Nitric oxide (NO) is a hormone that connects numerous reactions in plant cells under normal and environmental stress conditions. The application of 100 µM NO as sodium nitroprusside (SNP; NO donor) applied individually or in combination with  N or  S in different combinations (i.e. 100 mg N or S kg-1 soil applied at the time of sowing [100 N + 100S]0d or with split, 50 mg N or S kg-1 soil at the time of sowing and similar dose at 20 d after sowing [50 N + 50S]0d  + [50 N + 50S]20d ) was tested to alleviate salt stress in mustard (Brassica juncea L.). Application of 100 µM NO plus split application of N and S more significantly promoted stomatal behavior, photosynthetic and growth performance in the absence of salt stress and maximally alleviated effects of salt stress through increased N- and S-use efficiency, proline and antioxidant system. The combined application of N and S at the time of sowing was lesser effective in promoting photosynthesis and growth under salt or no salt stress conditions in presence or absence of NO. The study suggests that salt stress effects on the photosynthetic performance are mitigated more efficiently when NO was applied together with the split application of N and S given at two stages, and the photosynthetic activity was promoted under salt stress through increased N and S assimilation and antioxidant system. This strategy may be adopted in agricultural system for overcoming salt stress effects on performance of mustard.

Treatment of nitric oxide supplemented with nitrogen and sulfur regulates photosynthetic performance and stomatal behavior in mustard under salt stress.

Heavy metal toxicity is one of the most devastating abiotic stresses. Heavy metals cause serious damage to plant growth and productivity, which is a major problem for sustainable agriculture. It adversely affects plant molecular physiology and biochemistry by generating osmotic stress, ionic imbalance, oxidative stress, membrane disorganization, cellular toxicity, and metabolic homeostasis. To improve and stimulate plant tolerance to heavy metal stress, the application of biostimulants can be an effective approach without threatening the ecosystem. Melatonin (N-acetyl-5-methoxytryptamine), a biostimulator, plant growth regulator, and antioxidant, promotes plant tolerance to heavy metal stress by improving redox and nutrient homeostasis, osmotic balance, and primary and secondary metabolism. It is important to perceive the complete and detailed regulatory mechanisms of exogenous and endogenous melatonin-mediated heavy metal-toxicity mitigation in plants to identify potential research gaps that should be addressed in the future. This review provides a novel insight to understand the multifunctional role of melatonin in reducing heavy metal stress and the underlying molecular mechanisms.

Melatonin Modulates Plant Tolerance to Heavy Metal Stress: Morphological Responses to Molecular Mechanisms

Photosynthesis is the largest organic synthesis on Earth, salinity limits crop yield and quality worldwide directly or indirectly related to the decrease in photosynthetic efficiency. The mechanism by which photosynthetic apparatus responds to salt stress is extremely complex and varies with plant genotype, developmental stage, the history of the plant cell and duration of stress imposed. Recent studies have partially revealed the mechanisms from different levels: molecular, physiological and biochemical, morphological; but there is currently no unified mechanism to explain the effect of stress on photosynthesis. This study comprehensively reviews the adaptive mechanism of photosynthetic apparatus under salt stress, summarises methods for increasing the resistance and provides a practical way to increase grain yield in saline soils.

Research on the adaptive mechanism of photosynthetic apparatus under salt stress: New directions to increase crop yield in saline soils

Photochemistry and proteomics of mulberry (Morus alba L.) seedlings under NaCl and NaHCO3 stress.

Proline is a versatile plant metabolite, which is produced in large amounts in plants exposed to osmotic and oxidative stress. Proline has been shown to provide protection against various reactive oxygen species (ROS), such as hydrogen peroxide and hydroxyl radicals. On the other hand, its protective effect against singlet oxygen has been debated, and it is considered ineffective against superoxide. Here we used various methods for the detection of singlet oxygen (electron paramagnetic resonance, EPR, spin trapping by 2,2,6,6-tetramethyl-4-piperidone, fluorescence probing by singlet oxygen sensor green, SOSG, and oxygen uptake due to chemical trapping) and superoxide (oxygen uptake due to oxygen reduction) in vitro and in isolated thylakoids. We demonstrated that proline does quench both singlet oxygen and superoxide in vitro. By comparing the effects of chemical scavengers and physical quenchers, we concluded that proline eliminates singlet oxygen via a physical mechanism, with a bimolecular quenching rate of ca. 1.5-4 10(6) M-1 s(-1). Our data also show that proline can eliminate superoxide in vitro in a process that is likely to proceed via an electron transfer reaction. We could also show that proline does quench both singlet oxygen and superoxide produced in isolated thylakoids. The scavenging efficiency of proline is relatively small on a molar basis, but considering its presence in high amounts in plant cells under stress conditions it may provide a physiologically relevant contribution to ROS scavenging, supplementing other nonenzymatic ROS scavengers of plant cells.

Proline is a quencher of singlet oxygen and superoxide both in in vitro systems and isolated thylakoids.

Optimization of photosynthetic performance and protection against abiotic stress are essential to sustain plant growth. Photorespiratory metabolism can help plants to adapt to abiotic stress. The beneficial role of photorespiration under abiotic stress is further strengthened by cyclic electron flow (CEF) and alternative oxidase (AOX) pathways. We have attempted to critically assess the literature on the responses of these three phenomena—photorespiration, CEF and AOX, to different stress situations. We emphasize that photorespiration is the key player to protect photosynthesis and upregulates CEF as well as AOX. Then these three processes work in coordination to protect the plants against photoinhibition and maintain an optimal redox state in the cell, while providing ATP for metabolism and protein repair. H2O2 generated during photorespiratory metabolism seems to be an important signal to upregulate CEF or AOX. Further experiments are necessary to identify the signals originating from CEF or AOX to modulate photorespiration. The mutants deficient in CEF or AOX or both could be useful in this regard. The mutual interactions between CEF and AOX, so as to keep their complementarity, are also to be examined further.

Photorespiration is complemented by cyclic electron flow and the alternative oxidase pathway to optimize photosynthesis and protect against abiotic stress

Oxidative stress can occur in different parts of plant cells We employed two oxidants that induce reactive oxygen species (ROS) in different intracellular compartments: methyl viologen (MV, in chloroplasts) and menadione (MD, in mitochondria) The responses of pea (Pisum sativum) leaf discs to MV or MD after 4-h incubation in dark or moderate (300 μE m−2 s−1) or high light (1200 μE m−2 s−1) were examined Marked increase in ROS levels was observed, irrespective of compartment targeted The levels of proline, a compatible solute, increased markedly much more than that of ascorbate or glutathione during oxidative/photo-oxidative stress, emphasizing the importance of proline Further, the activities and transcripts of enzymes involved in biosynthesis or oxidation of proline were studied An upregulation of biosynthesis and downregulation of oxidation was the basis of proline accumulation Pyrroline-5-carboxylate synthetase (P5CS, involved in biosynthesis) and proline dehydrogenase (PDH, involved in oxidation) were the key enzymes regulated under oxidative stress Since these two enzymes—P5CS and PDH—are located in chloroplasts and mitochondria, respectively, we suggest that proline metabolism can help to mediate inter-organelle interactions and achieve redox homeostasis under photo-oxidative stress

Oxidative stress induced in chloroplasts or mitochondria promotes proline accumulation in leaves of pea (Pisum sativum): another example of chloroplast-mitochondria interactions.

Photorespiration, an essential component of plant metabolism, is concerted across four subcellular compartments, namely, chloroplast, peroxisome, mitochondrion, and the cytoplasm. It is unclear how the pathway located in different subcellular compartments respond to stress occurring exclusively in one of those. We attempted to assess the inter-organelle interaction during the photorespiratory pathway. For that purpose, we induced oxidative stress by menadione (MD) in mitochondria and photo-oxidative stress (high light) in chloroplasts. Subsequently, we examined the changes in selected photorespiratory enzymes, known to be located in other subcellular compartments. The presence of MD upregulated the transcript and protein levels of five chosen photorespiratory enzymes in both normal and high light. Peroxisomal glycolate oxidase and catalase activities increased by 50% and 25%, respectively, while chloroplastic glycerate kinase and phosphoglycolate phosphatase increased by ~30%. The effect of MD was maximum in high light, indicating photo-oxidative stress was an influential factor to regulate photorespiration. Oxidative stress created in mitochondria caused a coordinative upregulation of photorespiration in other organelles. We provided evidence that reactive oxygen species are important signals for inter-organelle communication during photorespiration. Thus, MD can be a valuable tool to modulate the redox state in plant cells to study the metabolic consequences across membranes.

Ramesh B. Bapatla

Papers

Photorespiration is complemented by cyclic electron flow and the alternative oxidase pathway to optimize photosynthesis and protect against abiotic stress

Oxidative stress induced in chloroplasts or mitochondria promotes proline accumulation in leaves of pea (Pisum sativum): another example of chloroplast-mitochondria interactions.

Modulation of Photorespiratory Enzymes by Oxidative and Photo-Oxidative Stress Induced by Menadione in Leaves of Pea (Pisum sativum).

Photosynthesis is sensitive to nitric oxide and respiration sensitive to hydrogen peroxide: Studies with pea mesophyll protoplasts.

C 3 Plants