The nuclear-encoded chloroplast NADP-dependent malate dehydrogenase (NADP-MDH) is a key enzyme controlling the malate valve, to allow the indirect export of reducing equivalents. Arabidopsis thaliana (L.) Heynh. T-DNA insertion mutants of NADP-MDH were used to assess the role of the light-activated NADP-MDH in a typical C3 plant. Surprisingly, even when exposed to high-light conditions in short days, nadp-mdh knockout mutants were phenotypically indistinguishable from the wild type. The photosynthetic performance and typical antioxidative systems, such as the Beck–Halliwell–Asada pathway, were barely affected in the mutants in response to high-light treatment. The reactive oxygen species levels remained low, indicating the apparent absence of oxidative stress, in the mutants. Further analysis revealed a novel combination of compensatory mechanisms in order to maintain redox homeostasis in the nadp-mdh plants under high-light conditions, particularly an increase in the NTRC/2-Cys peroxiredoxin (Prx) system in chloroplasts. There were indications of adjustments in extra-chloroplastic components of photorespiration and proline levels, which all could dissipate excess reducing equivalents, sustain photosynthesis, and prevent photoinhibition in nadp-mdh knockout plants. Such metabolic flexibility suggests that the malate valve acts in concert with other NADPH-consuming reactions to maintain a balanced redox state during photosynthesis under high-light stress in wild-type plants.

HighlightCytokinin induces functional stay-greenness by maintaining the Chl a/b ratios and the stability of photosynthetic complexes during dark-induced senescence in rice.

The bioenergetic processes of photosynthesis and respiration are mutually beneficial. Their interaction extends to photorespiration, which is linked to optimize photosynthesis. The interplay of these three pathways is facilitated by two major phenomena: sharing of energy/metabolite resources and maintenance of optimal levels of reactive oxygen species (ROS). The resource sharing among different compartments of plant cells is based on the production/utilization of reducing equivalents (NADPH, NADH) and ATP as well as on the metabolite exchange. The responsibility of generating the cellular requirements of ATP and NAD(P)H is mostly by the chloroplasts and mitochondria. In turn, besides the chloroplasts, the mitochondria, cytosol and peroxisomes are common sinks for reduced equivalents. Transporters located in membranes ensure the coordinated movement of metabolites across the cellular compartments. The present review emphasizes the beneficial interactions among photosynthesis, dark respiration and photorespiration, in relation to metabolism of C, N and S. Since the bioenergetic reactions tend to generate ROS, the cells modulate chloroplast and mitochondrial reactions, so as to ensure that the ROS levels do not rise to toxic levels. The patterns of minimization of ROS production and scavenging of excess ROS in intracellular compartments are highlighted. Some of the emerging developments are pointed out, such as model plants, orientation/movement of organelles and metabolomics.

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Inconsistency in the environment exposes plants to various abiotic stresses. This results in damage of a plant’s cellular components due to excessive accumulation of unstable reactive oxygen. Besides, it also disrupts enzymatic/non-enzymatic detoxification mechanisms in plants making them more sensitive. Salinity is one such abiotic stress which disrupts regular physiological mechanisms in plants. In this study, we examined the effects of salinity using NaCl in four different genotypes of Gerbera jamesonii cv Bolus, an important ornamental plant of family Compositae. We hypothesized that, upon treatment with NaCl (50, 100, 150 200mM concentration), alterations in the morphological features along with elevated levels of H2O2, MDA, proline, and degradation of chlorophyll will be observed. The enzymatic antioxidant defenses were also hypothesized to differ among genotypes based on their level of tolerance. These parameters were monitored on the 5th and 20th day of NaCl treatment and results were recorded. The observations suggest that 1. the Lattara genotype of Gerbera is sensitive to NaCl and 2. Faith is tolerant, while 3. Alcatras and Basic are moderately tolerant. These findings accompanied by further research on the physiological parameters responsible for attaining salinity tolerance may help in developing salt-tolerant varieties in Gerbera. Future studies on decoding molecular networks associated with the antioxidative defense system in Gerbera can help improve breeding and create novel germplasm in various ornamentally important plants besides Gerbera.

Salinity adversely affects various plants’ metabolic processes, negatively influencing their productivity and crop yield. Gerbera jamesonii cv. Bolus is a commercially important ornamental plant cultivated globally throughout the year for its cut flower production in polyhouses. During polyhouse cultivation, repeated fertigation may cause salinity in Gerbera, affecting flower quality and yield, and functional alterations in the basal level of cellular antioxidative defence systems. Though several factors induce salinity in general, we focussed on NaCl as it is one of the major components of fertigation. In salt-stressed plants, monitoring altered levels of antioxidative defense systems may help better understand their physiological changes. However, little to no studies were attempted on salinity-induced oxidative damage in Gerbera till date. In the current study, we verified the salt sensitivity level of Gerbera with varying concentrations of NaCl (0-200 mM) using in vitro leaf disc approach. We measured various antioxidative enzymatic/non enzymatic defense systems besides MDA and chlorophyll content. Treatment with higher salt concentrations (above100 mM NaCl) exhibited severe bleaching in leaf discs, followed by elevated levels of H2O2, lipid peroxidation and proline. Besides, our study also revealed a decrease in the total chlorophyll content; activities of superoxide dismutase, catalase, glutathione reductase, and ascorbate peroxidase. The observed results showed that Gerbera might not tolerate higher levels of NaCl as it could be detrimental to its cellular activities. Future studies on decoding molecular networks associated with salinity stress and antioxidative defense systems may help in developing salt-tolerant varieties in Gerbera and several other ornamental plants of Asteraceae.

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