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Summary: Fruit rind plays a pivotal role in alleviating water loss and disease and particularly in cracking resistance as well as the transportability, storability and shelf‐life quality of the fruit. High susceptibility to cracking due to low rind hardness is largely responsible for severe annual yield losses of fresh fruits such as watermelon in the field and during the postharvest process. However, the candidate gene controlling the rind hardness phenotype remains unclear to date. Herein, we report, for the first time, an ethylene‐responsive transcription factor 4 (ClERF4) associated with variation in rind hardness via a combinatory genetic map with bulk segregant analysis (BSA). Strikingly, our fine‐mapping approach revealed an InDel of 11 bp and a neighbouring SNP in the ClERF4 gene on chromosome 10, conferring cracking resistance in F2 populations with variable rind hardness. Furthermore, the concomitant kompetitive/competitive allele‐specific PCR (KASP) genotyping data sets of 104 germplasm accessions strongly supported candidate ClERF4 as a causative gene associated with fruit rind hardness variability. In conclusion, our results provide new insight into the underlying mechanism controlling rind hardness, a desirable trait in fresh fruit. Moreover, the findings will further enable the molecular improvement of fruit cracking resistance in watermelon via precisely targeting the causative gene relevant to rind hardness, ClERF4. [ABSTRACT FROM AUTHOR]

Although boron deficiency (BD) is a drastic disorder to the agriculture crops, its effects on watermelon still unknown. Therefore, the present study investigates the effect of BD (0 and 0.5 mg L−1) on different morphological, physiological and biochemical traits, and mineral uptake during early seedlings stage of watermelon. B-deficiency induced leaf chlorosis in watermelon initiated from the leaf margins and tips. Despite that, BD increased shoot length, and root and shoot dry weight of BD-watermelon seedlings. BD decreased leaf chlorophyll and carotenoid contents, and photosynthetic parameters without affecting quantum yield of PSII. BD significantly inhibited total soluble protein accumulation, while leaf proline content was unaffected. A significant increase in antioxidant enzyme activities in response to higher hydrogen peroxide (H2O2) generation could possibly reduced lipid peroxidation and promoted BD-watermelon plant growth. BD influenced mineral uptake variously: the induction of phosphorus (P) and zinc (Zn), and the preservation of potassium (K), calcium (Ca) and molybdenum (Mo) levels in the leaves could maintain watermelon growth; while, the inhibition of magnesium (Mg) uptake seemed to be responsible for suppressed leaf chlorophyll and photosynthesis. These results would further help in understanding the physiological pathways and mechanisms associated with BD especially in watermelon, and provide database for fertilizer recommendations in B-deficient areas. [ABSTRACT FROM AUTHOR]

Watermelon (Citrullus lanatus) is a globally important Cucurbitaceae crop in which grafting is commonly used to improve stress tolerance and enhance nutrient utilization. However, the mechanism underlying grafting-enhanced nutrient assimilation remains unclear. Here, we demonstrate the possible involvement of a novel Cucurbitaceae miRNA, ClmiR86, in grafting-enhanced phosphate-starvation tolerance via CALCINEURIN B-LIKE INTERACTING PROTEIN KINASE 5 (ClCIPK5) suppression in watermelon. Transcript analyses revealed that the induction of ClmiR86 expression was correlated with the downregulation of ClCIPK5 in squash-grafted watermelon under phosphate starvation. In addition, the differential expression of ClmiR86 in various watermelon genotypes was consistent with their phosphate utilization efficiency. Furthermore, ClmiR86 overexpression in Arabidopsis enhanced root growth and phosphate uptake under phosphate starvation and promoted inflorescence elongation under normal conditions. These results suggest that the ClmiR86–ClCIPK5 axis is involved in phosphate starvation response as well as grafting-enhanced growth vigor and phosphate assimilation. The present study provides valuable insights for investigating long-distance signaling and nutrient utilization in plants. [ABSTRACT FROM AUTHOR]

Strigolactone (SL) plays essential roles in regulating plant growth, development, and stress response. This study was conducted to evaluate the effect of exogenous SL on watermelon resistance against herbicides penoxsulam (PXL) and bensulfuron-methyl (BSM). These herbicides were found to negatively impact watermelon root growth and photosynthetic pigments, probably due to the ultrastructural damage and cell death in leaf and root tissues under herbicide stresses. The activation of SL-related gene expression suggested that the SL pathway may mitigate herbicide toxicity in watermelon. The exogenous SL dose-dependently reversed the PXL- or BSM-induced antioxidant activity, suggesting that SL may participate in maintaining antioxidant enzyme homeostasis under herbicide stresses. The up-regulation of herbicide metabolization and detoxification-related genes (cytochrome P450 and acetolactate synthase) by exogenous SL also in part explained how this phytohormone alleviates herbicide toxicity in watermelon. Our findings will provide valuable information underlying the regulatory effects of SL on herbicide tolerance in Cucurbitaceae crops. [ABSTRACT FROM AUTHOR]

Although Al3+-toxicity is one of the limiting factors for crop production in acidic soils, little is known about the Al3+-tolerance mechanism in watermelon, a fairly acid-tolerant crop. This work aimed to identify the interaction between the H 2 O 2 scavenging pathway and auxin glycosylation relevant to watermelon Al3+-tolerance. By analyzing expressions of hormone-related ClUGTs and antioxidant enzyme genes in Al3+-tolerant (ZJ) and Al3+-sensitive (NBT) cultivars, we identified ClUGT75s (B1, B2, and D1) and ClSOD1-2-ClCAT as crucial components associated with Al3+-tolerance. Al3+-stress significantly increased H 2 O 2 content by 92.7% in NBT and 42.3% in ZJ, accompanied by less Al3+-, auxin (IAA and IBA), and MDA contents in ZJ than NBT. These findings coincided with significant ClSOD1-2 expression and stable dismutation activity in NBT than ZJ. Hence, higher H 2 O 2 content in the root apex of NBT than ZJ correlated with a significant increase in auxin content and ClSOD1-2 up-regulation. Moreover, Al3+-activated ClUGT75D1 and ClUGT75B2 in ZJ coincided with no considerable change in IBA content, suggesting that glycosylation-mediated changes in IBA content might be relevant to Al3+-tolerance in watermelon. Furthermore, exogenous H 2 O 2 and IBA indicated ClUGT75D1 modulating IBA is likely dependent on H 2 O 2 background. We hypothesize that a higher H 2 O 2 level in NBT represses ClUGT75 , resulting in increased auxin than those in ZJ roots. Thus, excess in both H 2 O 2 and auxin aggravated the inhibition of root elongation under Al3+-stress. Our findings provide insights on the permissive action of H 2 O 2 in the mediation of auxin glycosylation by ClUGT75 in root apex for Al3+-tolerance in watermelon. • Accumulation of H 2 O 2 and auxin upon Al3+-stress inhibits primary root elongation. • ClSOD1-2-ClCATs pathways contribute to less H 2 O 2 in Al3+-tolerant watermelon. • IBA glycosylation by ClUGT75 contributes to Al3+-tolerance in watermelon. • A balanced level of H 2 O 2 is necessary for activated expression of ClUGT75 in root tip. • H 2 O 2 signaling affect auxin glycosylation via ClUGT75 under Al3+-stress. [ABSTRACT FROM AUTHOR]

Crop plants, such as watermelon, suffer from severe Aluminum (Al3+)-toxicity in acidic soils with their primary root elongation being first arrested. However, the significance of apoplastic or symplastic Al3+-toxicity in watermelon root is scarcely reported. In this work, we identified a medium fruit type (ZJ) and a small fruit type (NBT) as Al+3-tolerant and sensitive based on their differential primary root elongation rate respectively, and used them to show the effects of symplastic besides apoplastic Al distribution in the watermelon's root. Although the Al content was higher in the root of NBT than ZJ, Al+3 allocated in their apoplast, vacuole and plastid fractions were not significantly different between the two cultivars. Thus, only a few proportion of Al+3 differentially distributed in the nucleus and mitochondria corresponded to interesting differential morphological and physiological disorders recorded in the root under Al+3-stress. The symplastic amount of Al+3 substantially induced the energy efficient catalase pathway in ZJ, and the energy consuming ascorbate peroxidase pathway in NBT. These findings coincided with obvious starch granule visibility in the root ultra-structure of ZJ than NBT , suggesting a differential energy was used in supporting the root elongation and nutrient uptake for Al+3-tolerance in the two cultivars. This work provides clues that could be further investigated in the identification of genetic components and molecular mechanisms associated with Al+3-tolerance in watermelon. • Al+3 tolerant and sensitive watermelons were identified by primary root elongation. • The Al content was higher in the root of Al+3 sensitive than tolerant. • Different intracellular localization of Al was found in two watermelon cultivars. • Al+3 tolerant employed energy efficient catalase pathway under Al+3 stress. • Al+3 sensitive used energy consuming ascorbate peroxidase pathway under Al+3 stress. [ABSTRACT FROM AUTHOR]

Highlights • Irradiated and normal pollen induced similar hormone-response in watermelon fruit-set. • Quick response of auxin signaling and downstream network is crucial for fruit-set. • GAs is insufficient to induce parthenocarpy but function in fruit-growth stage. • Cytokinin could induce parthenocarpic fruit-set and subsequent growth in watermelon. Abstract Parthenocarpy, the natural or artificial induced fruit production without fertilization of ovules, confers various agricultural benefits, such as stable crop production in fluctuating environments, as well as increased fruit quality and shelf-life of some fleshy fruits. Whereas, phytohormones are known as important mediators of the fertilization-independent fruiting. Here, diploid parthenocarpic watermelon was produced by pollination with partially functional pollen, which was irradiated with soft-X-rays. Ovule transcriptome analyses demonstrated both irradiated and normal pollen induced complex hormone-regulation during fruit-set, and quick response of auxin signaling and downstream network may be crucial for fruit-set. Dynamic investigation of phytohormones in fertilized ovaries suggested soft-X-irradiated pollen induced the accumulation of gibberellins (GAs), cytokinins (CKs) and auxin as normal pollen did. Exogenous application of hormones or inhibitors suggested auxin may specifically act in fruit-set, whereas GAs is insufficient to induce parthenocarpy but function in fruit-growth stage. Single treatment of 1-(2-chloro-4-pyridyl)-3-phenylurea (CPPU, an active CK) could induce parthenocarpic fruit-set and subsequent growth, which might partly dependent on induced auxin accumulation in fruits. Our results will contribute to the potential breeding of vegetative (autonomous) parthenocarpic watermelon cultivar via genetic engineering, as well as provide useful insights into the mechanisms underlying parthenocarpy in cucurbit crops. [ABSTRACT FROM AUTHOR]

• Only one AOX gene (ClAOX) present in the watermelon genome. • ClAOX belongs to AOX2 sub-family but with a similar gene structure of AtAOX1a. • One SNP in ClAOX leads to the change from N to K in the 96th amino acids. • The naturally existed SNP in ClAOX differing the plant tolerance to cold stress. Alternative oxidase (AOX) is a mitochondrial enzyme encoded by a small nuclear gene family, which contains the two subfamilies, AOX1 and AOX2. In the present study on watermelon (Citrullus lanatus), only one ClAOX gene, belonging to AOX2 subfamily but having a similar gene structure to AtAOX1a , was found in the watermelon genome. The expression analysis suggested that ClAOX had the constitutive expression feature of AOX2 subfamily, but was cold inducible, which is normally considered an AOX1 subfamily feature. Moreover, one single nucleotide polymorphism (SNP) in ClAOX sequence, which led to the change from Lys (N) to Asn (K) in the 96th amino acids, was found among watermelon subspecies. Ectopic expression of two ClAOX alleles in the Arabidopsis aox1a knock-out mutant indicated that ClAOX K-expressing plants had stronger cold tolerance than aox1a mutant and ClAOX N-expressing plants. Our findings suggested watermelon genome contained a single ClAOX that possessed the expression features of both AOX1 and AOX2 subfamilies. A naturally existing SNP in ClAOX differentiated the cold tolerance of transgenic Arabidopsis plants, impling a possibility this gene might be a functional marker for stress-tolerance breeding. [ABSTRACT FROM AUTHOR]