Integrated transcriptomic and metabolomic analyses of a wax deficient citrus mutant exhibiting jasmonic acid-mediated defense against fungal pathogens

Abstract

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.

Lipidomic and transcriptomic analysis reveals reallocation of carbon flux from cuticular wax into plastid membrane lipids in a glossy “Newhall” navel orange mutant

“…Reallocation of carbon flux from cuticular wax into plastid membrane lipids in MT Previously, He et al 4 reported that a decreased wax content at the mature stage results in the glossy surface of the mutant. Our research further demonstrates that wax biosynthesis in MT had already been inhibited at the fruit expansion stage, and the inhibition reached the highest level in the mature stage.…”

“…The total wax was extracted according to a previously reported method 4,23 . Intact fruits were immersed in chloroform for 2 min, twice.…”

“…Measurement of JA phytohormones in "Newhall" navel orange and N. benthamiana Measurement of JA was performed according to a previously reported method (He et al 4 ). One hundred milligrams of each samples was ground and mixed with 800 μl of solvent containing methanol/H 2 O/acetic acid (80:19:1, v/v/v) and internal standards for JA determination (DHJA; Olomouc, Czech Republic).…”

“…Therefore, the cultivation of fruits with desirable colors, high glossiness, fine internal quality, and strong antibacterial properties is of considerable importance. As the first external barrier of plants that come into contact with the environment, epicuticular wax has been reported to play important roles in water retention, gas exchange 1,2 , and surface glossiness of citrus fruits 3,4 .…”

“…To date, most molecular studies related to lipids in citrus have only focused on the cuticle 3,4,22 , without highlighting the importance of membrane lipids. Based on the previously reported glossy mutant of the "Newhall" navel orange 4 , we performed lipidomic and transcriptomic analysis on both immature and mature stages of this glossy mutant (MT) and the wild-type "Newhall" navel orange (WT) to ascertain the carbon exchanges between wax and plastid lipids. We also attempted to elucidate the possible reasons underlying the enhanced JA levels in MT by the functional characterization of three upregulated lipase genes.…”