Abstract
Cedar-apple rust (Gymnosporangium yamadai Miyabe) is a fungal disease that causes substantial injury to apple trees and results in fruit with reduced size and quality and a lower commercial value. The molecular mechanisms underlying the primary and secondary metabolic effects of rust spots on the leaves of Malus apple cultivars are poorly understood. Using HPLC, we found that the contents of flavonoid compounds, especially anthocyanin and catechin, were significantly increased in rust-infected symptomatic tissue (RIT). The expression levels of structural genes and MYB transcription factors related to flavonoid biosynthesis were one- to seven-fold higher in the RIT. Among these genes, CHS, DFR, ANS, FLS and MYB10 showed more than a 10-fold increase, suggesting that these genes were expressed at significantly higher levels in the RIT. Hormone concentration assays showed that the levels of abscisic acid (ABA), ethylene (ETH), jasmonate (JA) and salicylic acid (SA) were higher in the RIT and were consistent with the expression levels of McNCED, McACS, McLOX and McNPR1, respectively. Our study explored the complicated crosstalk of the signal transduction pathways of ABA, ETH, JA and SA; the primary metabolism of glucose, sucrose, fructose and sorbitol; and the secondary metabolism of flavonoids involved in the rust resistance of Malus crabapple leaves.
Highlights
Gymnosporangium yamadai Miyabe, which is known as Cedar-apple rust, causes serious diseases and significant economic losses to apple cultivars (Lee et al, 2016)
We found that infected leaf tissues of different cultivars in an orchard displayed different color phenotypes during a rust disease outbreak in 2008–2010, with certain cultivars exhibiting red spots and the others showing yellow spots on the leaf surface
Rust spot areas on the infected leaves were obviously different among the pre-observed cultivars; we selected two typical cultivars, one with red spots and the other with yellow spots, to investigate the mechanisms underlying the different coloring of the rust spots and the differences between cultivars
Summary
Gymnosporangium yamadai Miyabe, which is known as Cedar-apple rust, causes serious diseases and significant economic losses to apple cultivars (Lee et al, 2016). Flavonols and proanthocyanidins have been suggested to play a role in suppressing the growth of E. coli bacteria and greatly reducing the ability of these bacteria to initiate an infection (Ge et al, 2013; Nguyen et al, 2016; Shang et al, 2017). In tissues, these substances may act as antioxidants and significantly contribute to the scavenging of free radicals produced via metabolic processes in plants, whereas at the organ surface, visible coloring phenotypes may be displayed because of the accumulation of flavonoids during exposure to pathogen infection or environmental stress. Visible coloring caused by flavonoids could represent a potential indicator of stress in plants for use in early diagnosis and prevention strategies; the coloring pattern of different species of plants is diverse and the underlying mechanisms are complex (Mierziak et al, 2014; Peixoto et al, 2016)
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