Adaptive strategies of plants to conserve internal phosphorus under P deficient condition to improve P utilization efficiency.

Phosphorus (P) is one of the limiting factors for plant growth and productivity due to its slow diffusion and immobilization in the soil which necessitates application of phosphatic fertilizers to meet the crop demand and obtain maximum yields. However, plants have evolved mechanisms to adapt to low P stress conditions either by increasing acquisition (alteration of belowground processes) or by internal inorganic P (Pi) utilization (cellular Pi homeostasis) or both. In this review, we have discussed the adaptive strategies that conserve the use of P and maintain cellular Pi homeostasis in the cytoplasm. These strategies involve modification in membrane lipid composition, flavanol/anthocyanin level, scavenging and reutilization of Pi adsorbed in cell wall pectin, remobilization of Pi during senescence by enzymes like RNases and purple acid phosphatases, alternative mitochondrial electron transport, and glycolytic pathways. The remobilization of Pi from senescing tissues and its internal redistribution to various cellular organelles is mediated by various Pi transporters. Although much efforts have been made to enhance P acquisition efficiency, an understanding of the physiological mechanisms conserving internal Pi and their manipulation would be useful for plants that can utilize P more efficiently to produce optimum growth per unit P uptake.

Multipronged Phenotyping Approaches to Characterize Sugarcane Root Systems.

Roots are the primary conductors of water and nutrients and play a vital role in sustaining growth and yield under stressful environments. The study of plant roots poses methodological difficulties in in situ assessment and sampling, which is especially true for sugarcane (Saccharum spp.). Traditional methods during the 1920s documented the genotypic variation in sugarcane root systems, after which few studies were reported on sugarcane root traits per se until recently. In addition to morphology, rhizosphere characteristics, including allelopathic effects and/or affinity for microbial symbiosis, determine plant establishment and survival. Ultimately, root systems define the above-ground productivity of sugarcane. With the impetus for climate-resilient varieties, it is becoming more relevant to explore and utilize the variability in root system traits of sugarcane. This paper describes multipronged approaches for sugarcane root phenotyping, including field excavation by trench sampling, the use of a root core sampler, raised platforms for root sampling, and raising plants under hydroponic culture, employed by a team of scientists at the Indian Council of Agricultural Research-Sugarcane Breeding Institute (ICAR-SBI). Field excavation by trench sampling is imperative to assess the plant roots in their natural growing environment. The use of raised platforms simulating field conditions and a root core sampler are alternative approaches, with a considerable reduction in time, uniform sample size, and less loss of root material. Hydroponic plant culture allows the study of morphology, anatomical features, and rhizosphere biology, including the exudation of organic compounds and microbial interactions. Data generated from different experiments using diverse sampling methods add to the wealth of information on the root system traits of sugarcane.

Integrated Approach in Genomic Selection to Accelerate Genetic Gain in Sugarcane.

Marker-assisted selection (MAS) has been widely used in the last few decades in plant breeding programs for the mapping and introgression of genes for economically important traits, which has enabled the development of a number of superior cultivars in different crops. In sugarcane, which is the most important source for sugar and bioethanol, marker development work was initiated long ago; however, marker-assisted breeding in sugarcane has been lagging, mainly due to its large complex genome, high levels of polyploidy and heterozygosity, varied number of chromosomes, and use of low/medium-density markers. Genomic selection (GS) is a proven technology in animal breeding and has recently been incorporated in plant breeding programs. GS is a potential tool for the rapid selection of superior genotypes and accelerating breeding cycle. However, its full potential could be realized by an integrated approach combining high-throughput phenotyping, genotyping, machine learning, and speed breeding with genomic selection. For better understanding of GS integration, we comprehensively discuss the concept of genetic gain through the breeder's equation, GS methodology, prediction models, current status of GS in sugarcane, challenges of prediction accuracy, challenges of GS in sugarcane, integrated GS, high-throughput phenotyping (HTP), high-throughput genotyping (HTG), machine learning, and speed breeding followed by its prospective applications in sugarcane improvement.

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.

Comparative Analysis of Soybean Root Proteome Reveals Molecular Basis of Differential Carboxylate Efflux under Low Phosphorus Stress.

Carboxylate efflux from roots is a crucial and differential response of soybean genotypes to low phosphorus (P) stress. Exudation of carboxylic acids including oxalate, citrate, succinate and fumarate was induced under low P stress, particularly in P-efficient soybean genotypes. Enhancement of root length, surface area and volume further improved P acquisition under low P stress. To understand the molecular basis of carboxylate efflux under low P stress, the root proteome of contrasting genotypes (P-efficient: EC-232019 and P-inefficient: EC-113396) was compared. Among a total of 325 spots, 105 (32%) were differentially abundant proteins (DAPs) between sufficient (250 µM) and low P (4 µM) levels. Abundance of 44 (14%) proteins decreased by more than two-fold under low P stress, while 61 (19%) proteins increased by more than two-fold. Protein identification and annotation revealed that the DAPs were involved in a myriad of functions including carboxylic acid synthesis, carbohydrate, protein and lipid metabolism. Proteins with significant abundance included malate dehydrogenase, isocitrate dehydrogenase, phosphoglucomutase, phosphoglycerate mutase, fructokinase, enolase, phosphoglycerate kinase, triosephosphate isomerase, alcohol dehydrogenase, glucan water dikinase, glutamine synthetase and argininosuccinate lyase. Inferences from proteomic analysis suggests the crosstalk between various metabolic pathways implicated in conferring superior P acquisition efficiency under stress.

Nitrogen-Deficiency Stress Induces Protein Expression Differentially in Low-N Tolerant and Low-N Sensitive Maize Genotypes.

Nitrogen (N) is essential for proper plant growth and its application has proven to be critical for agricultural produce. However, for unavoidable economic and environmental problems associated with excessive use of N-fertilizers, it is an urgent demand to manage application of fertilizers. Improving the N-use efficiency (NUE) of crop plants to sustain productivity even at low N levels is the possible solution. In the present investigation, contrasting low-N sensitive (HM-4) and low-N tolerant (PEHM-2) genotypes were identified and used for comparative proteome-profiling of leaves under optimum and low N as well as restoration of low N on 3rd (NR3) and 5th (NR5) days after re-supplying N. The analysis of differential expression pattern of proteins was performed by 2-D gel electrophoresis. Significant variations in the expression of proteins were observed under low N, which were genotype specific. In the leaf proteome, 25 spots were influenced by N treatment and four spots were different between the two genotypes. Most of the proteins that were differentially accumulated in response to N level and were involved in photosynthesis and metabolism, affirming the relationship between N and carbon metabolism. In addition to this, greater intensity of some defense proteins in the low N tolerant genotype was found that may have a possible role in imparting it tolerance under N starvation conditions. The new insights generated on maize proteome in response to N-starvation and restoration would be useful toward improvement of NUE in maize.