Expression of potential reference genes in response to macronutrient stress in rice and soybean.

Given the complexity of nutrient stress responses and the availability of a few validated reference genes, we aimed to identify robust and stable reference genes for macronutrient stress in rice and soybean. Ten potential reference genes were evaluated using geNorm, NormFinder, BestKeeper, Comparative ΔCt method, and RefFinder algorithms under low and completely starved conditions of nitrogen (N), phosphorus (P), potassium (K), and sulphur (S). Results revealed distinct sets of reference gene pairs, showing stable expression under different experimental conditions. The gene pairs TIP41/UBC(9/10/18) and F-box/UBC10 were most stable in rice and soybean, respectively under N stress. Under P stress, UBC9/UBC10 in rice and F-Box/UBC10 in soybean were most stable. Similarly, TIP41/UBC10 in rice and RING FINGER/UBC9 in soybean were the best gene pairs under K stress while F-Box/TIP41 in rice and UBC9/UBC10 in soybean were the most stable gene pairs under S stress. These reference gene pairs were validated by quantifying the expression levels of high-affinity transporters like NRT2.1/NRT2.5, PT1, AKT1, and SULTR1 for N, P, K, and S stress, respectively. This study reiterates the importance of choosing reference genes based on crop species and the experimental conditions, in order to obtain concrete answers to missing links of gene regulation in response to macronutrient deficiencies.

Differential response of hexaploid and tetraploid wheat to interactive effects of elevated [CO2] and low phosphorus.

KEY MESSAGE: Hexaploid wheat is more responsive than tetraploid to the interactive effects of elevated [CO2] and low P in terms of carboxylate efflux, enzyme activity and gene expression (TaPT1 and TaPAP). Availability of mineral nutrients to plants under changing climate has become a serious challenge to food security and economic development. An understanding of how elevated [CO2] influences phosphorus (P) acquisition processes at the whole-plant level would be critical in selecting cultivars as well as to maintain optimum yield in limited-P conditions. Wheat (Triticum aestivum and T. durum) grown hydroponically with sufficient and low P concentration were exposed to elevated and ambient [CO2]. Improved dry matter partitioning towards root resulted in increased root-to-shoot ratio, root length, volume, surface area, root hair length and density at elevated [CO2] with low P. Interaction of low P and [CO2] induced activity of enzymes (phosphoenolpyruvate carboxylase, malate dehydrogenase and citrate synthase) in root tissue resulting in twofold increase in carboxylates and acid phosphatase exudation. Physiological absorption capacity of roots showed that plants alter their uptake kinetics by increasing affinity (low Km) in response to elevated [CO2] under low P supply. Increased relative expression of genes, purple acid phosphatase (TaPAP) and high-affinity Pi transporter (TaPT1) in roots induced by elevated [CO2] and low P supported our physiological observations. Hexaploid wheat (PBW-396) being more responsive to elevated [CO2] at low P supply as compared to tetraploid (PDW-233) necessitates the ploidy effect to be explored further which might be advantageous under changing climate.

Root exudation potential in contrasting soybean genotypes in response to low soil phosphorus availability is determined by photo-biochemical processes.

Low phosphorus (P) availability elicits efflux of organic substances viz. carboxylic acids, phenolics, proteins, amino acids, sugars and other low molecular weight compounds in many leguminous crops including soybean (Glycine max (L.) Merr.). The potential for root exudation varies widely among soybean genotypes, as synthesis and secretion of root exudates place additional burden on the carbon demand of the plant. Hence, efficient photosynthetic machinery may attribute to the differential root exudation potential of soybean genotypes in response to low soil P availability. An attempt was made to understand the varietal differences in photo-biochemical processes of soybean genotypes identified previously with contrasting root exudation potential under low P (Vengavasi and Pandey, 2016). Genotypes EC-232019 (P-efficient) and EC-113396 (P-inefficient) were grown in soil with low (2 mg P kg-1 soil) and sufficient (25 mg P kg-1 soil) P levels under natural environment and observations were recorded at anthesis. The genotype EC-232019 exhibited higher maximal carboxylation rate (Vcmax), maximal photosynthesis (Amax), apparent quantum efficiency (Φ), mesophyll conductance (gm), triose phosphate utilization rate (TPU), photochemical quenching (qP) and electron transport rate (ETR), along with higher chlorophyll a, total chlorophyll and total carotenoid concentration as compared to the P-inefficient EC-113396. Low P-induced reduction in maximal electron transport rate (Jmax) and Φ was higher in EC-113396 rather than EC-232019, suggesting superior photo-biochemical efficiency in the latter. The observed variation in P uptake and growth responses might be attributed in part to the improved photo-biochemical processes exhibited by the P-efficient genotype EC-232019.