Flooding Causes Dramatic Compositional Shifts and Depletion of Putative Beneficial Bacteria on the Spring Wheat Microbiota

Flooding affects both above- and below-ground ecosystem processes, and it represents a substantial threat for crop and cereal productivity under climate change. Plant-associated microbiota play a crucial role in plant growth and fitness, but we still have a limited understanding of the response of the crop-microbiota complex under extreme weather events, such as flooding. Soil microbes are highly sensitive to abiotic disturbance, and shifts in microbial community composition, structure and functions are expected when soil conditions are altered due to flooding events (e.g., anoxia, pH alteration, changes in nutrient concentration). Here, we established a pot experiment to determine the effects of flooding stress on the spring wheat-microbiota complex. Since plant phenology could be an important factor in the response to hydrological stress, flooding was induced only once and at different plant growth stages (PGSs), such as tillering, booting and flowering. After each flooding event, we measured in the control and flooded pots several edaphic and plant properties and characterized the bacterial community associated to the rhizosphere and roots of wheat plant using a metabarcoding approach. In our study, flooding caused a significant reduction in plant development and we observed dramatic shifts in bacterial community composition at each PGS in which the hydrological stress was induced. However, a more pronounced disruption in community assembly was always shown in younger plants. Generally, flooding caused a (i) significant increase of bacterial taxa with anaerobic respiratory capabilities, such as members of Firmicutes and Desulfobacterota, (ii) a significant reduction in Actinobacteria and Proteobacteria, (iii) depletion of several putative plant-beneficial taxa, and (iv) increases of the abundance of potential detrimental bacteria. These significant differences in community composition between flooded and control samples were correlated with changes in soil conditions and plant properties caused by the hydrological stress, with pH and total N as the soil, and S, Na, Mn, and Ca concentrations as the root properties most influencing microbial assemblage in the wheat mircobiota under flooding stress. Collectively, our findings demonstrated the role of flooding on restructuring the spring wheat microbiota, and highlighted the detrimental effect of this hydrological stress on plant fitness and performance.

Exogenous ABA foliar spray treatment alleviates flooding stress in adzuki bean (Vigna angularis) during the seedling stage

Abscisic acid (ABA) plays an important role in regulating activities of antioxidant enzymes, hormone levels, accumulation of reactive oxygen species under stress. To understand the effects of exogenous ABA treatment on flooding tolerance at seedling stage in adzuki bean (Vigna angularis), the water-sensitive (WS) variety ‘Tianjinhong’ (TJH) and the water-tolerant (WT) variety ‘Longxiaodou 4’ (LXD 4) were foliar sprayed exogenous ABA with 20 μM, followed by flooding stress for 5 days under pot conditions. Our results revealed that under flooding stress at seedling stage, the contents of H2O2 and MDA, proline and soluble protein in adzuki bean leaves significantly increased, the contents of ABA, IAA and SA significantly increased, and the activities of SOD, POD and CAT also significantly increased. Flooding treatment for 5 days resulted in a significant decrease in single pot yield of LXD 4 by 8.40% and TJH by 9.91%. Foliar spray of 20 μM exogenous ABA could resist flooding stress, considerably increased the contents of proline and soluble protein, significantly decreased the contents of H2O2 and MDA, significantly increased the activities of SOD and POD, increased the contents of ABA and SA, and significantly inhibited the increase of IAA content in adzuki bean leaves. Spraying exogenous ABA significantly increased the yield of ‘LXD’ 4 by 6.95% after 4 days of flooding treatment, and ‘TJH’ by 4.46%. To sum up, there were significant differences in physiological stress among different varieties of adzuki bean under flooding stress, and spraying exogenous ABA could effectively alleviate the effects of flooding stress on physiology and yield of adzuki bean. The results of this study provided a theoretical basis for further studying the physiological mechanism of adzuki bean resisting flooding stress at seedling stage and improving the yield of adzuki bean under flooding stress. Thus, foliar spraying exogenous ABA effectively improved submergence tolerance by enhancing the activity of protective enzymes and osmoregulation. These results provided novel insights and were expected to aid in the development of more effective stress resistance cultivation methods in adzuki bean production.

Screening of ‘King’ Mandarin Hybrids as Tolerant Citrus Rootstocks to Flooding Stress

This work compares the tolerance to long-term anoxia conditions (35 days) of five new citrus ‘King’ mandarin (Citrus nobilis L. Lour) × Poncirus trifoliata ((L.) Raf.) hybrids (named 0501XX) and Carrizo citrange (CC, Citrus sinensis (L.) Osb. × Poncirus trifoliata (L.) Raf.), the widely used citrus rootstock in Spain. Growth parameters, chlorophyll concentration, gas exchange and fluorescence parameters, water relations in leaves, abscisic acid (ABA) concentration, and PIP1 and PIP2 gene expressions were assessed. With a waterlogging treatment, the root system biomass of most hybrids went down, and the chlorophyll a and b concentrations substantially dropped. The net CO2 assimilation rates (An) and stomatal conductance (gs) lowered significantly due to flooding, and the transpiration rate (E) closely paralleled the changes in gs. The leaf water and osmotic potentials significantly increased in most 0501 hybrids. As a trend, flooding stress lowered the ABA concentration in roots from most hybrids, but increased in the leaves of CC, 05019 and 050110. Under the control treatment (Ct) conditions, most 0501 hybrids showed higher PIP1 and PIP2 expressions than the control rootstock CC, but were impaired due to the flooding conditions in 05019 and 050110. From this study, we conclude that 0501 genotypes develop some adaptive responses in plants against flooding stress such as (1) stomata closure to prevent water loss likely mediated by ABA levels, and (2) enhanced water and osmotic potentials and the downregulation of those genes regulating aquaporin channels to maintain water relations in plants. Although these traits seemed especially relevant in hybrids 050110 and 050125, further experiments must be done to determine their behavior under field conditions, particularly their influence on commercial varieties and their suitability as flooding-tolerant hybrids for replacing CC, one of the main genotypes that is widely used as a citrus rootstock in Spain, under these conditions.

The Roles of Competition and Facilitation in Producing Zonation Along an Experimental Flooding Gradient: A Tale of Two Tails with Ten Freshwater Marsh Plants

Plant zonation is conspicuous in wetlands. The cause is frequently assumed to be the direct physiological effects of physical factors (termed ‘stress’), however many experiments show that competition and facilitation also cause zonation patterns. We conducted a field experiment with freshwater marsh emergent plants to test the causes of zonation along a single stress gradient: flooding duration. We constructed an experimental wetland with ten flooding levels to ensure that the environmental conditions represented the full range of potential flooding levels, from never flooded to continually flooded. We planted ten common marsh plants with varied ecology along the flooding duration gradient. We grew them alone and in mixture for three years and measured changes in the minimum and maximum limits, the mode and the range of distribution, and interaction importance. The mode of distribution did not shift, whether species were grown alone or with neighbours. We found strong effects of competition under low flooding stress. We found no effects from facilitation under high flooding stress. Flooding duration alone controlled the lower limits of plants. The effects of competition were intense enough to eliminate half of the species within three growing seasons. Our experiment showed that competition and physical stresses, but not facilitation, controls the zonation of emergent macrophytes along a flooding duration gradient, at least in freshwater wetlands. Models guiding wetland restoration need to include competition as well as flood duration as causal factors, but not facilitation.

Voltage-Dependent Anion-Selective Channels and Other Mitochondrial Membrane Proteins Form Diverse Complexes in Beetroots Subjected to Flood-Induced Programmed Cell Death

In plants, programmed cell death (PCD) is involved in both the development and the response to biotic and abiotic aggressions. In early stages of PCD, mitochondrial membranes are made permeable by the formation of permeability transition pores, whose protein composition is debated. Cytochrome c (cyt c) is then released from mitochondria, inducing the degradation of chromatin characteristic of PCD. Since flooding stress can produce PCD in several plant species, the first goal of this study was to know if flooding stress could be used to induce PCD in Beta vulgaris roots. To do this, 2-month-old beet plants were flood-stressed from 1 to 5 days, and the alterations indicating PCD in stressed beetroot cells were observed with a confocal fluorescence microscope. As expected, nuclei were deformed, and chromatin was condensed and fragmented in flooded beetroots. In addition, cyt c was released from mitochondria. After assessing that flood stress induced PCD in beetroots, the composition of mitochondrial protein complexes was observed in control and flood-stressed beetroots. Protein complexes from isolated mitochondria were separated by native gel electrophoresis, and their proteins were identified by mass spectrometry. The spectra count of three isoforms of voltage-dependent anion-selective channels (VDACs) increased after 1 day of flooding. In addition, the size of the complexes formed by VDAC was higher in flood-stressed beetroots for 1 day (∼200 kDa) compared with non-stressed ones (∼100 kDa). Other proteins, such as chaperonin CPN60-2, also formed complexes with different masses in control and flood-stressed beetroots. Finally, possible interactions of VDAC with other proteins were found performing a cluster analysis. These results indicate that mitochondrial protein complexes formed by VDAC could be involved in the process of PCD in flood-stressed beetroots. Data are available via ProteomeXchange with identifier PXD027781.