Cadmium (Cd) is one of the main pollutants in paddy fields, and its accumulation in rice (Oryza sativa L.) and subsequent transfer to food chain is a global environmental issue. This paper reviews the toxic effects, tolerance mechanisms, and management of Cd in a rice paddy. Cadmium toxicity decreases seed germination, growth, mineral nutrients, photosynthesis, and grain yield. It also causes oxidative stress and genotoxicity in rice. Plant response to Cd toxicity varies with cultivars, growth condition, and duration of Cd exposure. Under Cd stress, stimulation of antioxidant defense system, osmoregulation, ion homeostasis, and over production of signaling molecules are important tolerance mechanisms in rice. Several strategies have been proposed for the management of Cd-contaminated paddy soils. One such approach is the exogenous application of hormones, osmolytes, and signaling molecules. Moreover, Cd uptake and toxicity in rice can be decreased by proper application of essential nutrients such as nitrogen, zinc, iron, and selenium in Cd-contaminated soils. In addition, several inorganic (liming and silicon) and organic (compost and biochar) amendments have been applied in the soils to reduce Cd stress in rice. Selection of low Cd-accumulating rice cultivars, crop rotation, water management, and exogenous application of microbes could be a reasonable approach to alleviate Cd toxicity in rice. To draw a sound conclusion, long-term field trials are still required, including risks and benefit analysis for various management strategies.

Cadmium stress in rice: toxic effects, tolerance mechanisms, and management: a critical review.

Drought and salinity are the main abiotic stresses limiting crop yield and quality worldwide. Improving food production in drought- and salt-prone areas is the key to meet the increasing food demands in near future. It has been widely reported that silicon (Si), a second most abundant element in soil, could reduce drought and salt stress in plants. Here, we reviewed the emerging role of Si in enhancing drought and salt tolerance in plants and highlighted the mechanisms through which Si could alleviate both drought and salt stress in plants. Silicon application increased plant growth, biomass, photosynthetic pigments, straw and grain yield, and quality under either drought or salt stress. Under both salt and drought stress, the key mechanisms evoked are nutrient elements homeostasis, modification of gas exchange attributes, osmotic adjustment, regulating the synthesis of compatible solutes, stimulation of antioxidant enzymes, and gene expression in plants. In addition, Si application decreased Na+ uptake and translocation while increased K+ uptake and translocation under salt stress. However, these mechanisms vary with plant species, genotype, growth conditions, duration of stress imposed, and so on. This review article highlights the potential for improving plant resistance to drought and salt stress by Si application and provides a theoretical basis for application of Si in saline soils and arid and semiarid regions worldwide. This review article also highlights the future research needs about the role of Si under drought stress and in saline soils.

Mechanisms of silicon-mediated alleviation of drought and salt stress in plants: a review.

Cuticular wax, the first protective layer of above ground tissues of many plant species, is a key evolutionary innovation in plants. Cuticular wax safeguards the evolution from certain green algae to flowering plants and the diversification of plant taxa during the eras of dry and adverse terrestrial living conditions and global climate changes. Cuticular wax plays significant roles in plant abiotic and biotic stress tolerance and has been implicated in defense mechanisms against excessive ultraviolet radiation, high temperature, bacterial and fungal pathogens, insects, high salinity and low temperature. As one of the most detrimental abiotic stresses, drought limits plant growth and productivity, which poses huge threats to global food security and health of terrestrial ecosystem. The composition, biochemistry, structure, biosynthesis and transport of plant cuticular wax have been reviewed extensively. However, the molecular and evolutionary mechanisms of cuticular wax in plants in response to drought stress are still lacking. In this review, we focus on potential mechanisms, from evolutionary, molecular, and physiological aspects, that control cuticular wax and its roles in plant drought tolerance. We also raise key research questions and propose important directions to be resolved in the future, leading to potential applications of cuticular wax for water use efficiency in agricultural and environmental sustainability.

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Molecular and Evolutionary Mechanisms of Cuticular Wax for Plant Drought Tolerance.

In plant sciences, the prodigious significance of micronutrient is unavoidable since plant relies primarily on micronutrient as it has profound influence on array of plant activities. Although micronutrients are abundantly present in the soil but plants usually acquire them in relatively trace amounts; hence, regarded as tracer element. B, Cu, Fe, Mn, Zn are such micronutrients required in minute amounts by plants but inexorably play an eminent role in plant growth and development. Plant metabolism, nutrient regulation, reproductive growth, chlorophyll synthesis, production of carbohydrates, fruit and seed development, etc., are such effective functions performed by micronutrients. These tracer elements when present at adequate level, elevate the healthy growth in plant physiological, biochemical and metabolic characteristics while their deficiency promotes abnormal growth in plants. Prevalence of micronutrient deficiency has become more common in recent years and the rate of their reduction has further been increased by the perpetual demands of modern crop cultivars, high soil erosion, etc. On the basis of present existing condition, it is not difficult to conclude that, the regular increment of micronutrient deficiency will be mostly responsible for the remarkable degradation in substantiality of agricultural crops somewhere in near future and so that this issue has now been the subject of intensified research among the breeder, ingenuities and expertise of science. These micronutrients can also be proven toxic when present at accelerated concentrations and such toxicity level endangers the plant growth. Taking this into consideration, the current review unfolds the phenomenal participation of micronutrients in plant sciences and gives a brief overview of the current understanding of main features concerning several micronutrient acquisitions in agricultural crop plants.

Micronutrients and their diverse role in agricultural crops: advances and future prospective

Drought is a severe environmental constraint to plant productivity. Being a multidimensional stress, it triggers a wide variety of plant responses ranging from physiological, biochemical to molecular levels. One of the inevitable consequences of drought stress is an increase in reactive oxygen species (ROS) production in different cellular compartments, namely the chloroplasts and mitochondria. This enhanced ROS production is, however, kept under tight control by a versatile and cooperative antioxidant system that modulates intracellular ROS content and sets the redoxstatus of the cell. Furthermore, ROS production under stresses functions as an alarm signal that triggers defence or acclimation. Specific signal transduction pathways involve, e.g., H2O2 as a secondary messenger. ROS signalling under drought is linked to abscisic acid (ABA) and Ca2+ fluxes. At molecular levels, several drought-responsive genes, transcription factors, aquaporins, late embryogenesis abundant proteins, heat shock proteins, and dehydrins have been identified. This review discusses recent understanding on molecular responses and protective mechanisms of drought stress.

Molecular responses to drought stress in plants

A proteomic analysis combining photosynthesis-related key enzymes measurement and mRNA level analysis was used to investigate the variations of proteins involved in the photosynthetic apparatus in flag leaves of high-yield rice LYPJ during senescence under field conditions. The stability of chlorophyll-protein complexes of thylakoid membranes was measured by BN-PAGE system. Our findings revealed that the LHCII complexes were more stable than F0F1-ATPase and Cyb
 6
 /f complexes. Thylakoid membranes proteins during the senescence stage were separated by two-dimensional gel electrophoresis. We identified 13 photosynthesis-related proteins (a fivefold or greater variation), including electron transport, Calvin–Benson cycle and chloroplast protease during senescence of flag leaves. Moreover, our data showed that Ca2+-ATPase was more stable during senescence of flag leaves in rice, compared to Mg2+-ATPase. In enzymes activities analyses, it was observed that a little increment of C4 photosynthetic enzymes activities at the beginning of flag leaves senescence might compensate, to some degree, for the larger reductions in Rubisco activity, confirmed by quantitative RT-PCR analysis.

Analysis of thylakoid membrane protein and photosynthesis- related key enzymes in super high-yield hybrid rice LYPJ grown in field condition during senescence stage

In the present study, photosynthetic responses induced by cadmium stress in chlorophyll biosynthesis, photochemical activities, the stability of thylakoid membranes chlorophyll-protein complexes and the chloroplast ultrastructure of the cereal crop Oryza sativa L. were characterized. Cadmium inhibited the biosynthesis of chlorophyll by interfering with activity of δ-aminolevulinic acid dehydratase in the rice seedlings. For the photochemical activities analyses, the extent of the decrease in photosystem II activity was much greater than that in the PS I activity. The variations in the chlorophyll a fluorescence parameters also indicated that cadmium toxicity drastically affected the photochemistry of PS II. Biochemical analyses by BN-PAGE and protein immunoblot showed that cadmium toxicity considerably affected the stability of PS II-core, cytb
 6
 /f, RuBisCO, PSI + LHCI and LHCII (Trimeric). We observed the rate of the thylakoid membranes protein degradation, was mainly at the level of RbcL, PsaA, Lhca1 and D1. In addition, the damages to chloroplast structure and thylakoid stacking analyzed by transmission electron microscopy were indicative of general disarray in the photosynthetic functions exerted by cadmium toxicity. These results are valuable for understanding the biological consequences of heavy metals contamination particularly in soils devoted to organic agriculture.

Photosynthetic responses of Oryza sativa L. seedlings to cadmium stress: physiological, biochemical and ultrastructural analyses.

Iron deficiency is an important abiotic stress that limits productivity of crops all over the world. We selected a hybrid rice (Oryza sativa L.), LYPJ, which is super high-yield and widely cultured in China, to investigate changes in the components and structure of thylakoid membranes and photosynthetic performance in response to iron deficiency. Our results demonstrated that photosystem I (PSI) is the primary target for iron deficiency, while the changes in photosystem II (PSII) are important for rebuilding a balance in disrupted energy utilization and dissipation caused by differential degradation of photosynthetic components. The result of immunoblot analysis suggested that the core subunit PsaA declined drastically, while PsbA remained relatively stable. Furthermore, several organizational changes of the photosynthetic apparatus were found by BN-PAGE, including a marked decrease in the PSI core complexes, the Cytb 6 /f complex, and the trimeric form of the LHCII antenna, consistent with the observed unstacking grana. The fluorescence induction analysis indicated a descending PSII activity with energy dissipation enhanced markedly. In addition, we proposed that the crippled CO2 assimilation could be compensated by the enhanced of phosphoenolpyruvate carboxylase (PEPC), which is suggested by the decreased ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) and photosynthetic efficiency.

Fe deficiency induced changes in rice (Oryza sativa L.) thylakoids

Despite the increasing occurrence of ultraviolet-B (UV-B) radiation, its molecular mechanism is poorly documented in higher plants compared to other environmental stress. In present study, the influence of supplemental UV-B radiation on photosynthetic performance and antioxidant enzymes in rice (Oryza sativa L.) was investigated. Supplemental UV-B radiation reduced net photosynthetic rate in rice flag leaves during senescence stage. By means of the JIP-test, it was found that the potential of processing light energy through the photosynthetic machinery was slightly inhibited by the increased thermal dissipation. Furthermore, 18 thylakoid membrane protein spots were differentially expressed (5-fold or greater variation compared to the control) in supplemental UV-B-treated rice. These identified proteins were involved in various cellular responses and metabolic processes including photosynthesis, stress defense, Calvin cycle, and others of unknown functions. Taken together, these results suggested that physiological changes that resulted from supplemental UV-B radiation were linked to the light reaction, carbon metabolism, and antioxidant enzymes in rice leaves during senescence stage.

Kang Li

Papers

Analysis of thylakoid membrane protein and photosynthesis- related key enzymes in super high-yield hybrid rice LYPJ grown in field condition during senescence stage

Photosynthetic responses of Oryza sativa L. seedlings to cadmium stress: physiological, biochemical and ultrastructural analyses.

Fe deficiency induced changes in rice (Oryza sativa L.) thylakoids

Responses of photosynthetic properties and antioxidant enzymes in high-yield rice flag leaves to supplemental UV-B radiation during senescence stage.