Application of boron reduces vanadium toxicity by altering the subcellular distribution of vanadium, enhancing boron uptake and enhancing the antioxidant defense system of watermelon.

Vanadium (V) is the fifth most abundant transition metal, elevated levels of V are hazardous to plants. Boron (B) is an essential micronutrient for plants and can mitigate heavy metal toxicity. However, the mechanism used by B to promote tolerance to vanadium is unknown. In this study, a combination of physiological and gene expression analysis was used to explain mechanism of B (75 µM) induced V (40 mg L-1) stress tolerance in watermelon. V stress severely reduced root and shoot growth and increased the accumulation of ROS. B application improved tolerance to V by enhancing the expression of B transporter genes (ClaNIP5;1-1, ClaNIP5;1-2, ClaBOR4) that facilitated B uptake and transport while restricting V transport in plant tissues. At cellular level, the higher V retention in leaves was achieved by cell wall chelation, whereas, the higher V exclusion in vacuole of root cell was driven by elevated vacuolar H+-ATPase, H+-PPase activities, and transcript level of ClaVHP1;1, ClaPDR12-1 and ClaPDR12-2 genes facilitated by B application. Moreover, B application reduced tissue ROS cascade by enhancing antioxidant enzymatic activity and expression of superoxide dismutase (ClaCSD1-1, ClaCSD1-2, ClaCSD3, ClaMSD1) and catalase (ClaCAT2-1, ClaCAT2-2) genes that enhanced the defense mechanism of the V treated plants, improved root and shoot growth and tolerance index of watermelon. In conclusion, we demonstrate that ameliorative effect of B in tolerance to V of watermelon was based on B homeostasis and improved antioxidant defense system. These findings might help to increase watermelon production in V polluted soils.

Genome-wide expression profiling of leaves and roots of watermelon in response to low nitrogen.

BACKGROUND: Nitrogen (N) is a key macronutrient required for plant growth and development. In this study, watermelon plants were grown under hydroponic conditions at 0.2 mM N, 4.5 mM N, and 9 mM N for 14 days. RESULTS: Dry weight and photosynthetic assimilation at low N (0.2 mM) was reduced by 29 and 74% compared with high N (9 mM). The photochemical activity (Fv/Fm) was also reduced from 0.78 at high N to 0.71 at low N. The N concentration in the leaf, stem, and root of watermelon under low N conditions was reduced by 68, 104, and 108%, respectively compared with 9 mM N treatment after 14 days of N treatment. In the leaf tissues of watermelon grown under low N conditions, 9598 genes were differentially expressed, out of which 4533 genes (47.22%) were up-regulated whereas, 5065 genes (52.78%) were down-regulated compared with high N. Similarly in the root tissues, 3956 genes were differentially expressed, out of which 1605 genes were up-regulated (40.57%) and 2351 genes were down-regulated (59.43%), compared with high N. Our results suggest that leaf tissues are more sensitive to N deficiency compared with root tissues. The gene ontology (GO) analysis showed that the availability of N significantly affected 19 biological processes, 8 cell component metabolic pathways, and 3 molecular functions in the leaves; and 13 biological processes, 12 molecular functions, and 5 cell component metabolic pathways in the roots of watermelon. The low affinity nitrate transporters, high affinity nitrate transporters, ammonium transporters, genes related with nitrogen assimilation, and chlorophyll and photosynthesis were expressed differentially in response to low N. Three nitrate transporters (Cla010066, Cla009721, Cla012765) substantially responded to low nitrate supply in the root and leaf tissues. Additionally, a large number of transcription factors (1365) were involved in adaptation to low N availability. The major transcription factor families identified in this study includes MYB, AP2-EREBP, bHLH, C2H2 and NAC. CONCLUSION: Candidate genes identified in this study for nitrate uptake and transport can be targeted and utilized for further studies in watermelon breeding and improvement programs to improve N uptake and utilization efficiency.

Boron: Functions and Approaches to Enhance Its Availability in Plants for Sustainable Agriculture.

Boron (B) is an essential trace element required for the physiological functioning of higher plants. B deficiency is considered as a nutritional disorder that adversely affects the metabolism and growth of plants. B is involved in the structural and functional integrity of the cell wall and membranes, ion fluxes (H⁺, K⁺, PO43−, Rb⁺, Ca2+) across the membranes, cell division and elongation, nitrogen and carbohydrate metabolism, sugar transport, cytoskeletal proteins, and plasmalemma-bound enzymes, nucleic acid, indoleacetic acid, polyamines, ascorbic acid, and phenol metabolism and transport. This review critically examines the functions of B in plants, deficiency symptoms, and the mechanism of B uptake and transport under limited B conditions. B deficiency can be mitigated by inorganic fertilizer supplementation, but the deleterious impact of frequent fertilizer application disrupts soil fertility and creates environmental pollution. Considering this, we have summarized the available information regarding alternative approaches, such as root structural modification, grafting, application of biostimulators (mycorrhizal fungi (MF) and rhizobacteria), and nanotechnology, that can be effectively utilized for B acquisition, leading to resource conservation. Additionally, we have discussed several new aspects, such as the combination of grafting or MF with nanotechnology, combined inoculation of arbuscular MF and rhizobacteria, melatonin application, and the use of natural and synthetic chelators, that possibly play a role in B uptake and translocation under B stress conditions.

Comparative analysis of pumpkin rootstocks mediated impact on melon sensory fruit quality through integration of non-targeted metabolomics and sensory evaluation.

Melon fruits are popular because of sweet taste and pleasant aroma. Grafting has been extensively used for melons to alleviate abiotic stresses and control soil borne diseases. However, use of grafting for vegetable fruit quality improvement is less studies. In modern age fruit quality particularly sensory quality characteristics have key importance from consumer eye lens. We performed liquid chromatography-mass spectrometry and metabonomic analysis to examine sensory fruit quality of melon grafted onto ten different pumpkin rootstocks. Bases on the result of our study, 478 metabolites were detected and 184 metabolites consisting of lipids, amino acids and organic oxygen compounds were differentially expressed in grafted melon fruits. The results from metabolomic, physiochemical and sensory analysis explain the differences in melon fruit flavor from two contrasting rootstocks. In conclusion the fruits from Tianzhen No. 1 rootstock exhibited better organoleptic characteristics and higher soluble sugars content [glucose (19.87 mg/g), fructose (19.68 mg/g) and sucrose (169.45 mg/g)] compared with other rootstocks used in this study. Moreover, the contents of bitterness causing amino acids such as L-arginine, L-asparagine, Histidinyl-histidine and Acetyl-DL-valine were found lower in Tianzhen No. 1-grafted melon fruits compared with Sizhuang No. 12-grafted melon fruits. These fruit quality characteristics made Tianzhen No. 1 rootstock suitable for commercial cultivation of Yuniang melon.

High relative humidity improve chilling tolerance by maintaining leaf water potential in watermelon seedlings.

Low temperature is a major environmental factor that severely impairs plant growth and productivity. Although the response to low temperature stress is well studied, the mechanisms of chilling tolerance are still not well understood. Here, we describe experiments that aimed to determine whether relative humidity (RH) contribute to chilling tolerance by regulating leaf water potential in watermelon seedlings. Plants exposed to chilling stress (10 °C/5 °C day/night) were severely wilted, and the water potential in their leaves was decreased. We found that maintaining high RH when plants were subjected to chilling-stress conditions attenuated the reduction in leaf water potential, reduced electrolyte leakage, improved the stability of photosynthesis, and alleviated chilling damage. Pretreatment with ABA increased chilling tolerance in low RH conditions but became ineffective in high RH conditions. Analysis of endogenous ABA content indicated that water potential mediated chilling tolerance was independent of ABA. Analysis of stomatal resistance indicated that the maintenance of water potential was related to stomatal resistance but that the balance between water absorption and loss is more important. In conclusion, high RH maintained leaf water potential and cell turgor, maintained better cell morphology, improved stomatal conductance and thus, ultimately improved the chilling tolerance of watermelon seedlings.

Pumpkin rootstock improves the growth and development of watermelon by enhancing uptake and transport of boron and regulating the gene expression.

Boron (B) is an essential trace element that plays a vital role in metabolic and physiological functions of higher plants. The adequate supply of B is important for plant growth and development. Grafting is a technique used to improve the ion uptake and plant growth. In this study, a commercial watermelon cultivar "Zaojia 8424" [Citrullus lanatus (Thunb.) Matsum. and Nakai.] was grafted onto pumpkin (Cucurbita maxima × Cucurbita moschata) rootstock cv. "Qingyan Zhenmu No.1" with an aim to investigate the response of grafted plants to different levels of B supply (0.25 µM, 25 µM and 75 µM B) in the nutrient solution. Self-grafted watermelon plants were used as control. Pumpkin rootstock improved the plant growth, chlorophyll and carotenoid contents, photosynthetic assimilation, stomatal conductance, transpiration rate, B accumulation and up-regulated the expression of NIP5;1, NIP6;1 and B transporter (BOR2, BOR4) genes in the roots and leaves at 25 µM B compared with self-grafted watermelon plants. Moreover, pumpkin rootstock reduced the oxidative stress and cell damage by reducing H2O2 and MDA contents, and down-regulating the expression of PDCD2-1, PDCD2-2 genes. Moreover, it enhanced the antioxidant activity of watermelon by up-regulating the expression of SOD1, SOD2, CAT2-1, and CAT2-2 genes. Based on these observations, we concluded that pumpkin rootstock has ability to improve the plant growth of watermelon by enhancing the B uptake. This study may help adjust the B concentration in the nutrient medium for watermelon production where pumpkin grafted plants are utilized.

Melatonin pretreatment improves vanadium stress tolerance of watermelon seedlings by reducing vanadium concentration in the leaves and regulating melatonin biosynthesis and antioxidant-related gene expression.

Vanadium (V) is an important heavy metal with ubiquitous presence in the Earth's crust, but limited information is available as to its effect on plants and management strategies. Melatonin is a widely studied biomolecule; it acts as an antioxidant and a signaling molecule that enhances the abiotic stress tolerance of plants. Melatonin improves copper, zinc, and cadmium tolerance in plants. In this study, we investigated the response of watermelon seedlings to V stress and the potential role of melatonin in enhancing V stress tolerance of watermelon seedlings. The results showed that seedlings pretreated with melatonin (0.1µM) exposed to V (50mg/L) had a higher relative chlorophyll content (SPAD index), photosynthetic assimilation, and plant growth compared with non-melatonin pretreated seedlings. Melatonin pretreatment lowered leaf and stem V concentrations by reducing V transport from root to shoot. Melatonin pretreatment enhanced superoxide dismutase (SOD) and catalase (CAT) activities, and reduced the hydrogen peroxide (H2O2) and malondialdehyde (MDA) content of watermelon seedlings, by regulating melatonin biosynthesis and gene expression for superoxide dismutase, peroxidase, ascorbate peroxidase, glutathione peroxidase, and glutathione S-transferase. So far as we know, these results are the first evidence that melatonin improves plant growth of watermelon seedlings under vanadium stress conditions. Considering these observations, melatonin can be utilized to reduce the availability of V to plants, and improve plant growth and V stress tolerance.