalization of ROS between the WT and transgenic lines. Stress tolerance assays of these plants revealed that OsRR26 functions as a negative regulator of salinity stress tolerance across different developmental stages in rice. Physiological and biochemical analyses unveiled that the knockdown of OsRR26 enhances salinity tolerance, characterized by improved chlorophyll retention and the accumulation of soluble sugars, K+ content, and amino acids, particularly proline.

A translational regulator MHZ9 modulates ethylene signaling in rice

Ethylene plays essential roles in rice growth, development and stress adaptation. Translational control of ethylene signaling remains unclear in rice. Here, through analysis of an ethylene-response mutant mhz9, we identified a glycine-tyrosine-phenylalanine (GYF) domain protein MHZ9, which positively regulates ethylene signaling at translational level in rice. MHZ9 is localized in RNA processing bodies. The C-terminal domain of MHZ9 interacts with OsEIN2, a central regulator of rice ethylene signaling, and the N-terminal domain directly binds to the OsEBF1/2 mRNAs for translational inhibition, allowing accumulation of transcription factor OsEIL1 to activate the downstream signaling. RNA-IP seq and CLIP-seq analyses reveal that MHZ9 associates with hundreds of RNAs. Ribo-seq analysis indicates that MHZ9 is required for the regulation of ~ 90% of genes translationally affected by ethylene. Our study identifies a translational regulator MHZ9, which mediates translational regulation of genes in response to ethylene, facilitating stress adaptation and trait improvement in rice.

The Journey from Two-Step to Multi-Step Phosphorelay Signaling Systems

Background

The two-component signaling (TCS) system is an important signal transduction machinery in prokaryotes and eukaryotes, excluding animals, that uses a protein phosphorylation mechanism for signal transmission.

Conclusion

Prokaryotes have a primitive type of TCS machinery, which mainly comprises a membrane-bound sensory histidine kinase (HK) and its cognate cytoplasmic response regulator (RR). Hence, it is sometimes referred to as two-step phosphorelay (TSP). Eukaryotes have more sophisticated signaling machinery, with an extra component - a histidine-containing phosphotransfer (HPT) protein that shuttles between HK and RR to communicate signal baggage. As a result, the TSP has evolved from a two-step phosphorelay (His–Asp) in simple prokaryotes to a multi-step phosphorelay (MSP) cascade (His–Asp–His–Asp) in complex eukaryotic organisms, such as plants, to mediate the signaling network. This molecular evolution is also reflected in the form of considerable structural modifications in the domain architecture of the individual components of the TCS system. In this review, we present TCS system's evolutionary journey from the primitive TSP to advanced MSP type across the genera. This information will be highly useful in designing the future strategies of crop improvement based on the individual members of the TCS machinery.

Advances in Sensing, Response and Regulation Mechanism of Salt Tolerance in Rice

Soil salinity is a serious menace in rice production threatening global food security. Rice responses to salt stress involve a series of biological processes, including antioxidation, osmoregulation or osmoprotection, and ion homeostasis, which are regulated by different genes. Understanding these adaptive mechanisms and the key genes involved are crucial in developing highly salt-tolerant cultivars. In this review, we discuss the molecular mechanisms of salt tolerance in rice—from sensing to transcriptional regulation of key genes—based on the current knowledge. Furthermore, we highlight the functionally validated salt-responsive genes in rice.

What do archaeal and eukaryotic histidine kinases sense?

Signal transduction systems configured around a core phosphotransfer step between a histidine kinase and a cognate response regulator protein occur in organisms from all domains of life. These systems, termed two-component systems, constitute the majority of multi-component signaling pathways in Bacteria but are less prevalent in Archaea and Eukarya. The core signaling domains are modular, allowing versatility in configuration of components into single-step phosphotransfer and multi-step phosphorelay pathways, the former being predominant in bacteria and the latter in eukaryotes. Two-component systems regulate key cellular regulatory processes that provide adaptive responses to environmental stimuli and are of interest for the development of antimicrobial therapeutics, biotechnology applications, and biosensor engineering. In bacteria, two-component systems have been found to mediate responses to an extremely broad array of extracellular and intracellular chemical and physical stimuli, whereas in archaea and eukaryotes, the use of two-component systems is more limited. This review summarizes recent advances in exploring the repertoire of sensor histidine kinases in the Archaea and Eukarya domains of life.

Acclimation to salt modifies the activation of several osmotic stress-activated lipid signalling pathways in Chlamydomonas

Osmotic stress rapidly activates several phospholipid signalling pathways in the unicellular alga Chlamydomonas. In this report, we have studied the effects of salt-acclimation on growth and phospholipid signalling. Growing cells on media containing 100 mM NaCl increased their salt-tolerance but did not affect the overall phospholipid content, except that levels of phosphatidylinositol phosphate (PIP) and phosphatidylinositol 4,5-bisphosphate [PI(4,5)P₂] were reduced by one-third. When these NaCl-acclimated cells were treated with increasing concentrations of salt, the same lipid signalling pathways as in non-acclimated cells were activated. This was witnessed as increases in phosphatidic acid (PA), lyso-phosphatidic acid (L-PA), diacylglycerol pyrophosphate (DGPP), PI(4,5)P₂ and its isomer PI(3,5)P₂. However, all dose-dependent responses were shifted to higher osmotic-stress levels, and the responses were lower than in non-acclimated cells. When NaCl-acclimated cells were treated with other osmotica, such as KCl and sucrose, the same effects were found, illustrating that they were due to hyperosmotic rather than hyperionic acclimation. The results indicate that acclimation to moderate salt stress modifies stress perception and the activation of several downstream pathways.

Isolation, Characterization and Transcriptome Analysis of a Cytokinin Receptor Mutant Osckt1 in Rice

Cytokinins play important roles in regulating plant development, including shoot and root meristems, leaf longevity, and grain yield. However, the in planta functions of rice cytokinin receptors have not been genetically characterized yet. Here we isolated a rice mutant, Osckt1, with enhanced tolerance to cytokinin treatment. Further analysis showed that Osckt1 was insensitive to aromatic cytokinins but responded normally to isoprenoid and phenylurea-type cytokinins. Map-based cloning revealed that the mutation occurred in a putative cytokinin receptor gene, histidine kinase 6 (OsHK6). OsCKT1 was found to be expressed in various tissues throughout the plant and the protein was located in the endoplasmic reticulum. In addition, whole-genome gene expression profiling analysis showed that OsCKT1 was involved in cytokinin regulation of a number of biological processes, including secondary metabolism, sucrose and starch metabolism, chlorophyll synthesis, and photosynthesis. Our results demonstrate that OsCKT1 plays important roles in cytokinin perception and control of root development in rice.

Human microtubule affinity-regulating kinase 4 is stable at extremes of pH

MAP/microtubule affinity-regulating kinase 4 (MARK4) is a member of adenosine monophosphate-activated protein kinases, directly associated with cancer and neurodegenerative diseases. Here, we have cloned, expressed, and purified two variants of MARK4 [the kinase domain (MARK4-F2), and kinase domain along with 59 N-terminal residues (MARK4-F1)] and compared their stability at varying pH range. Structural and functional changes were observed by incubating both forms of MARK4 in buffers of different pH. We measured the secondary structure of MARK4 using circular dichroism and tertiary structure by measuring intrinsic fluorescence and absorbance properties along with the size of proteins by dynamic light scattering. We observed that at extremes of pH (below pH 3.5 and above pH 9.0), MARK4 is quite stable. However, a remarkable aggregate formation was observed at intermediate pH (between pH 3.5 and 9.0). To further validate this result, we have modeled both forms of MARK4 and performed molecular dynamics simulation for 15 ns. The spectroscopic observations are in excellent agreement with the findings of molecular dynamics simulation. We also performed ATPase activity at varying pH and found a significant correlation of structure of MARK4 with its enzyme activity. It is interesting to note that both forms of MARK4 are showing a similar pattern of structure changes with reference to pH.

OsHK3 is a crucial regulator of abscisic acid signaling involved in antioxidant defense in rice

In this study, the role of the rice (Oryza sativa L.) histidine kinase OsHK3 in abscisic acid (ABA)-induced antioxidant defense was investigated. Treatments with ABA, H2 O2 , and polyethylene glycol (PEG) induced the expression of OsHK3 in rice leaves, and H2 O2 is required for ABA-induced increase in the expression of OsHK3 under water stress. Subcellular localization analysis showed that OsHK3 is located in the cytoplasm and the plasma membrane. The transient expression analysis and the transient RNA interference test in rice protoplasts showed that OsHK3 is required for ABA-induced upregulation in the expression of antioxidant enzymes genes and the activities of antioxidant enzymes. Further analysis showed that OsHK3 functions upstream of the calcium/calmodulin-dependent protein kinase OsDMI3 and the mitogen-activated protein kinase OsMPK1 to regulate the activities of antioxidant enzymes in ABA signaling. Moreover, OsHK3 was also shown to regulate the expression of nicotinamide adenine dinucleotide phosphate oxidase genes, OsrbohB and OsrbohE, and the production of H2 O2 in ABA signaling. Our data indicate that OsHK3 play an important role in the regulation of ABA-induced antioxidant defense and in the feedback regulation of H2 O2 production in ABA signaling.