Naturally, resistant crop germplasms are important resources for managing the issues of agricultural product safety and environment deterioration. We found a spontaneous mutant of ‘Newhall’ navel orange (Citrus sinensis Osbeck) (MT) with broad-spectrum protections against fungal pathogens in the orchard, postharvest-storage, and artificial inoculation conditions. To understand the defense mechanism of MT fruit, we constructed a genome-scale metabolic network that integrated metabolome and transcriptome datasets. The coordinated transcriptomic and metabolic data were enriched in two sub-networks, showing the decrease in very long chain fatty acid (by 41.53%) and cuticular wax synthesis (by 81.34%), and increase in the synthesis of jasmonic acid (JA) (by 95.23%) and JA-induced metabolites such as 5-dimethylnobietin (by 28.37%) in MT. Furthermore, cytological and biochemical analyses confirmed that the response to fungal infection in MT was independent of wax deficiency and was correlated with the levels of jasmonates, and the expression of plant defensin gene PDF1.2. Results of exogenous application of MeJA and JA inhibitors such as propyl gallate proved that JA-mediated defense contributes to the strong tolerance against pathogens in MT. Our results indicated that jasmonate biosynthesis and signaling are stimulated by the fatty acid redirection of MT, and participate in the tolerance of pathogenic fungi.
Cuticular wax on plant aerial surfaces plays a vital role in the defense against various stresses, and the genes related to wax metabolism have been well documented in several model plants. However, there is very limited research on the key enzymes and transcription factors (TFs) associated with carbon chain distribution and wax biosynthesis in citrus fruit. In this study, an analysis of wax metabolites indicated that even carbon-chain (C24-C28) metabolites are the dominant wax components in citrus fruit, and a 3-ketoacyl-CoA synthase (KCS) family gene (CsKCS20) plays an important role in the carbon chain distribution during wax biosynthesis in a wax-deficient mutant (MT). Expression of CsKCS20 in yeast indicated that CsKCS20 can catalyze the biosynthesis of C22 and C24 very-long-chain fatty acids (VLCFAs). In addition, transcriptome and sequence analysis indicated that the differential expression of CsKCS20 between the wild-type (WT) and MT fruit can be partly attributed to the regulation of CsMYB96, which was further confirmed by yeast one-hybrid (Y1H) assays, electrophoretic mobility shift assays (EMSAs) and dual luciferase assays. The functions of CsMYB96 and CsKCS20 in wax biosynthesis were further validated by heterologous expression in Arabidopsis. In summary, this study elucidates the important roles of CsKCS20 and CsMYB96 in regulating VLCFA elongation and cuticular wax biosynthesis, which provides new directions for the improvement of citrus fruit wax quality in genetic breeding programs.
Quality deterioration and senescence are the main causes of loss of fruit postharvest. Here, the effects of 200 lmol L À1 strigolactone GR24 on the physicochemical characters, organoleptic quality, and antioxidant capacity of postharvest orange fruit were studied. GR24 increased fruit firmness (9.2%), citrus color index (22.34%), titratable acidity (11.7%), total soluble solid (13.13%), and soluble sugar content (8.17%) irrespective of the rates of weight loss (1.22%) and respiration (24.13%), indicating an improvement in organoleptic quality. GR24 also decreased the accumulation of hydrogen peroxide (30.08%) and malondialdehyde (27.59%), but increased the levels of antioxidants (ascorbate, glutathione, and total phenol) and enzyme activities (catalase, ascorbate peroxidase, and glutathione reductase), indicating an enhancement in antioxidant capacity. Moreover, GR24 delayed fruit senescence process by stimulating the boost of the antioxidant system during 0-63 days and delaying the quality deterioration at 77-91 days.
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