Role of C4 photosynthetic enzyme isoforms in C3 plants and their potential applications in improving agronomic traits in crops

As compared to C3, C4 plants have higher photosynthetic rates and better tolerance to high temperature and drought. These traits are highly beneficial in the current scenario of global warming. Interestingly, all the genes of the C4 photosynthetic pathway are present in C3 plants, although they are involved in diverse non-photosynthetic functions. Non-photosynthetic isoforms of carbonic anhydrase (CA), phosphoenolpyruvate carboxylase (PEPC), malate dehydrogenase (MDH), the decarboxylating enzymes NAD/NADP-malic enzyme (NAD/NADP-ME), and phosphoenolpyruvate carboxykinase (PEPCK), and finally pyruvate orthophosphate dikinase (PPDK) catalyze reactions that are essential for major plant metabolism pathways, such as the tricarboxylic acid (TCA) cycle, maintenance of cellular pH, uptake of nutrients and their assimilation. Consistent with this view differential expression pattern of these non-photosynthetic C3 isoforms has been observed in different tissues across the plant developmental stages, such as germination, grain filling, and leaf senescence. Also abundance of these C3 isoforms is increased considerably in response to environmental fluctuations particularly during abiotic stress. Here we review the vital roles played by C3 isoforms of C4 enzymes and the probable mechanisms by which they help plants in acclimation to adverse growth conditions. Further, their potential applications to increase the agronomic trait value of C3 crops is discussed.

Elevated CO2 and Water Stress in Combination in Plants: Brothers in Arms or Partners in Crime?

The changing dynamics in the climate are the primary and important determinants of agriculture productivity. The effects of this changing climate on overall productivity in agriculture can be understood when we study the effects of individual components contributing to the changing climate on plants and crops. Elevated CO₂ (eCO₂) and drought due to high variability in rainfall is one of the important manifestations of the changing climate. There is a considerable amount of literature that addresses climate effects on plant systems from molecules to ecosystems. Of particular interest is the effect of increased CO₂ on plants in relation to drought and water stress. As it is known that one of the consistent effects of increased CO₂ in the atmosphere is increased photosynthesis, especially in C₃ plants, it will be interesting to know the effect of drought in relation to elevated CO₂. The potential of elevated CO₂ ameliorating the effects of water deficit stress is evident from literature, which suggests that these two agents are brothers in arms protecting the plant from stress rather than partners in crime, specifically for water deficit when in isolation. The possible mechanisms by which this occurs will be discussed in this minireview. Interpreting the effects of short-term and long-term exposure of plants to elevated CO₂ in the context of ameliorating the negative impacts of drought will show us the possible ways by which there can be effective adaption to crops in the changing climate scenario.

Comparative transcriptomics method to infer gene coexpression networks and its applications to maize and rice leaf transcriptomes

Time-series transcriptomes of a biological process obtained under different conditions are useful for identifying the regulators of the process and their regulatory networks. However, such data are 3D (gene expression, time, and condition), and there is currently no method that can deal with their full complexity. Here, we developed a method that avoids time-point alignment and normalization between conditions. We applied it to analyze time-series transcriptomes of developing maize leaves under light-dark cycles and under total darkness and obtained eight time-ordered gene coexpression networks (TO-GCNs), which can be used to predict upstream regulators of any genes in the GCNs. One of the eight TO-GCNs is light-independent and likely includes all genes involved in the development of Kranz anatomy, which is a structure crucial for the high efficiency of photosynthesis in C₄ plants. Using this TO-GCN, we predicted and experimentally validated a regulatory cascade upstream of SHORTROOT1, a key Kranz anatomy regulator. Moreover, we applied the method to compare transcriptomes from maize and rice leaf segments and identified regulators of maize C₄ enzyme genes and RUBISCO SMALL SUBUNIT2 Our study provides not only a powerful method but also novel insights into the regulatory networks underlying Kranz anatomy development and C₄ photosynthesis.

Primary carbon metabolism-related changes in cucumber exposed to single and sequential treatments with salt stress and bacterial infection

This study examines how salt stress interacts with bacterial infection at the metabolic level. We measured chlorophyll a fluorescence as well as profiles of phosphoenolpyruvate carboxylase (PEPC), NADP-malic enzyme (NADP-ME), NADP-isocitrate dehydrogenase (NADP-ICDH) and fumarase activities, malic and citric acids contents and the expression of NADP-ICDH and NADP-ME in the organ-dependent (root vs leaves) response of cucumber plants exposed to individual or sequential action of salt stress (50 mM or 100 mM NaCl) and Pseudomonas syringae pv lachrymans (Psl). NaCl treatment, Psl infection and the combination of these stresses caused disturbances in the activity of photosystem II which were suggested to specifically transmit the stress signals. PEPC and NADP-ME were induced in cucumber plants under stress, confirming that in C3 plants they function in defence responses. The profiles of malate and citrate contents, PEPC as well as NADP-ICDH and NADP-ME activities and gene expression in response to a combination of salt and pathogen stresses differed from those provoked by individual stress with respect to the direction, intensity and timing. The results indicated that the most pronounced defence response related to the readjustment of the carbon metabolism was observed in the leaves of plants exposed to combined stress. Intense activity changes of NADPH-generating enzymes, NADP-ICDH and NADP-ME, characterized the tailored response to combined stress and could be important for the integration of defence mechanisms between organs.