Effects of water deficit stress on growth, water relations and osmolyte accumulation in Medicago truncatula and M. laciniata populations

Brassinosteroids Regulate the Water Deficit and Latex Yield of Rubber Trees

Brassinolide (BR) is an important plant hormone that regulates the growth and development of plants and the formation of yield. The yield and quality of latex from Hevea brasiliensis are regulated by phytohormones. The understanding of gene network regulation mechanism of latex formation in rubber trees is still very limited. In this research, the rubber tree variety CATAS73397 was selected to analyze the relationship between BR, water deficit resistance, and latex yield. The results showed that BR improves the vitality of rubber trees under water deficit by increasing the rate of photosynthesis, reducing the seepage of osmotic regulatory substances, increasing the synthesis of energy substances, and improving the antioxidant system. Furthermore, BR increased the yield and quality of latex by reducing the plugging index and elevating the lutoid bursting index without decreasing mercaptan, sucrose, and inorganic phosphorus. This was confirmed by an increased expression of genes related to latex flow. RNA-seq analysis further indicated that DEG encoded proteins were enriched in the MAPK signaling pathway, plant hormone signal transduction and sucrose metabolism. Phytohormone content displayed significant differences, in that trans-Zeatin, ethylene, salicylic acid, kinetin, and cytokinin were induced by BR, whereas auxin, abscisic acid, and gibberellin were not. In summary, the current research lays a foundation for comprehending the molecular mechanism of latex formation in rubber trees and explores the potential candidate genes involved in natural rubber biosynthesis to provide useful information for further research in relevant areas.

Comprehensive resistance evaluation of 15 blueberry cultivars under high soil pH stress based on growth phenotype and physiological traits

High soil pH is one of the main abiotic factors that negatively affects blueberry growth and cultivation. However, no comprehensive evaluation of the high soil pH tolerance of different blueberry cultivars has been conducted. Herein, 16 phenotypic and physiological indices of 15 blueberry cultivars were measured through pot experiments, and the high-pH soil tolerance coefficient (HSTC) was calculated based on these indices to comprehensively evaluate the high-soil-pH tolerance of plants. The results demonstrated that high soil pH stress inhibited blueberry 77.growth, and MDA, soluble sugar (SS), and soluble protein (SP) levels increased in leaves. Moreover, in all cultivars, CAT activity in the antioxidant system was enhanced, whereas SOD activity was reduced, and the relative expression levels of the antioxidant enzyme genes SOD and CAT showed similar changes. In addition, the leaf chlorophyll relative content (SPAD), net photosynthetic rate (P_n), transpiration rate (E), and stomatal conductance (G_s) decreased, while changes in the intercellular CO₂ concentration (C_i) were noted in different cultivars. Finally, according to the comprehensive evaluation value D obtained from the combination of principal component analysis (PCA) and membership function (MF), the 15 blueberry cultivars can be divided into 4 categories: high soil pH-tolerant type ['Briteblue' (highest D value 0.815)], intermediate tolerance type ('Zhaixuan 9', 'Zhaixuan 7', 'Emerald', 'Primadonna', 'Powderblue' and 'Chandler'), low high soil pH-tolerant type ('Brightwell', 'Gardenblue', 'Plolific' and 'Sharpblue') and high soil pH-sensitive type ['Legacy', 'Bluegold', 'Baldwin' and 'Anna' (lowest D value 0.166)]. Stepwise linear regression analysis revealed that plant height, SS, E, leaf length, C_i, SOD, and SPAD could be used to predict and evaluate the high soil pH tolerance of blueberry cultivars.

Tolerant mechanism of model legume plant Medicago truncatula to drought, salt, and cold stresses

Legume plants produce one-third of the total yield of primary crops and are important food sources for both humans and animals worldwide. Frequent exposure to abiotic stresses, such as drought, salt, and cold, greatly limits the production of legume crops. Several morphological, physiological, and molecular studies have been conducted to characterize the response and adaptation mechanism to abiotic stresses. The tolerant mechanisms of the model legume plant Medicago truncatula to abiotic stresses have been extensively studied. Although many potential genes and integrated networks underlying the M. truncatula in responding to abiotic stresses have been identified and described, a comprehensive summary of the tolerant mechanism is lacking. In this review, we provide a comprehensive summary of the adaptive mechanism by which M. truncatula responds to drought, salt, and cold stress. We also discuss future research that need to be explored to improve the abiotic tolerance of legume plants.

From Soil Amendments to Controlling Autophagy: Supporting Plant Metabolism under Conditions of Water Shortage and Salinity

Crop resistance to environmental stress is a major issue. The globally increasing land degradation and desertification enhance the demand on management practices to balance both food and environmental objectives, including strategies that tighten nutrient cycles and maintain yields. Agriculture needs to provide, among other things, future additional ecosystem services, such as water quantity and quality, runoff control, soil fertility maintenance, carbon storage, climate regulation, and biodiversity. Numerous research projects have focused on the food–soil–climate nexus, and results were summarized in several reviews during the last decades. Based on this impressive piece of information, we have selected only a few aspects with the intention of studying plant–soil interactions and methods for optimization. In the short term, the use of soil amendments is currently attracting great interest to cover the current demand in agriculture. We will discuss the impact of biochar at water shortage, and plant growth promoting bacteria (PGPB) at improving nutrient supply to plants. In this review, our focus is on the interplay of both soil amendments on primary reactions of photosynthesis, plant growth conditions, and signaling during adaptation to environmental stress. Moreover, we aim at providing a general overview of how dehydration and salinity affect signaling in cells. With the use of the example of abscisic acid (ABA) and ethylene, we discuss the effects that can be observed when biochar and PGPB are used in the presence of stress. The stress response of plants is a multifactorial trait. Nevertheless, we will show that plants follow a general concept to adapt to unfavorable environmental conditions in the short and long term. However, plant species differ in the upper and lower regulatory limits of gene expression. Therefore, the presented data may help in the identification of traits for future breeding of stress-resistant crops. One target for breeding could be the removal and efficient recycling of damaged as well as needless compounds and structures. Furthermore, in this context, we will show that autophagy can be a useful goal of breeding measures, since the recycling of building blocks helps the cells to overcome a period of imbalanced substrate supply during stress adjustment.

How Could the Use of Crop Wild Relatives in Breeding Increase the Adaptation of Crops to Marginal Environments?

Alongside the use of fertilizer and chemical control of weeds, pests, and diseases modern breeding has been very successful in generating cultivars that have increased agricultural production several fold in favorable environments. These typically homogeneous cultivars (either homozygous inbreds or hybrids derived from inbred parents) are bred under optimal field conditions and perform well when there is sufficient water and nutrients. However, such optimal conditions are rare globally; indeed, a large proportion of arable land could be considered marginal for agricultural production. Marginal agricultural land typically has poor fertility and/or shallow soil depth, is subject to soil erosion, and often occurs in semi-arid or saline environments. Moreover, these marginal environments are expected to expand with ongoing climate change and progressive degradation of soil and water resources globally. Crop wild relatives (CWRs), most often used in breeding as sources of biotic resistance, often also possess traits adapting them to marginal environments. Wild progenitors have been selected over the course of their evolutionary history to maintain their fitness under a diverse range of stresses. Conversely, modern breeding for broad adaptation has reduced genetic diversity and increased genetic vulnerability to biotic and abiotic challenges. There is potential to exploit genetic heterogeneity, as opposed to genetic uniformity, in breeding for the utilization of marginal lands. This review discusses the adaptive traits that could improve the performance of cultivars in marginal environments and breeding strategies to deploy them.

Addressing the challenge of cold stress resilience with the synergistic effect of Rhizobium inoculation and exogenous melatonin application in Medicago truncatula

Cold stress is an adverse environmental condition that limits the growth and yield of leguminous plants. Thus, discovering an effective way of ameliorating cold-mediated damage is important for sustainable legume production. In this study, the combined use of Rhizobium inoculation (RI) and melatonin (MT) pretreatment was investigated in Medicago truncatula plants under cold stress. Eight-week-old seedlings were divided into eight groups: (i) CK (no stress, noninoculated, no MT), (ii) RI (Rhizobium inoculated), (iii) MT (75 μM melatonin), (iv) RI+MT, (v) CS (cold stress at 4 °C without Rhizobium inoculation and melatonin), (vi) CS+RI, (vii) CS+MT, and (viii) CS+RI+MT. Plants were exposed to cold stress for 24 hrs. Cold stress decreased photosynthetic pigments and increased oxidative stress. Pretreatment with RI and MT alone or combined significantly improved root activity and plant biomass production under cold stress. Interestingly, chlorophyll contents increased by 242.81% and MDA levels dramatically decreased by 34.22% in the CS+RI+MT treatment compared to the CS treatment. Moreover, RI+MT pretreatment improved the antioxidative ability by increasing the activities of peroxidase (POD; 8.45%), superoxide dismutase (SOD; 50.36%), catalase (CAT; 140.26%), and ascorbate peroxidase (APX; 42.63%) over CS treated plants. Additionally, increased osmolyte accumulation, nutrient uptake, and nitrate reductase activity due to the combined use of RI and MT helped the plants counteract cold-mediated damage by strengthening the nonenzymatic antioxidant system. These data indicate that pretreatment with a combined application of RI and MT can attenuate cold damage by enhancing the antioxidation ability of legumes.

Physiological changes and transcriptome profiling in Saccharum spontaneum L. leaf under water stress and re-watering conditions

As the polyploidy progenitor of modern sugarcane, Saccharum spontaneum is considered to be a valuable resistance source to various biotic and abiotic stresses. However, little has been reported on the mechanism of drought tolerance in S. spontaneum. Herein, the physiological changes of S. spontaneum GXS87-16 at three water-deficit levels (mild, moderate, and severe) and after re-watering during the elongation stage were investigated. RNA sequencing was utilized for global transcriptome profiling of GXS87-16 under severe drought and re-watered conditions. There were significant alterations in the physiological parameters of GXS87-16 in response to drought stress and then recovered differently after re-watering. A total of 1569 differentially expressed genes (DEGs) associated with water stress and re-watering were identified. Notably, the majority of the DEGs were induced by stress. GO functional annotations and KEGG pathway analysis assigned the DEGs to 47 GO categories and 93 pathway categories. The pathway categories were involved in various processes, such as RNA transport, mRNA surveillance, plant hormone signal transduction, and plant-pathogen interaction. The reliability of the RNA-seq results was confirmed by qRT-PCR. This study shed light on the regulatory processes of drought tolerance in S. spontaneum and identifies useful genes for genetic improvement of drought tolerance in sugarcane.

Photosynthetic performance of quinoa (Chenopodium quinoa Willd.) after exposure to a gradual drought stress followed by a recovery period

Drought is an abiotic scourge, one of the major environmental stress factors that adversely affect plant growth and photosynthesis machinery through a disruption of cell organelles, arrangement thylakoid membranes and the electron transport chain. Herein, we probed the effect of drought stress on photosynthetic performance of Chenopodium quinoa Willd. Beforehand, plants were subjected to water deficit (as 15% Field Capacity, FC) for one (D-1W) or two weeks (D-2W), and were then re-watered at 95% FC for 2 weeks. Light and electron microscopy analysis of leaves showed no apparent changes in mesophyll cell organization and chloroplast ultrastructure after one week of drought stress, while a swelling of thylakoids and starch accumulation were observed after the prolonged drought (D-2W). The latter induced a decrease in both PSI and PSII quantum yields which was accompanied by an increase in F₀ (minimum fluorescence) and a decline in F_m (maximum fluorescence). Drought stress influenced the fluorescence transients, where the major changes at the OJIP prompt FI level were detected in the OJ and IP phases. Prolonged drought induced a decrease in chl a fluorescence at IP phase which was readjusted and established back after re-watering and even more an increase was observed after 2 weeks of recovery. The maximum quantum yield of primary photochemistry (φ_Po) was unaffected by the different drought stress regimes. Drought induced an increase in the ABS/RC and DI₀/RC ratios which was concurrent to a stable φ_Po (maximum quantum yield of PSII primary photochemistry). A substantial decrease in PI_(ABS) was detected especially, during severe drought stress (D-2W) suggesting a drop in the PSII efficiency and the level of electron transport through the plastoquinone pool (PQ pool) towards oxidized PSI RCs (P₇₀₀⁺). The immunoblot analysis of the main PSII proteins revealed considerable changes in the D1, D2, CP47, OEC, PsbQ and LHCII proteins under drought. These changes depend on the stress duration and recovery period. The main message of this investigation is the elevated recovery capacities of PSII and PSI photochemical activities after re-watering.

Morpho-Physiology and Antioxidant Enzyme Activities of Transgenic Rice Plant Overexpressing ABP57 under Reproductive Stage Drought Condition

MR219 transgenic rice line which overexpressed an auxin-binding protein (ABP57) and its wild-type cultivar, MR219, were screened under well-watered (WW) and drought stress (DS) conditions at the early reproductive stage. This study was conducted with the standard planting distance and under a normal environment to assess the yield advantages based on the field conditions. The aim of this study was to understand the response of these rice genotypes towards DS at morpho-physiological, biochemical, and agronomical levels. It was found that the DS had affected all these levels of the genotypes studied; however, the transgenic plant showed a higher number of tillers, flag leaf area, biomass, relative water content, total chlorophyll content, and antioxidative defense mechanism than the MR219 under DS. Compared to its wild-type, the transgenic plant showed an increased leaf photosynthetic rate by 7% under WW and 11% under DS. The transgenic plant also showed higher yields than MR219 under the WW (10%) and DS (59%). The results propose that drought tolerance is significantly improved in the MR219 transgenic rice line. It may develop a new opportunity for the drought-tolerant rice breeding programme via overexpression of ABP57.

Evaluation of pulse crops' functional diversity supporting food production

Pulses, defined as legumes which produce dry seed used for human consumption, are plants of great agronomic value, at the food system level as much as the field level but their diversity has been largely underused. This study aimed at analyzing existing data on cultivated pulse species in the literature to provide a broad and structured description of pulses’ interspecific functional diversity. We used a functional trait-based approach to evaluate how pulse diversity could support food production in agroecosystems constrained by low water and nutrient availability and exposed to high weed pressure. We gathered data for 17 functional traits and six agroecosystem properties for 43 pulse species. Our analytical framework highlights the correlations and combinations of functional traits that best predict values of six agroecosystem properties defined as ecosystem services estimates. We show that pulse diversity has been structured both by breeding and by an environmental gradient. The covariance space corresponding to agroecosystem properties was structured by three properties: producers, competitors, stress-tolerant species. The distribution of crop species in this functional space reflected ecological adaptive strategies described in wild species, where the size-related axis of variation is separated from variation of leaf morpho-physiological traits. Six agroecosystem properties were predicted by different combinations of traits. However, we identified ubiquitous plant traits such as leaflet length, days to maturity, seed weight, and leaf nitrogen content, that discriminated agroecosystem properties and allowed us to gather individual species into three clusters, representative of the three strategies highlighted earlier. Implications for pulses provisioning of services in agroecosystems are discussed.

The intertwined metabolism during symbiotic nitrogen fixation elucidated by metabolic modelling

Genome-scale metabolic network models can be used for various analyses including the prediction of metabolic responses to changes in the environment. Legumes are well known for their rhizobial symbiosis that introduces nitrogen into the global nutrient cycle. Here, we describe a fully compartmentalised, mass and charge-balanced, genome-scale model of the clover Medicago truncatula, which has been adopted as a model organism for legumes. We employed flux balance analysis to demonstrate that the network is capable of producing biomass components in experimentally observed proportions, during day and night. By connecting the plant model to a model of its rhizobial symbiont, Sinorhizobium meliloti, we were able to investigate the effects of the symbiosis on metabolic fluxes and plant growth and could demonstrate how oxygen availability influences metabolic exchanges between plant and symbiont, thus elucidating potential benefits of inter organism amino acid cycling. We thus provide a modelling framework, in which the interlinked metabolism of plants and nodules can be studied from a theoretical perspective.

Reproductive Strategies in Mediterranean Legumes: Trade-Offs between Phenology, Seed Size and Vigor within and between Wild and Domesticated Lupinus Species Collected along Aridity Gradients

To investigate wild and domesticated Mediterranean annual reproductive strategies, common garden comparisons of Old World lupins collected along aridity gradients were initiated. These are excellent candidates for ecophysiology, being widely distributed across contrasting environments, having distinct domestication histories, from ancient Lupinus albus to recently domesticated Lupinus angustifolius and Lupinus luteus, facilitating the study of both natural and human selection. Strong trade-offs between seed size, early vigor and phenology were observed: vigor increasing, and flowering becoming earlier with increasing seed size. Despite large specific differences in all these traits, natural and human selection have operated in very similar ways in all 3 species. In wild material, as collection environments became drier and hotter, phenology became earlier, while seed size, early vigor and reproductive investment increased. Wild and domesticated germplasm separated along similar lines. Within similar habitats, domesticated material was consistently earlier, with larger seeds, greater early vigor and higher reproductive investment than wild, suggesting selection for both early establishment and timely maturity/drought escape in both domesticated and wild low rainfall ecotypes. Species differences reflected their distribution. Small and soft-seeded, low vigor L. luteus had a late, rainfall-responsive phenology specifically adapted to long season environments, and a narrow coastal distribution. L. angustifolius was much more conservative; more hard-seeded, flowering and maturing much earlier, with a wide Mediterranean distribution. L. albus flowered earlier but matured much later, with longer reproductive phases supporting much larger seed sizes and early vigor than either L. luteus or L. angustifolius. This ruderal/competitive combination appears to give L. albus a broad adaptive capacity, reflected in its relatively wider Mediterranean/North African distribution.

Contrasting adaptive strategies to terminal drought-stress gradients in Mediterranean legumes: phenology, productivity, and water relations in wild and domesticated Lupinus luteus L

Our understanding of within-species annual plant adaptation to rainfall gradients is fragmented. Broad-scale ecological applications of Grime's C-S-R triangle are often superficial, while detailed drought physiology tends to be narrow, focusing on elite cultivars. The former lack the detail to explain how plants respond, while the latter provide little context to investigate trade-offs among traits, to explain where/why these might be adaptive. Ecophysiology, combining the breadth of the former with the detail of the latter, can resolve this disconnect and is applied here to describe adaptive strategies in the Mediterranean legume Lupinus luteus. Wild and domesticated material from low- and high-rainfall environments was evaluated under contrasting terminal drought. These opposing environments have selected for contrasting, integrated, adaptive strategies. Long-season, high-rainfall habitats select for competitive (C) traits: delayed phenology, high above- and below-ground biomass, productivity, and fecundity, leading to high water-use and early stress onset. Terminal drought-prone environments select for the opposite: ruderal (R) traits that facilitate drought escape/avoidance but limit reproductive potential. Surprisingly, high-rainfall ecotypes generate lower critical leaf water potentials under water deficit, maintaining higher relative water content than the latter. Given that L. luteus evolved in sandy, low-water-holding capacity soils, this represents a bet-hedging response to intermittent self-imposed water-deficits associated with a strongly C-selected adaptive strategy that is therefore redundant in R-selected low-rainfall ecotypes. Domesticated L. luteus is even more R-selected, reflecting ongoing selection for early maturity. Introgression of appropriate C-selected adaptive traits from wild germplasm may widen the crop production range.

Interactive effects of salinity and phosphorus availability on growth, water relations, nutritional status and photosynthetic activity of barley (Hordeum vulgare L.)

The interactive effects of salinity and phosphorus availability on growth, water relations, nutritional status and photosynthetic activity were investigated in barley (Hordeum vulgare L. cv. Manel). Seedlings were grown hydroponically under low or sufficient phosphorus (P) supply (5 or 180 μmol KH(2) PO(4) plant(-1) week(-1) , respectively), with or without 100 mm NaCl. Phosphorus deficiency or salinity significantly decreased whole plant growth, leaf water content, leaf osmotic potential and gas exchange parameters, with a more marked impact of P stress. The effect of both stresses was not additive since the response of plants to combined salinity and P deficiency was similar to that of plants grown under P deficiency alone. In addition, salt-treated plants exposed to P deficiency showed higher salt tolerance compared to plants grown with sufficient P supply. This was related to plant ability to significantly increase root:shoot DW ratio, root length, K(+)/Na(+) ratio, leaf proline and soluble sugar concentrations and total non-enzymatic antioxidant capacity, together with restricting Na(+) accumulation in the upper leaves. As a whole, our results indicate that under concomitant exposure to both salt and P deficiency, the impact of the latter constraint is pre-dominant.