Abstract
Colletotrichum fructicola infects pear leaves, resulting in two major symptoms: tiny black spots (TS) followed by severe early defoliation and big necrotic lesions (BnL) without apparent damage depending on the pathotypes. How the same fungal species causes different symptoms remains unclear. To understand the molecular mechanism underlying the resulting diseases and the diverse symptoms, two C. fructicola pathogenetic strains (PAFQ31 and PAFQ32 responsible for TS and BnL symptoms, respectively) were inoculated on Pyrus pyrifolia leaves and subjected to transcriptome sequencing at the quiescent stage (QS) and necrotrophic stage (NS), respectively. In planta, the genes involved in the salicylic acid (SA) signaling pathway were upregulated at the NS caused by the infection of each strain. In contrast, the ethylene (ET), abscisic acid (ABA), and jasmonic acid (JA) signaling pathways were specifically related to the TS symptoms caused by the infection of strain PAFQ31, corresponding to the yellowish and early defoliation symptoms triggered by the strain infection. Correspondingly, SA was accumulated in similar levels in the leaves infected by each strain at NS, but JA was significantly higher in the PAFQ31-infected as measured using high-performance liquid chromatography. Weighted gene co-expression network analysis also reveals specific genes, pathways, phytohormones, and transcription factors (TFs) associated with the PAFQ31-associated early defoliation. Taken together, these data suggest that specific metabolic pathways were regulated in P. pyrifolia in response to the infection of two C. fructicola pathotypes resulting in the diverse symptoms: JA, ET, and ABA accumulated in the PAFQ31-infected leaves, which negatively affected the chlorophyll metabolism and photosynthesis pathways while positively affecting the expression of senescence-associated TFs and genes, resulted in leaf yellowing and defoliation; whereas SA inhibited JA-induced gene expression in the PAFQ32-infected leaves, which led to hypersensitive response-like reaction and BnL symptoms.
Highlights
Pear fruits have rich nutritional value and other beneficial food values and are ranked as the third most important temperate fruit crop after grapes and apples and are widely cultivated in six continents with major production in China, United States, Italy, Argentina, and Spain, containing five commercially cultivated species of Pyrus bretschneideri, P. communis, P. pyrifolia, P. sinkiangensis, and P. ussuriensis
Strain PAFQ32 did not induce small brown necrotic lesions until 40 dpi, and subsequently, the small lesions expanded into big necrotic lesions (BnL) symptoms under appropriate conditions but no defoliation was observed for these diseased leaves (Figure 1A6)
Both intracellular and subcuticular intramural infections were observed in apple leaves during the biotrophic development stage as infected by C. fructicola (Shang et al, 2020), which caused tiny black spots (TS) symptoms accompanied by severe defoliation of apple plants similar to those of the pear plants observed in this study (Zhang et al, 2015; Fu et al, 2019; Shang et al, 2020)
Summary
Pear fruits have rich nutritional value and other beneficial food values and are ranked as the third most important temperate fruit crop after grapes and apples and are widely cultivated in six continents with major production in China, United States, Italy, Argentina, and Spain, containing five commercially cultivated species of Pyrus bretschneideri, P. communis, P. pyrifolia, P. sinkiangensis, and P. ussuriensis Pear anthracnose caused by Colletotrichum spp., is one of the destructive diseases in major pear-cultivation areas of China (Fu et al, 2019) It occurs in pear’s growth and fruit maturation periods and mainly damages leaves and fruits, resulting in great economic losses (Wu et al, 2010; Zhang et al, 2015). Of these species, Colletotrichum fructicola has been characterized as the most predominant species related to the pear anthracnose in China (Fu et al, 2019). Since most Colletotrichum species only cause BrL/BnL symptoms on infected crops (Cannon et al, 2012; Huang et al, 2013; Liu et al, 2015; Munir et al, 2016), it is imperative to understand how C. fructicola induces the TS symptoms and early defoliation besides the BrL/BnL on Pyrus species
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