Abscisic acid mediated differential growth responses of root and shoot of Vigna radiata (L.) Wilczek seedlings under water stress

The characterization of plant derived-carbon dots and its responses on chlorophyll a fluorescence kinetics, radical accumulation in guard cells, cellular redox state and antioxidant system in chromium stressed-Lactuca sativa

Based on their chemical structure and catalytic features, carbon dots (CDs) demonstrate great advantages for agricultural systems. The improvements in growth, photosynthesis, nutrient assimilation and resistance are provided by CDs treatments under control or adverse conditions. However, there is no data on how CDs can enhance the tolerance against chromium toxicity on gas exchange, photosynthetic machinery and ROS-based membrane functionality. The present study was conducted to evaluate the impacts of the different concentrations of orange peel derived-carbon dots (50-100-200-500 mg L-1 CD) on growth, chlorophyll fluorescence, phenomenological fluxes between photosystems, photosynthetic performance, ROS accumulation and antioxidant system under chromium stress (Cr, 100 μM chromium (VI) oxide) in Lactuca sativa. CDs removed the Cr-reduced changes in growth (RGR), water content (RWC) and proline (Pro) content. Compared to stress, CD exposures caused an alleviation in carbon assimilation rate, stomatal conductance, transpiration rate, carboxylation efficiency, chlorophyll fluorescence (Fv/Fm) and potential photochemical efficiency (Fv/Fo). Cr toxicity disrupted the energy fluxes (ABS/RC, TRo/RC, ETo/RC and DIo/RC), quantum yields and, efficiency (ΨEo and φRo), dissipation of energy (DIo/RC) and performance index (PIABS and PItotal). An amelioration in these parameters was provided by CD addition to Cr-applied plants. Stressed plants had high activities of superoxide dismutase (SOD), peroxidase (POX) and ascorbate peroxidase (APX), which could not prevent the increase of H2O2 and lipid peroxidation (TBARS content). While all CDs induced SOD and catalase (CAT) in response to stress, POX and enzyme/non-enzymes related to ascorbate-glutathione (AsA-GSH) cycle (APX, monodehydroascorbate reductase (MDHAR) and dehydroascorbate reductase (DHAR), the contents of AsA and, GSH) were activated by 50-100-200 mg L-1 CD. CDs were able to protect the AsA regeneration, GSH/GSSG and GSH redox status. The decreases in H2O2 content might be attributed to the increased activity of glutathione peroxidase (GPX). Therefore, all CD applications minimized the Cr stress-based disturbances (TBARS content) by controlling ROS accumulation, antioxidant system and photosynthetic machinery. In conclusion, CDs have the potential to be used as a biocompatible inducer in removing the adverse effects of Cr stress in lettuce plants.

Understanding role of roots in plant response to drought: Way forward to climate-resilient crops

Drought stress leads to a significant amount of agricultural crop loss. Thus, with changing climatic conditions, it is important to develop resilience measures in agricultural systems against drought stress. Roots play a crucial role in regulating plant development under drought stress. In this review, we have summarized the studies on the role of roots and root-mediated plant responses. We have also discussed the importance of root system architecture (RSA) and the various structural and anatomical changes that it undergoes to increase survival and productivity under drought. Various genes, transcription factors, and quantitative trait loci involved in regulating root growth and development are also discussed. A summarization of various instruments and software that can be used for high-throughput phenotyping in the field is also provided in this review. More comprehensive studies are required to help build a detailed understanding of RSA and associated traits for breeding drought-resilient cultivars.

Comparative Transcriptome Analysis of Tolerant and Sensitive Genotypes of Common Bean (Phaseolus vulgaris L.) in Response to Terminal Drought Stress

We conducted a genome-wide transcriptomic analysis of three drought tolerant and sensitive genotypes of common bean to examine their transcriptional responses to terminal drought stress. We then conducted pairwise comparisons between the root and leaf transcriptomes from the resulting tissue based on combined transcriptomic data from the tolerant and sensitive genotypes. Our transcriptomic data revealed that 491 (6.4%) DEGs (differentially expressed genes) were upregulated in tolerant genotypes, whereas they were downregulated in sensitive genotypes; likewise, 396 (5.1%) DEGs upregulated in sensitive genotypes were downregulated in tolerant genotypes. Several transcription factors, heat shock proteins, and chaperones were identified in the study. Several DEGs in drought DB (data Base) overlapped between genotypes. The GO (gene ontology) terms for biological processes showed upregulation of DEGs in tolerant genotypes for sulfate and drug transmembrane transport when compared to sensitive genotypes. A GO term for cellular components enriched with upregulated DEGs for the apoplast in tolerant genotypes. These results substantiated the temporal pattern of root growth (elongation and initiation of root growth), and ABA-mediated drought response in tolerant genotypes. KEGG (kyoto encyclopedia of genes and genomes) analysis revealed an upregulation of MAPK (mitogen activated protein kinase) signaling pathways and plant hormone signaling pathways in tolerant genotypes. As a result of this study, it will be possible to uncover the molecular mechanisms of drought tolerance in response to terminal drought stress in the field. Further, genome-wide transcriptomic analysis of both tolerant and sensitive genotypes will assist us in identifying potential genes that may contribute to improving drought tolerance in the common bean.

Screening for drought tolerance potential of nine cocoa (Theobroma cacao L.) genotypes from Ghana

This study was conducted with a view to screen nine genotypes of Cacao from The Cocoa Research Institute of Ghana (CRIG) to test their abilities to withstand drought stress conditions using parameters such as leaf relative water content (RWC), proline accumulation in leaves and trichomes. The experimental design consisted of three replicates of the genotypes used and these were laid out in Complete Randomized Block Design (CRBD) to determine the drought tolerance potentials of the nine genotypes of cocoa at the seedling stage. Two water treatments were used which involved withholding water from one day after full saturation with water prior to the first appearance of drought symptoms (FADS) and watering every two days to the completion of the experiment. Results in this research revealed that proline was found to gather in water-stressed seedlings, and the differences in the mean proline amounts in the genotypes was found to be significant. Genotype T63/971 x Sca9 had the most elevated concentration of free proline at FADS (4 μg/g DW) followed by genotype T60 x Pound10 (3.5 μg/g DW) whereas genotype PA150 × 9006 had the smallest amount of accumulated proline in leaves. Genotype PA150 × 6020 had the highest RWC and SMC of 65% and 1.5% respectively at FADS whilst genotype PA7 x 6035 had the lowest RWC of 43%. There was a direct relationship between the amounts of free proline of genotypes T63/971 x SCA9 and T60 x POUND10 and their respective RWC of the leaves. Genotypes T63/971 x SCA9 and PA150 × 9006 had the highest and lowest numbers of trichomes respectively. Inference from this study revealed that T63/971 x SCA9 and T60 x POUND10 genotypes appear to be the most drought-tolerant genotypes in view of their relatively high values of free proline content, leaf RWC, trichomes and lower values of soil water use (SMC).

The apple columnar gene candidate MdCoL and the AP2/ERF factor MdDREB2 positively regulate ABA biosynthesis by activating the expression of MdNCED6/9

MdCoL, which encodes a putative 2OG-Fe(II) oxygenase, is a strong candidate gene for control of the columnar growth phenotype in apple. However, the mechanism by which MdCoL produces the columnar trait is unclear. Here, we show that MdCoL influences abscisic acid (ABA) biosynthesis through its interactions with the MdDREB2 transcription factor. Expression analyses and transgenic tobacco studies have confirmed that MdCoL is likely a candidate for control of the columnar phenotype. Furthermore, the ABA level in columnar apple trees is significantly higher than that in standard apple trees. A protein interaction experiment has showed that MdCoL interacts with MdDREB2. Transient expression and electrophoretic mobility shift assays have demonstrated that MdDREB2 binds directly to the DRE motif in the MdNCED6 and MdNCED9 (MdNCED6/9) gene promoters, thereby activating the transcription of these ABA biosynthesis genes. In addition, a higher ABA content has been detected following co-overexpression of MdCoL-MdDREB2 when compared with the overexpression of MdCoL or MdDREB2 alone. Taken together, our results indicate that an interaction between MdCoL and MdDREB2 promotes the expression of MdNCED6/9 and increases ABA levels, a phenomenon that may underlie the columnar growth phenotype in apple.

Abscisic acid modulates differential physiological and biochemical responses of roots, stems, and leaves in mung bean seedlings to cadmium stress

Experiments were conducted to determine how exogenous abscisic acid (ABA) mediates the tolerance of plants to cadmium (Cd) exposure. Cd stress strongly reduced all the growth parameters of mung bean seedlings. Cd significantly increased ascorbate peroxidase (APX) and catalase (CAT) activities in roots and stems, and peroxidase (POD) activities in roots, stems, and leaves of mung bean seedlings. Cd caused remarkable increases in the levels of leaf chlorophyll and carotenoid, root polyphenols, and malondialdehyde (MDA) and proline in the three organs. However, Cd greatly decreased leaf CAT activity, root and leaf ascorbic acid (AsA) levels, and stem and leaf polyphenol levels. Foliar application of ABA partially alleviated Cd toxicity on the seedlings. ABA could restore most of the changed biochemical parameters caused by Cd, suggesting that ABA played roles in the protection of membrane lipid peroxidation and the modulation of antioxidative defense systems in response to Cd stress. Our results also implied the differential physiological and biochemical responsive patterns of roots, stems, and leaves to Cd and ABA in mung bean seedlings. The great changes in many biochemical parameters in roots suggested that roots were the first to be affected by Cd and play pivotal roles in response to Cd, especially in chelating Cd and reducing Cd absorption.

The NADPH-oxidase LsRbohC1 plays a role in lettuce (Lactuca sativa) seed germination

Reactive oxygen species (ROS) are increasingly recognized as non-enzymatic players in the processes of radicle elongation growth and endosperm weakening during seed germination. NADPH oxidases (EC 1.6.3.1), also known as respiratory burst oxidase homologues (Rbohs), are key enzymes for the production of ROS. We previously reported that ROS are involved in the radicle elongation growth and endosperm weakening during lettuce seed germination. However, the function of the gene(s) encoding Rbohs during lettuce seed germination remains to be elucidated. In this study, one lettuce Rboh gene LsRbohC1 was cloned, and over-expression and RNAi-lines of this gene were generated. It was found that LsRbohC1 was abundantly expressed in germinating seeds, especially in the endosperm cap and hypocotyl. Over-expression/knock-down of this gene observably increased/decreased the production of superoxide radicals in the radicle and endosperm cap, and significantly promoted/delayed the germination process. The results suggest that LsRbohC1 plays a role in lettuce seed germination.

Effects of residual plastic-film mulch on field corn growth and productivity

Plastic-film mulching has played an important role to promote agricultural production in arid areas; however, due to its inefficient recycling capacity, large amounts of residues have been accumulated in soils, causing negative impacts on crop growth and on the environment. To investigate these effects on water use efficiency, a two-years field experiment was carried out, applying different levels of plastic-film residues, from 0 to 600 kg ha^-1. Results show that these residues have a negative impact on root and shoot growth at several growth stages of corn crop, particularly if above 300 kg ha^-1. Root length and weight density decrease with the amount of residues throughout the majority of crop season. Plastic-film residues of about 600 kg ha^-1 are responsible for the decrease of the biomass root to shoot ratio during the tasseling stage. Moreover, during physiological maturity, root and shoot revealed the highest sensitivity, and the least negative effects on the root system. Results also show that crop water consumption has a slight decrease with the plastic-film residues, though there was also a significant decrease of the yield and the water use efficiency reduction. This information allows to state that it is determinant to learn how to deal with the problem, adjusting the irrigation and crop management to avoid yield impacts. It would also be important to find an efficient procedure to mechanically collect the residues in the soil, and to apply new biodegradable film mulching.

Abscisic acid mediated proline biosynthesis and antioxidant ability in roots of two different rice genotypes under hypoxic stress

BACKGROUND

Abscisic acid (ABA) and proline play important roles in rice acclimation to different stress conditions. To study whether cross-talk exists between ABA and proline, their roles in rice acclimation to hypoxia, rice growth, root oxidative damage and endogenous ABA and proline accumulation were investigated in two different rice genotypes ('Nipponbare' (Nip) and 'Upland 502' (U502)).

RESULTS

Compared with U502 seedlings, Nip seedlings were highly tolerant to hypoxic stress, with increased plant biomass and leaf photosynthesis and decreased root oxidative damage. Hypoxia significantly stimulated the accumulation of proline and ABA in the roots of both cultivars, with a higher ABA level observed in Nip than in U502, whereas the proline levels showed no significant difference in the two cultivars. The time course variation showed that the root ABA and proline contents under hypoxia increased 1.5- and 1.2-fold in Nip, and 2.2- and 0.7-fold in U502, respectively, within the 1 d of hypoxic stress, but peak ABA production (1 d) occurred before proline accumulation (5 d) in both cultivars. Treatment with an ABA synthesis inhibitor (norflurazon, Norf) inhibited proline synthesis and simultaneously aggravated hypoxia-induced oxidative damage in the roots of both cultivars, but these effects were reversed by exogenous ABA application. Hypoxia plus Norf treatment also induced an increase in glutamate (the main precursor of proline). This indicates that proline accumulation is regulated by ABA-dependent signals under hypoxic stress. Moreover, genes involved in proline metabolism were differentially expressed between the two genotypes, with expression mediated by ABA under hypoxic stress. In Nip, hypoxia-induced proline accumulation in roots was attributed to the upregulation of OsP5CS2 and downregulation of OsProDH, whereas upregulation of OsP5CS1 combined with downregulation of OsProDH enhanced the proline level in U502.

CONCLUSION

These results suggest that the high tolerance of the Nip cultivar is related to the high ABA level and ABA-mediated antioxidant capacity in roots. ABA acts upstream of proline accumulation by regulating the expression of genes encoding the key enzymes in proline biosynthesis, which also partly improves rice acclimation to hypoxic stress. However, other signaling pathways enhancing tolerance to hypoxia in the Nip cultivar still need to be elucidated.

Possible interaction of ROS, antioxidants and ABA to survive osmotic stress upon acclimation in Vigna radiata L. Wilczek seedlings

Acclimation is a process of adjustment to gradual environmental change that enables plants to survive further stress by triggering some tolerance mechanism possibly involving ABA, ROS and oxidative metabolism. Here we have studied acclimation responses in terms of the performances with regard to physiological (growth and relative water content) and biochemical (chlorophyll, carotenoids, protein, malondialdehyde, sugar content) attributes, ABA production and stomatal sensitivity to exogenous ABA, extracellular ROS production and activation of antioxidant system. Our study reveals that repeated exposure to short-term mild water stress simulated by polyethylene glycol (PEG-6000) induces acclimation in mung bean (Vigna radiata L. Wilczek) seedlings. Acclimation induced tolerance was associated with reduced leaf size and enhanced root growth, accumulation of soluble sugar as osmoprotectant, maintenance of water potential, lessening of membrane damage as indicated by lower MDA content. Acclimated mung bean seedlings have shown greater degree of tolerance through increased production of and enhanced sensitivity to ABA (as reflected by faster stomatal closure), enhanced production of extracellular O₂^.- and H₂O₂ and the elevated activities of antioxidative enzymes to control the oxidative burst. Taken together, the results convey that acclimated seedlings minimize osmotic stress-induced damage through a possible network of ABA, ROS and antioxidants.