Abiotic stresses and soil nutrient limitations are major environmental conditions that reduce plant growth, productivity and quality. Plants have evolved mechanisms to perceive these environmental challenges, transmit the stress signals within cells as well as between cells and tissues, and make appropriate adjustments in their growth and development in order to survive and reproduce. In recent years, significant progress has been made on many fronts of the stress signaling research, particularly in understanding the downstream signaling events that culminate at the activation of stress- and nutrient limitation-responsive genes, cellular ion homeostasis, and growth adjustment. However, the revelation of the early events of stress signaling, particularly the identification of primary stress sensors, still lags behind. In this review, we summarize recent work on the genetic and molecular mechanisms of plant abiotic stress and nutrient limitation sensing and signaling and discuss new directions for future studies.

Plant abiotic stress response and nutrient use efficiency.

Arsenic (As) and cadmium (Cd) are two toxic elements that have a relatively high risk of transfer from paddy soil to rice grain. Rice is a major dietary source of these two elements for populations consuming rice as a staple food. Reducing their accumulation in rice grain is important for food safety and human health. We review recent progress in understanding the biogeochemical processes controlling As and Cd bioavailability in paddy soil, the mechanisms of their uptake, translocation and detoxification in rice plants, and strategies to reduce their accumulation in rice grain. Similarities and differences between the two elements are emphasized. Some knowledge gaps are also identified. The concentrations of As and Cd in rice grain vary by three orders of magnitude, depending on the bioavailability of the two elements in soil, rice genotype and growing conditions. The redox potential in paddy soil has a profound but opposite effect on As and Cd bioavailability, whereas soil pH affects Cd bioavailability more than As bioavailability. A number of key genes involved in As and Cd uptake, translocation, sequestration, and detoxification in rice have been characterized. Allelic variations of several genes underlying the variations in Cd accumulation have been identified, but more remains to be elucidated, especially for As. Two types of strategies can be used to reduce As and Cd accumulation, reducing their bioavailability in soil or their uptake and translocation in rice. Reducing the accumulation of both As and Cd in rice simultaneously remains a great challenge.

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Arsenic and cadmium accumulation in rice and mitigation strategies

Arsenic is the most prevalent environmental toxic substance and ranks first on the US Environmental Protection Agency's Superfund List Arsenic is a carcinogen and a causative agent of numerous human diseases Paradoxically arsenic is used as a chemotherapeutic agent for treatment of acute promyelocytic leukemia Inorganic arsenic has two biological important oxidation states: As(V) (arsenate) and As(III) (arsenite) Arsenic uptake is adventitious because the arsenate and arsenite are chemically similar to required nutrients Arsenate resembles phosphate and is a competitive inhibitor of many phosphate-utilizing enzymes Arsenate is taken up by phosphate transport systems In contrast, at physiological pH, the form of arsenite is As(OH)(3), which resembles organic molecules such as glycerol Consequently, arsenite is taken into cells by aquaglyceroporin channels Arsenic efflux systems are found in nearly every organism and evolved to rid cells of this toxic metalloid These efflux systems include members of the multidrug resistance protein family and the bacterial exchangers Acr3 and ArsB ArsB can also be a subunit of the ArsAB As(III)-translocating ATPase, an ATP-driven efflux pump The ArsD metallochaperone binds cytosolic As(III) and transfers it to the ArsA subunit of the efflux pump Knowledge of the pathways and transporters for arsenic uptake and efflux is essential for understanding its toxicity and carcinogenicity and for rational design of cancer chemotherapeutic drugs

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Pathways of arsenic uptake and efflux.

Mechanisms underlying the photocatalytic degradation pathway of ciprofloxacin with heterogeneous TiO2

Arsenic (As) is a non-essential toxic metalloid whose elevated concentration in rice grains is a serious issue both for rice yield and quality, and for human health. The rice-As interactions, hence, have been studied extensively in past few decades. A deep understanding of factors influencing As uptake and transport from soil to grains can be helpful to tackle this issue so as to minimize grain As levels. Arsenic uptake at the root surface by rice plants depends on factors like iron plaque and radial oxygen loss. There is involvement of a number of transporters viz., phosphate transporters and aquaglyceroporins in the uptake and transport of different As species and in the movement to subcellular compartments. These processes are also affected by sulfur availability and consequently on the level of thiol (-SH)-containing As binding peptides viz., glutathione (GSH) and phytochelatins (PCs). Further, the role of phloem in As movement to the grains is also suggested. This review presents a detailed map of journey of As from soil to the grains. The implications for the utilization of available knowledge in minimizing As in rice grains are presented.

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The Journey of Arsenic from Soil to Grain in Rice.

Soil contamination with arsenic (As) can cause phytotoxicity and elevated As accumulation in rice grain. Here, we used a forward genetics approach to investigate the mechanism of arsenate (As(V)) tolerance and accumulation in rice. A rice mutant hypersensitive to As(V), but not to As(III), was isolated. Genomic resequencing and complementation tests were used to identify the causal gene. The function of the gene, its expression pattern and subcellular localization were characterized. OsHAC4 is the causal gene for the As(V)-hypersensitive phenotype. The gene encodes a rhodanase-like protein that shows As(V) reductase activity when expressed in Escherichia coli. OsHAC4 was highly expressed in roots and was induced by As(V). In OsHAC4pro-GUS transgenic plants, the gene was expressed exclusively in the root epidermis and exodermis. OsHAC4-eGFP was localized in the cytoplasm and the nucleus. Mutation in OsHAC4 resulted in decreased As(V) reduction in roots, decreased As(III) efflux to the external medium and markedly increased As accumulation in rice shoots. Overexpression of OsHAC4 increased As(V) tolerance and decreased As accumulation in rice plants. OsHAC4 is an As(V) reductase that is critical for As(V) detoxification and for the control of As accumulation in rice. As(V) reduction, followed by As(III) efflux, is an important mechanism of As(V) detoxification.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/nph.14572

OsHAC4 is critical for arsenate tolerance and regulates arsenic accumulation in rice.

Molecular mechanisms of phosphate transport and signaling in higher plants.

Phosphorus (P) is an essential macronutrient for plant growth and development, but the molecular mechanism determining how plants sense external inorganic phosphate (Pi) levels and reprogram transcriptional and adaptive responses is incompletely understood. In this study, we investigated the function of OsSPX6 (hereafter SPX6), an uncharacterized member of SPX domain (SYG1, Pho81 and XPR1)-containing proteins in rice, using reverse genetics and biochemical approaches. Transgenic plants overexpressing SPX6 exhibited decreased Pi concentrations and suppression of phosphate starvation-induced (PSI) genes. By contrast, transgenic lines with decreased SPX6 transcript levels or spx6 mutant showed significant Pi accumulation in the leaf and upregulation of PSI genes. Overexpression of SPX6 genetically suppressed the overexpression of PHOSPHATE STARVATION RESPONSE REGULATOR 2 (PHR2) in terms of the accumulation of high Pi content. Moreover, direct interaction of SPX6 with PHR2 impeded PHR2 translocation into the nucleus, and inhibited PHR2 binding to the P1BS (PHR1 binding sequence) element. SPX6 protein was degraded in leaves under Pi-deficient conditions, whereas it accumulated in roots. We conclude that rice SPX6 is another important negative regulator in Pi starvation signaling through the interaction with PHR2. SPX6 shows different responses to Pi starvation in shoot and root, which differ from those of other SPX proteins.

Rice SPX6 negatively regulates the phosphate starvation response through suppression of the transcription factor PHR2

Membraneless organelles (MLO) regulate diverse biological processes in a spatiotemporally controlled manner spanning from inside to outside of the cells. The plasma membrane (PM) at the cell surface serves as a central platform for forming multi-component signaling hubs that sense mechanical and chemical cues during physiological and pathological conditions. During signal transduction, the assembly and formation of membrane-bound MLO are dynamically tunable depending on the physicochemical properties of the surrounding environment and partitioning biomolecules. Biomechanical properties of MLO-associated membrane structures can control the microenvironment for biomolecular interactions and assembly. Lipid-protein complex interactions determine the catalytic region's assembly pattern and assembly rate and, thereby, the amplitude of activities. In this review, we will focus on how cell surface microenvironments, including membrane curvature, surface topology and tension, lipid-phase separation, and adhesion force, guide the assembly of PM-associated MLO for cell signal transductions.

Two-dimensional molecular condensation in cell signaling and mechanosensing.

We study the three-dimensional (3D) polarization properties of a tightly focused partially coherent vector beam whose initial spatial coherence structure exhibits a lattice distribution. By examining the 3D degree of polarization and the polarimetric dimension of the tightly focused field, we demonstrate that this initial spatial coherence structure induces a 3D isotropically unpolarized beam lattice in the focal plane. Along the longitudinal direction, we observe the formation of nearly 3D unpolarized channels spanning 16 wavelengths in length near the focal region. We demonstrate that the spatial distribution of the 3D unpolarized lattice can be conveniently controlled through engineering the spatial coherence structure of the incident beam.

Xinlu Zhu

Papers

OsHAC4 is critical for arsenate tolerance and regulates arsenic accumulation in rice.

Molecular mechanisms of phosphate transport and signaling in higher plants.

Rice SPX6 negatively regulates the phosphate starvation response through suppression of the transcription factor PHR2

Two-dimensional molecular condensation in cell signaling and mechanosensing.

Generation of optical 3D unpolarized lattices in a tightly focused random beam.