MicroRNAs (miRNAs) are endogenous ∼23 nt RNAs that play important gene-regulatory roles in animals and plants by pairing to the mRNAs of protein-coding genes to direct their posttranscriptional repression. This review outlines the current understanding of miRNA target recognition in animals and discusses the widespread impact of miRNAs on both the expression and evolution of protein-coding genes.

MicroRNAs: Target Recognition and Regulatory Functions

Global warming and climate change are driving an alarming increase in the frequency and intensity of different abiotic stresses, such as droughts, heat waves, cold snaps, and flooding, negatively affecting crop yields and causing food shortages. Climate change is also altering the composition and behavior of different insect and pathogen populations adding to yield losses worldwide. Additional constraints to agriculture are caused by the increasing amounts of human-generated pollutants, as well as the negative impact of climate change on soil microbiomes. Although in the laboratory, we are trained to study the impact of individual stress conditions on plants, in the field many stresses, pollutants, and pests could simultaneously or sequentially affect plants, causing conditions of stress combination. Because climate change is expected to increase the frequency and intensity of such stress combination events (e.g., heat waves combined with drought, flooding, or other abiotic stresses, pollutants, and/or pathogens), a concentrated effort is needed to study how stress combination is affecting crops. This need is particularly critical, as many studies have shown that the response of plants to stress combination is unique and cannot be predicted from simply studying each of the different stresses that are part of the stress combination. Strategies to enhance crop tolerance to a particular stress may therefore fail to enhance tolerance to this specific stress, when combined with other factors. Here we review recent studies of stress combinations in different plants and propose new approaches and avenues for the development of stress combination- and climate change-resilient crops.

Developing climate-resilient crops: improving plant tolerance to stress combination.

Episodes of heat waves combined with drought can have a devastating impact on agricultural production worldwide. These conditions, as well as many other types of stress combinations, impose unique physiological and developmental demands on plants and require the activation of dedicated pathways. Here, we review recent RNA sequencing studies of stress combination in plants, and conduct a meta-analysis of the transcriptome response of plants to different types of stress combination. Our analysis reveals that each different stress combination is accompanied by its own set of stress combination-specific transcripts, and that the response of different transcription factor families is unique to each stress combination. The alarming rate of increase in global temperatures, coupled with the predicted increase in future episodes of extreme weather, highlight an urgent need to develop crop plants with enhanced tolerance to stress combination. The uniqueness and complexity of the physiological and molecular response of plants to each different stress combination, highlighted here, demonstrate the daunting challenge we face in accomplishing this goal. Dedicated efforts combining field experimentation, omics, and network analyses, coupled with advanced phenotyping and breeding methods, will be needed to address specific crops and particular stress combinations relevant to maintaining our future food chain secured.

Signal transduction networks during stress combination.

Elevated temperature and water deficit are the major abiotic factors restricting plant growth. While in nature these two stresses often occur at the same time; little is known about their combined effect on plants. Therefore, the main objective of the current study was to observe the effect of these two stresses on phenology, dry matter and seed yield in soybean. Two soybean genotypes JS 97-52 and EC 538828 were grown under green-house conditions which were maintained at different day/night temperatures of 30/22, 34/24, 38/26 and 42/28 °C with an average temperature of 26, 29, 32 and 35 °C, respectively. At each temperature, pots were divided into three sets, one set was unstressed while second and third set were subjected to water stress at vegetative and reproductive stage, respectively. As compared to 30/22 °C increase in temperature to 34/24 °C caused a marginal decline in leaf area, seed weight, total biomass, pods/pl, seeds/pl, harvest index, seeds/pod and 100 seed weight. The decline was of higher magnitude at 38/26 and 42/28 °C. Water stress imposed at two growth stages also significantly affected dry matter and yield. The highest average seed yield (10.9 g/pl) was observed at 30/22 °C, which was significantly reduced by 19, 42 and 64% at 34/24, 38/24 and 42/28 °C, respectively. Similarly, compared to unstressed plants (11.3 g/pl) there was 28 and 74% reduction in yield in plants stressed at vegetative and reproductive stage. Thus, both temperature and water stress affected the growth and yield but the effect was more severe when water stress was imposed at higher temperatures. JS 97-52 was more affected by temperature and water stress as compared to EC 538828. Though drought is the only abiotic factor that is known to affect the water status of plants, but the severity of the effect is highly dependent on prevailing temperature.

Impact of combined stress of high temperature and water deficit on growth and seed yield of soybean

Melatonin-induced growth promotion of maize seedlings is associated with maintaining coordination between gene expressions and activities of key enzymes involved in carbon and nitrogen metabolisms. Melatonin is a pleiotropic molecule that influences many diverse actions to enhance plant growth. The effect of melatonin on maintaining a necessary balance between carbon and nitrogen metabolisms that underpins the growth process in higher plants remains unclear. In this study, the influence of melatonin on nitrogen assimilation, mitochondrial respiration, and photosynthesis, which are major pathways related with carbon and nitrogen metabolism, was investigated on the basis of the seedling growth of maize. Melatonin applications (10, 100, and 1000 μmol L−1) significantly increased the growth parameters assessed by root elongation, plant height, leaf surface area, and the contents of protein, carbohydrate, and chlorophyll in comparison to the control seedlings. They also had a strong encouraging effect on the activities and gene expressions of enzymes (nitrate reductase, nitrite reductase, glutamine synthase, glutamate 2-oxoglutarate transferase, and NADH–glutamate dehydrogenase) involved in the nitrogen assimilation process. While melatonin applications elevated nitrate and nitrite concentrations, they markedly lowered ammonium content compared to control. Similarly, the activity of citrate synthase, the first enzyme of citric acid cycle providing carbon skeleton for nitrogen assimilation, was significantly augmented by melatonin applications. Moreover, melatonin considerably upregulated the gene expressions of citrate synthase and cytochrome oxidase, an enzyme responsible for ATP production. Remarkable increments were recorded at Rubisco activity and gene expressions of Rubisco and Rubisco activase in melatonin-treated seedlings. In conclusion, all these data put together reveal that melatonin-induced growth promotion of maize seedlings resulted from its coordinating effect on carbon and nitrogen metabolisms.

Melatonin promotes plant growth by maintaining integration and coordination between carbon and nitrogen metabolisms

Drought and heat stress are important abiotic stress restricting plant growth, while the two stresses often occur at the same time in nature and little is known about when these stresses occur in combination. Therefore, attempts were made to understand the impact of water stress imposed under different temperature conditions on photosynthesis, chlorophyll fluorescence, antioxidant enzymes, lipid peroxidation, chlorophyll, proline, free amino acid, epicuticular wax content and seed yield. Soybean genotype EC 538828 was grown under greenhouse conditions at day/night temperatures of 30/22, 34/24, 38/26 and 42/28 °C. At each temperature, pots were divided into two sets, one set was unstressed while second was subjected to water stress at reproductive stage (beginning of seed fill). High temperature significantly declined the rate of photosynthesis, stomatal conductance, water use efficiency, Fv/Fm ratio, photochemical quenching, PhiPSII, electron transport rate, ascorbic acid, total free amino acids, chlorophyll content and seed yield. As against this with increase in temperature from 30/22 to 42/28 °C intercellular CO2, transpiration rate, vapor pressure deficit, non photochemical quenching, proline content, SOD, POD, APX, GR, MDA and epicuticular wax content were increased. Water stress when imposed at different temperature further aggravated the effects of temperature, and the combination of water stress and high temperature had more detrimental effect.

Interactive effect of temperature and water stress on physiological and biochemical processes in soybean

The arbuscular mycorrhizal (AM) fungi-rhizobia synergism is a promising approach for improving the growth and nutrition of soybean. It is, therefore, imperative to evaluate potential soybean rhizobia and AM fungi singly to identify their stress protectant physiological traits, enhance growth and nodulation of soybean and improve soil health. The efficacy of five root nodulating soybean rhizobia and an indigenous AM fungus, Glomus intraradices, was evaluated on soybean (cv JS 93-05) under microcosm conditions. In general, all the inoculated plants showed higher fresh shoot and root weight, and nodule number as compared to uninoculated control plants. The plants inoculated with Bradyrhizobium
 japonicum (strain USDA 110), B. liaoningense 17c (MTCC 10753) and AM fungus showed higher growth and nodulation. However, the plants inoculated with rhizobia 12c (unidentified), B. japonicum DE2-5a (MTCC 10751) and USDA 205 did enhance nodulation but found at par with the other inoculated plants. Interestingly, these inoculated plants found to have comparatively higher nitrogen and phosphorus uptake. B. japonicum (strain USDA 110), native slow growing rhizobia isolate DE2-5a and AM fungi were also found to stimulate proline content in shoots, and trehalase and fluorescein diacetate activities in the rhizosphere soil. Considering the growth and physiological responses of AM fungi and rhizobial strains (DE2-5a and 17c) to soybean, there is a need for further testing the synergistic responses to evolve better survival candidates under drought-stress conditions.

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Effect of Native Soybean Rhizobia and AM Fungi in the Improvement of Nodulation, Growth, Soil Enzymes and Physiological Status of Soybean Under Microcosm Conditions

In plant breeding programs, screening for drought-tolerance is often a bottleneck. An experiment was conducted in the field and rainout shelters to: (1) identify physiological traits in breeding programs that can be used as criteria for selecting drought tolerance soybean genotypes [Glycine max (L.) Merr], (2) evaluate genotypic differences to drought tolerance, and (3) identify genotypes with superior drought tolerance. Sixteen genotypes were evaluated in split plot design under irrigated and drought conditions. Various physiological traits were measured in irrigated and drought stressed plants such as canopy temperature, root length, specific leaf weight, photosynthetic rate, chlorophyll, and epicuticular wax content. As compared with irrigated conditions, the percent reduction in mean soybean yield under rainout shelter was 40%. The mean yields of soybean genotypes ranged from 1162 kg/ha (NRC 12) to 2610 kg/ha (JS 335) under irrigated conditions, whereas, under water stress conditions, yields ranged from 852 kg/ha (Samrat) to 1654 kg/ha (EC 538828). Genotypes EC 538828, JS 97-52, EC 456548, and EC 602288 had better avoidance to drought than other genotypes. The superior drought tolerance of the four genotypes was related to their low canopy temperature, deep root system, and high values for root/shoot weight ratio, specific leaf weight, chlorophyll content, photosynthetic rate, epicuticular wax content, and Photosystem II (PSII) efficiency. Therefore, when genetic diversity of these physiological traits is established in breeding programs, these traits can be used as a selection criterion for selecting drought tolerant genotypes.

Identification of drought tolerant genotypes using physiological traits in soybean.

Soybean anthracnose caused by Colletotrichum truncatum (Schw.) Andrus & W.D. Moore is a devastating disease in Central and Southern India. Chemical control of this disease is neither economical nor eco-friendly and genetic resistance is considered to be an efficient means for its management. Therefore, in the current study, a total of 225 soybean [Glycine max (L.) Merr.] germplasm accessions were evaluated for anthracnose resistance which resulted in the identification of five genotypes viz., EC 538828, EC 34372, EC 457254, AKSS 67 and Karune as highly resistant. Genetics of anthracnose resistance in three F2 populations derived from EC 34372 × JS 95-60, EC 457254 × JS 95-60 and AKSS 67 × JS 95-60 revealed that the resistance in all the three resistant parents was governed by two major genes interacting in complementary fashion. The χ2 test for goodness of fit revealed that F2 plants in each of the three populations segregated in 9 resistant: 7 susceptible ratio. This is the first report on genetics of anthracnose resistance in soybean and will aid soybean breeders to develop a strategic anthracnose resistance breeding program and to map the genes governing resistance. Research findings from this study indicated the potential role of exotic germplasm in Indian soybean improvement against anthracnose disease.

Virender Singh Bhatia

Papers

Impact of combined stress of high temperature and water deficit on growth and seed yield of soybean

Interactive effect of temperature and water stress on physiological and biochemical processes in soybean

Effect of Native Soybean Rhizobia and AM Fungi in the Improvement of Nodulation, Growth, Soil Enzymes and Physiological Status of Soybean Under Microcosm Conditions

Identification of drought tolerant genotypes using physiological traits in soybean.

Genetic inheritance and identification of germplasm sources for anthracnose resistance in soybean [ Glycine max (L.) Merr.]