Abstract
BackgroundMandarin ‘Shatangju’ is susceptible to Huanglongbing (HLB) and the HLB-infected fruits are small, off-flavor, and stay-green at the maturity period. To understand the relationship between pericarp color and HLB pathogen and the effect mechanism of HLB on fruit pericarp coloration, quantitative analyses of HLB bacterial pathogens and carotenoids and also the integrative analysis of metabolome and transcriptome profiles were performed in the mandarin ‘Shatangju’ variety with four different color fruits, whole green fruits (WGF), top-yellow and base-green fruits (TYBGF), whole light-yellow fruits (WLYF), and whole dark-yellow fruits (WDYF) that were infected with HLB.Resultsthe HLB bacterial population followed the order WGF > TYBGF > WLYF > WDYF. And there were significant differences between each group of samples. Regarding the accumulation of chlorophyll and carotenoid, the chlorophyll-a content in WGF was the highest and in WDYF was the lowest. The content of chlorophyll-b in WGF was significantly higher than that in other three pericarps. There were significant differences in the total content of carotenoid between each group. WGF and TYBGF pericarps were low in phytoene, γ-carotene, β-cryptoxanthin and apocarotenal, while other kinds of carotenoids were significantly higher than those in WDYF. And WLYF was only short of apocarotenal. We comprehensively compared the transcriptome and metabolite profiles of abnormal (WGF, TYBGF and WLYF) and normal (WDYF, control) pericarps. In total, 2,880, 2,782 and 1,053 differentially expressed genes (DEGs), including 121, 117 and 43 transcription factors were identified in the three comparisons, respectively. The qRT-PCR confirmed the expression levels of genes selected from transcriptome. Additionally, a total of 77 flavonoids and other phenylpropanoid-derived metabolites were identified in the three comparisons. Most (76.65 %) showed markedly lower abundances in the three comparisons. The phenylpropanoid biosynthesis pathway was the major enrichment pathway in the integrative analysis of metabolome and transcriptome profiles.ConclusionsSynthesizing the above analytical results, this study indicated that different color pericarps were associated with the reduced levels of some carotenoids and phenylpropanoids derivatives products and the down-regulation of proteins in flavonoids, phenylpropanoids derivatives biosynthesis pathway and the photosynthesis-antenna proteins.
Highlights
Mandarin ‘Shatangju’ is susceptible to Huanglongbing (HLB) and the HLB-infected fruits are small, offflavor, and stay-green at the maturity period
Synthesizing the above analytical results, this study indicated that different color pericarps were associated with the reduced levels of some carotenoids and phenylpropanoids derivatives products and the downregulation of proteins in flavonoids, phenylpropanoids derivatives biosynthesis pathway and the photosynthesisantenna proteins
All trees were infected by Candidatus Liberibacter asiaticus and fresh fruits were collected from ten HLB-infected trees and mixed together in January 2019. all fruits were classified into four types according to the coloration: whole green fruits (WGF), top-yellow and base-green fruits (TYBGF), whole light-yellow fruits (WLYF), and whole dark yellow fruits (WDYF)
Summary
Mandarin ‘Shatangju’ is susceptible to Huanglongbing (HLB) and the HLB-infected fruits are small, offflavor, and stay-green at the maturity period. ‘Shatangju’) is one of the superior native citrus varieties in Guangdong Province, China. The typical symptoms of HLB-infected fruits are that the fruits are small, lopsided, and off-flavor, and their stylar end remains green at the maturity period, such as in sweet orange and mandarin ‘Shatangju’ [4, 5]. Some fruits of one HLB-infected ‘Shatangju’ tree are similar to the fruits of HLB-uninfected tree in terms of quality and appearance [6]. The influence of HLB infection on citrus fruit yield and quality including internal and external quality have been reported [7]. The influencing mechanism of HLB infection was not well clarified
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