Partial and full root-zone drought stresses account for differentiate root-sourced signal and yield formation in primitive wheat

Distinct planting patterns exert legacy effects on the networks and assembly of root-associated microbiomes in subsequent crops

Soil legacy effects from previous crops can significantly influence plant-soil interactions in crop rotations. However, the microbial mechanism underlying this effect in subsequent root-associated compartments remains unclear. We investigated the effects of planting patterns (four-year continuous maize [MM], three-year winter wheat and one-year maize rotation [WM], and three-year potato and one-year maize rotation [PM]) on the microbial composition and structure of root-associated compartments, the effect of distinct crops on subsequent microbial co-occurrence patterns, and the assembly mechanism by which the root-associated compartments (bulk soil, rhizosphere, and roots) in subsequent crops regulate the microbiome habitat. Compared with MM, the relative abundance of Acidobacteria in WM was 29.7 % lower, whereas that of Bacteroidota in PM was 37.9 % higher in all three compartments. The co-occurrence patterns of the microbial communities exhibited varied responses to different planting patterns. Indicator taxon analysis revealed less shared and specific species in the root bacterial and fungal networks. The planting pattern elicited specific responses from modules within bacterial and fungal co-occurrence networks in all three compartments. Moreover, the planting patterns and root-associated compartments collectively drove the assembly process of root-associated microorganisms. The neutral model showed that, compared with MM, the stochasticity of bacterial assembly decreased under WM and PM but increased for fungal assembly. WM and PM increased the relative effects of the homogenized dispersal of fungal assemblies in roots. We conclude that previous crops exhibit marked legacy effects in the root-associated microbiome. Therefore, soil heritage should not be ignored when discussing microbiome recruitment strategies and co-occurrence patterns in subsequent crops.

Root-to-shoot signaling positively mediates source-sink relation in late growth stages in diploid and tetraploid wheat

Non-hydraulic root source signaling (nHRS) is a unique positive response to soil drying in the regulation of plant growth and development. However, it is unclear how the nHRS mediates the tradeoff between source and sink at the late growth stages and its adaptive mechanisms in primitive wheat. To address this issue, a root-splitting design was made by inserting solid partition in the middle of the pot culture to induce the occurrence of nHRS using four wheat cultivars (MO1 and MO4, diploid; DM22 and DM31, tetraploid) as materials. Three water treatments were designed as 1) both halves watered (CK), 2) holistic root system watered then droughted (FS), 3) one-half of the root system watered and half droughted (PS). FS and PS were designed to compare the role of the full root system and split root system to induce nHRS. Leaves samples were collected during booting and anthesis to compare the role of nHRS at both growth stages. The data indicated that under PS treatment, ABA concentration was significantly higher than FS and CK, demonstrating the induction of nHRS in split root design and nHRS decreased cytokinin (ZR) levels, particularly in the PS treatment. Soluble sugar and proline accumulation were higher in the anthesis stage as compared to the booting stage. POD activity was higher at anthesis, while CAT was higher at the booting stage. Increased ABA (nHRS) correlated with source-sink relationships and metabolic rate (i.e., leaf) connecting other stress signals. Biomass density showed superior resource acquisition and utilization capabilities in both FS and PS treatment as compared to CK in all plants. Our findings indicate that nHRS-induced alterations in phytohormones and their effect on source-sink relations were allied with the growth stages in primitive wheat.

Response of source-sink relationship to progressive water deficit in the domestication of dryland wheat

It is crucial to clarify the physiological responses of wheat (T. aestivum) plants to source-sink manipulation and assimilation transportation under drought stress during domestication of dryland wheat. In this research, a two-year field experiment was conducted using nine wheat cultivars in a semiarid site of northwest China. The source-sink manipulation treatments including defoliation of flag leaves and 50% removal of ears were applied at the anthesis stage under two levels of drought stress conditions i.e. progressive water supply (PWS) and rainfed drought treatment (RDT). Our results indicated that drought stress reduced the dry weight of leaves, sheaths and stems, as well as caused a significant yield reduction. High ploidy wheat exhibits a greater capacity to sustain higher grain yields when subjected to drought stress, primarily due to its stronger buffer capacity between source supply and sink demand. All wheat species with different ploidy levels had a certain degree of source limitation and sink restriction. During the domestication of wheat, the type of source and sink might be ploidy-dependent with progressive water deficit, but similar interactive relationships. The source-sink ratio of tetraploid species was the largest, while that of hexaploid species was the lowest.

Deficit irrigation strategies (PRD, SDI) and titanium nanoparticles improve water use efficiency and flower quality in greenhouse-grown cut roses

This study explored the use of deficit irrigation techniques for water management in the hydroponic greenhouse cultivation of cut roses. A factorial experiment was conducted using three irrigation treatments: full irrigation (FI), partial root drying (PRD), and sustained deficit irrigation (SDI), and three doses of titanium dioxide nanoparticle foliar application (0, 15, and 30 ppm) as stress alleviation. Results showed that drought stress increased biochemical parameters such as the plants' proline and total phenol content. Compared to SDI treatment, the PRD treatments have an increase in flower number by 40%. The PRD strategy has positive effects on drought tolerance by increasing osmotic and elastic adjustment. Therefore, higher relative water content and longer root length in PRD treatments were observed. Thus, Biomass water use efficiency significantly increased in PRD treatments compared to others. In the PRD treatment, yield WUE increases by 26% and 61% compared to FI and SDI, respectively. The results showed TiO2-NPs positively affected mitigating and even improving some traits in drought stress conditions. These results suggest the superiority of the PRD strategy, which improves growth characteristics and water use efficiency, leading to increased sustainability, reduced environmental impact of greenhouse toxic wastewater, and total profitability of the greenhouse.

Enhancement of osmotic stress tolerance in soybean seed germination by bacterial bioactive extracts

Soybean (Glycine max (L.) Merr.) is important to the global food industry; however, its productivity is affected by abiotic stresses such as osmosis, flooding, heat, and cold. Here, we evaluated the bioactive extracts of two biostimulant bacterial strains, Bacillus butanolivorans KJ40 and B. siamensis H30-3, for their ability to convey tolerance to osmotic stress in soybean seeds during germination. Soybean seeds were dip-treated in extracts of KJ40 (KJ40E) or H30-3 (H30-3E) and incubated with either 0% or 20% polyethylene glycol 6000 (PEG), simulating drought-induced osmotic stress. We measured malondialdehyde content as a marker for lipid peroxidation, as well as the activity of antioxidant enzymes, including catalase, glutathione peroxidase, and glutathione reductase, together with changes in sugars content. We also monitored the expression of genes involved in the gibberellic acid (GA)-biosynthesis pathway, and abscisic acid (ABA) signaling. Following osmotic stress in the extract-treated seeds, malondialdehyde content decreased, while antioxidant enzyme activity increased. Similarly, the expression of GA-synthesis genes, including GmGA2ox1 and GmGA3 were upregulated in KJ40E-dipped seeds at 12 or 6 h after treatment, respectively. The ABA signaling genes GmABI4 and GmDREB1 were upregulated in H30-3E- and KJ40E-treated seeds at 0 and 12 h after treatment under osmotic stress; however, GmABI5, GmABI4, and GmDREB1 levels were also elevated in the dip-treated seeds in baseline conditions. The GA/ABA ratio increased only in KJ40E-treated seeds undergoing osmotic stress, while glucose content significantly decreased in H30-3E-treated seeds at 24 h after treatment. Collectively, our findings indicated that dip-treatment of soybean seeds in KJ40E and H30-3E can enhance the seeds' resistance to osmotic stress during germination, and ameliorate cellular damage caused by secondary oxidative stress. This seed treatment can be used agriculturally to promote germination under drought stress and lead to increase crop yield and quality.

Genome-Wide Identification of DUF668 Gene Family and Expression Analysis under Drought and Salt Stresses in Sweet Potato [Ipomoea batatas (L.) Lam]

The domain of unknown function 668 (DUF668) is a gene family that plays a vital role in responses to adversity coercion stresses in plant. However, the function of the DUF668 gene family is not fully understood in sweet potato. In this study, bioinformatics methods were used to analyze the number, physicochemical properties, evolution, structure, and promoter cis-acting elements of the IbDUF668 family genes, and RNA-seq and qRT-PCR were performed to detect gene expression and their regulation under hormonal and abiotic stress. A total of 14 IbDUF668 proteins were identified in sweet potato, distributed on nine chromosomes. By phylogenetic analysis, IbDUF668 proteins can be divided into two subfamilies. Transcriptome expression profiling revealed that many genes from DUF668 in sweet potato showed specificity and differential expression under cold, heat, drought, salt and hormones (ABA, GA3 and IAA). Four genes (IbDUF668-6, 7, 11 and 13) of sweet potato were significantly upregulated by qRT-PCR under ABA, drought and NaCl stress. Results suggest that the DUF668 gene family is involved in drought and salt tolerance in sweet potato, and it will further provide the basic information of DUF668 gene mechanisms in plants.

Role of abscisic acid in modulating drought acclimation, agronomic characteristics and β-N-oxalyl-L-α,β-diaminopropionic acid (β-ODAP) accumulation in grass pea (Lathyrus sativus L.)

BACKGROUND

β-N-oxalyl-l-α,β-diaminopropionic acid (β-ODAP) is a physiological indicator in response to drying soil. However, how abscisic acid (ABA) modulates β-ODAP accumulation and its related agronomic characteristics in drought stressed grass pea (Lathyrus sativus L.) continue to be unclear. The present study aimed to evaluate the effects of ABA addition on drought tolerance, agronomic characteristics and β-ODAP content in grass pea under drought stress.

RESULTS

Exogenous ABA significantly promoted ABA levels by 19.3% and 18.3% under moderate and severe drought stress, respectively, compared to CK (without ABA, used as control check treatment). ABA addition activated earlier trigger of non-hydraulic root-sourced signal at 69.1% field capacity (FC) (65.5% FC in CK) and accordingly prolonged its operation period to 45.6% FC (49.0% FC in CK). This phenomenon was mechanically associated with the physiological mediation of ABA, where its addition significantly promoted the activities of leaf superoxide dismutase, catalase and peroxidase enzymes and the biosynthesis of leaf proline, simultaneously lowering the accumulation of malondialdehyde and hydrogen peroxide under moderate and severe stresses. Interestingly, ABA application significantly increased seed β-ODAP content by 21.7% and 21.3% under moderate and severe drought stress, but did not change leaf β-ODAP content. Furthermore, ABA application produced similar shoot biomass and grain yield as control groups.

CONCLUSION

Exogenous ABA improved the drought adaptability of grass pea and promoted the synthesis of β-ODAP in seeds but not in leaves. Our findings provide novel insights into the agronomic role of ABA in relation to β-ODAP enrichment in grass pea subjected to drought stress. © 2021 Society of Chemical Industry.

Drought stress-induced changes in redox metabolism of barley (Hordeum vulgare L.)

In the present investigation, influence of water stress on redox metabolism was evaluated in the flag leaf and grains of four barley (Hordeum vulgare L.) genotypes viz DWRB 101, 432 ICARDA, Jyoti and 430 ICARDA at 10th, 20th and 30th days after anthesis (DAA). Relative water content, electrolyte leakage, antioxidative enzymes and their related metabolites were studied during drought stress. Relative water content was well maintained in both the tissues of DWRB 101 and 432 ICARDA. The upregulation of catalase at 20th DAA while ascorbate peroxidase, glutathione reductase and dehydro reductase at 30th DAA in the flag leaf and grains of DWRB 101 and 432 ICARDA may be responsible for lesser increase in H2O2 content as compared to other genotypes. Moreover, the downregulation of superoxide dismutase was comparatively higher in Jyoti and 430 ICARDA. The redox homeostasis was well established during the stress in DWRB 101 and 432 ICARDA by maintaining comparatively higher ratios of ascorbate/dehydroascorbate and reduced/oxidized glutathione. Therefore, scrutiny of data indicated that DWRB 101 and 432 ICARDA may perform better under drought stress in comparison with Jyoti and 430 ICARDA.

Silicon induces adventitious root formation in rice under arsenate stress with involvement of nitric oxide and indole-3-acetic acid

Arsenic (As) negatively affects plant development. This study evaluates how the application of silicon (Si) can favor the formation of adventitious roots in rice under arsenate stress (AsV) as a mechanism to mitigate its negative effects. The simultaneous application of AsV and Si up-regulated the expression of genes involved in nitric oxide (NO) metabolism, cell cycle progression, auxin (IAA, indole-3-acetic acid) biosynthesis and transport, and Si uptake which accompanied adventitious root formation. Furthermore, Si triggered the expression and activity of enzymes involved in ascorbate recycling. Treatment with L-NAME (NG-nitro L-arginine methyl ester), an inhibitor of NO generation, significantly suppressed adventitious root formation, even in the presence of Si; however, supplying NO in the growth media rescued its effects. Our data suggest that both NO and IAA are essential for Si-mediated adventitious root formation under AsV stress. Interestingly, TIBA (2,3,5-triiodobenzoic acid), a polar auxin transport inhibitor, suppressed adventitious root formation even in the presence of Si and SNP (sodium nitroprusside, an NO donor), suggesting that Si is involved in a mechanism whereby a cellular signal is triggered and that first requires NO formation, followed by IAA biosynthesis.

Differentiate responses of tetraploid and hexaploid wheat (Triticum aestivum L.) to moderate and severe drought stress: a cue of wheat domestication

Differentiate mechanism of wheat species in response to contrasting drought stress gradients implies a cue of its long-term domestication. In the present study, three water regimes including well-watered control (WW, 80% field water capacity (FC)), moderate drought stress (MS, 50% FC,) and severe drought stress (SS, 30% FC) were designed to reveal different responses of eight wheat species (four tetraploid and four hexaploid) representing different breeding decades and genetic origins to drought stresses. The data indicated that 50% FC and 30% FC fell into the soil moisture threshold range of non-hydraulic and hydraulic root signal occurrence, respectively. In general, grain yield, grain number/spike weight per plant, aboveground biomass, harvest index (HI) and water use efficiency (WUE) were significantly higher in hexaploid species than those of tetraploid species under drought stress (P < .05). Particularly, non-hydraulic root signal was triggered and continuously operated under 50% FC, while hydraulic root signal was observed under 30% FC, respectively. Under 80% FC, the allometric exponent (ɑ) of M_aboveground vs M_root decreased from tetraploid to hexaploid (both were of <1), indicating that during the domestication, the hexaploid species allocated less biomass to root system. For the relationship of M_ear vs M_vegetative, the ɑ value was significantly greater in the hexaploid species, showing that hexaploid wheat distributed more biomass to ear than tetraploid to improve yield. Under 50% FC, this trend was enhanced. However, under 30% FC, there was no significant difference in the ɑ value between two species. Additionally, correlation analyses on yield formation affirmed the above results. Therefore, drought tolerance tended to be enhanced in hexaploid species under the pressure of artificial selection than that of tetraploid species. When drought stress exceeded a certain threshold, both species would be negatively seriously affected and followed a similar mechanism for better survival.

Root Adaptation via Common Genetic Factors Conditioning Tolerance to Multiple Stresses for Crops Cultivated on Acidic Tropical Soils

Crop tolerance to multiple abiotic stresses has long been pursued as a Holy Grail in plant breeding efforts that target crop adaptation to tropical soils. On tropical, acidic soils, aluminum (Al) toxicity, low phosphorus (P) availability and drought stress are the major limitations to yield stability. Molecular breeding based on a small suite of pleiotropic genes, particularly those with moderate to major phenotypic effects, could help circumvent the need for complex breeding designs and large population sizes aimed at selecting transgressive progeny accumulating favorable alleles controlling polygenic traits. The underlying question is twofold: do common tolerance mechanisms to Al toxicity, P deficiency and drought exist? And if they do, will they be useful in a plant breeding program that targets stress-prone environments. The selective environments in tropical regions are such that multiple, co-existing regulatory networks may drive the fixation of either distinctly different or a smaller number of pleiotropic abiotic stress tolerance genes. Recent studies suggest that genes contributing to crop adaptation to acidic soils, such as the major Arabidopsis Al tolerance protein, AtALMT1, which encodes an aluminum-activated root malate transporter, may influence both Al tolerance and P acquisition via changes in root system morphology and architecture. However, trans-acting elements such as transcription factors (TFs) may be the best option for pleiotropic control of multiple abiotic stress genes, due to their small and often multiple binding sequences in the genome. One such example is the C2H2-type zinc finger, AtSTOP1, which is a transcriptional regulator of a number of Arabidopsis Al tolerance genes, including AtMATE and AtALMT1, and has been shown to activate AtALMT1, not only in response to Al but also low soil P. The large WRKY family of transcription factors are also known to affect a broad spectrum of phenotypes, some of which are related to acidic soil abiotic stress responses. Hence, we focus here on signaling proteins such as TFs and protein kinases to identify, from the literature, evidence for unifying regulatory networks controlling Al tolerance, P efficiency and, also possibly drought tolerance. Particular emphasis will be given to modification of root system morphology and architecture, which could be an important physiological "hub" leading to crop adaptation to multiple soil-based abiotic stress factors.

Analyzing Spatio-Temporal Factors to Estimate the Response Time between SMOS and In-Situ Soil Moisture at Different Depths

A comprehensive understanding of temporal variability of subsurface soil moisture (SM) is paramount in hydrological and agricultural applications such as rainfed farming and irrigation. Since the SMOS (Soil Moisture and Ocean Salinity) mission was launched in 2009, globally available satellite SM retrievals have been used to investigate SM dynamics, based on the fact that useful information about subsurface SM is contained in their time series. SM along the depth profile is influenced by atmospheric forcing and local SM properties. Until now, subsurface SM was estimated by weighting preceding information of remotely sensed surface SM time series according to an optimized depth-specific characteristic time length. However, especially in regions with extreme SM conditions, the response time is supposed to be seasonally variable and depends on related processes occurring at different timescales. Aim of this study was to quantify the response time by means of the time lag between the trend series of satellite and in-situ SM observations using a Dynamic Time Warping (DTW) technique. DTW was applied to the SMOS satellite SM L4 product at 1 km resolution developed by the Barcelona Expert Center (BEC), and in-situ near-surface and root-zone SM of four representative stations at multiple depths, located in the Soil Moisture Measurements Station Network of the University of Salamanca (REMEDHUS) in Western Spain. DTW was customized to control the rate of accumulation and reduction of time lag during wetting and drying conditions and to consider the onset dates of pronounced precipitation events to increase sensitivity to prominent features of the input series. The temporal variability of climate factors in combination with crop growing seasons were used to indicate prevailing SM-related processes. Hereby, a comparison of long-term precipitation recordings and estimations of potential evapotranspiration (PET) allowed us to estimate SM seasons. The spatial heterogeneity of land use was analyzed by means of high-resolution images of Normalized Difference Vegetation Index (NDVI) from Sentinel-2 to provide information about the level of spatial representativeness of SMOS observations to each in-situ station. Results of the spatio-temporal analysis of the study were then evaluated to understand seasonally and spatially changing patterns in time lag. The time lag evolution describes a variable characteristic time length by considering the relevant processes which link SMOS and in-situ SM observation, which is an important step to accurately infer subsurface SM from satellite time series. At a further stage, the approach needs to be applied to different SM networks to understand the seasonal, climate- and site-specific characteristic behaviour of time lag and to decide, whether general conclusions can be drawn.

Plant growth under suboptimal water conditions: early responses and methods to study them

Drought stress forms a major environmental constraint during the life cycle of plants, often decreasing plant yield and in extreme cases threatening survival. The molecular and physiological responses induced by drought have been the topic of extensive research during the past decades. Because soil-based approaches to studying drought responses are often challenging due to low throughput and insufficient control of the conditions, osmotic stress assays in plates were developed to mimic drought. Addition of compounds such as polyethylene glycol, mannitol, sorbitol, or NaCl to controlled growth media has become increasingly popular since it offers the advantage of accurate control of stress level and onset. These osmotic stress assays enabled the discovery of very early stress responses, occurring within seconds or minutes following osmotic stress exposure. In this review, we construct a detailed timeline of early responses to osmotic stress, with a focus on how they initiate plant growth arrest. We further discuss the specific responses triggered by different types and severities of osmotic stress. Finally, we compare short-term plant responses under osmotic stress versus in-soil drought and discuss the advantages, disadvantages, and future of these plate-based proxies for drought.

Physiological, biochemical and gene-expressional responses to water deficit in apple subjected to partial root-zone drying (PRD)

Water scarcity is one of the major factors limiting apple production. Partial root-zone drying (PRD) is a water-saving irrigation technique necessary to improve the efficiency of irrigation techniques to optimize the amount of fruit produced with the volume of water used. The apple trees cv. Red Delicious were exposed to four treatments, including (1) control with 100% of the crop evapotranspiration (ETc) needs; (2) alternate partial root-zone drying with 75% of the ETc needs (APRD75); (3) fixed partial root-zone drying with 75% of the ETc needs (FPRD75); (4) fixed partial root-zone irrigation with 50% of the ETc needs (FPRD50) in a semiarid region of Iran. Results showed that leaf water potential (Ψ _leaf), and chlorophyll were significantly decreased in FPRD50 compared to control and other PRD treatments. APRD75 and FPRD75 treatments significantly enhanced (+) -catechin (+C), epicatechin (EC), chlorogenic acid (CGA), caffeic acid (CA) as well as increased water use efficiency (WUE) (by 30-40% compared to control) without significant reduction of yield. PRD reduced gibberellic acid (GA3) and kinetin, while, increased the abscisic acid (ABA) and salicylic acid (SA) levels. The abiotic stress-responsive transcription factors (TFs) MdoMYB121, MdoMYB155, MdbZIP2, and MdbZIP48 were highly expressed in all PRD treatments. Our results demonstrated that APRD75 and FPRD75 have the potential to stimulate antioxidant defense mechanisms, hormonal signaling pathways, and expression of drought-tolerance TFs to improve WUE while maintaining crop yield. Therefore, APRD75andFPRD75 with water savings as compared to full irrigation might be a suitable strategy for irrigation apple trees under water scarcity.