Advances in study of the interaction between apple and Colletotrichum gloeosporioides

Abstract: Colletotrichum gloeosporioides can cause apple bitter rot, and anthracnose leaf blight, resulting in affecting fruit yield and quality, and causing serious economic losses to the apple industry. According to the latest fungal classification system, the C. gloeosporioides species complex consists of 13 different species, including C. gloeosporioides, C. aenigma and C. fructicola et al. Among them, C. fructicola and C. gloeosporioides are important pathogenic fungi on various fruit trees. Meanwhile, C. gloeosporioides can also cause diseases on other fruit trees such as cherry, passion fruit, and kiwifruit. In order to better prevent and control diseases, we need to have a comprehensive understanding of the classification, pathogenic mechanisms, and host interaction mechanisms of the pathogens on apples. In the process of colonizing host tissue, a number of C. gloeosporioides genes participate in different phases of infection procedures, which include conidiation, appressorium morphogenesis, melanization and penetration, biotrophy, necrotrophy, and various transport activities. In recent years, research on the pathogenic molecular mechanism of C. gloeosporioides on apples has mainly focused on the cloning and analysis of pathogenic related genes, screening and identification of effector proteins, pathogenic enzymes, and colletotoxins of C. gloeosporioides. Fungi secrete enzymes such as pectin, keratin and cellulase could help them successfully infect their hosts. New studies have shown that the adapter protein gene GcAP1 can regulate the expression of endopolygalacturonase genes (CgPG1 and CgPG2), pectin lyase genes (pnl-1, pnl-2), and pectate lyase genes (pelA, pelB), and GcAP1 is an important virulencefactor of C. gloeosporioides. Currently, the successful application of PEG mediated genetic transformation and Agrobacterium mediated transformation in the study of C. gloeosporioides provides a basis for the development of pathogenic molecular mechanisms. It has been confirmed that the genes with different functions such as CgABCF2, CgCMK1, CgSET5, CgOpt1, CgNVF1, CgABCF2, CgChip6 are present in C. gloeosporioides, playing an important role in infecting apples. In addition, C2H2 transcription factors, cation stress response transcription factors CgSltA, CgCrzA, and CsHtf1 also play important roles in pathogen pathogenesis. During the infection process, C. gloeosporioides can also secrete a series of effectors to inhibit the host immune response, thereby promoting pathogen infection and colonization. Currently, scientists have analyzed the roles of effectors such as CfE12, CfEC92, and Sntf2 in C. gloeosporioides, laying the foundation for subsequent research on interactions of pathogen and host. In addition, C. gloeosporioides secrete toxins during the necrotrophic stage, causing necrosis of the host tissue. The research on apple disease resistance started relatively late, mainly focusing on germplasm resource identification, physiological and biochemical testing, disease resistance gene mining, plant hormone mediated disease resistance response, disease related transcription factors, and other mechanisms of action. Research has shown that after inoculation with anthrax fungus, the activities of superoxide dismutase (SOD), polyphenol oxidase (PPO), peroxidase (POD), catalase (CAT), and serotonin N-acetyltransferase (SNAT) in apple leaves increased, indicating that these enzymes are involved in the infection process of C. gloeosporioides. Plant hormones play an important role in plant defense and growth and development, and hormones related to plant immune responses include salicylic acid (SA), jasmonic acid (JA), ethylene (ET), abscisic acid (ABA), and so on. Research has shown that there are significant differences in the expression levels of SA synthesis related genes MdEDS1, MdPAD4, MdPAL and SA signal transduction related genes MdNPR1, MdPR1 and MdPR5 between resistant and susceptible varieties. There are differences in the resistance and susceptibility of different apple varieties to C. gloeosporioides. The Hanfu variety has been used to screen for resistance genes due to its high resistance to C. gloeosporioides. WRKY and NAC transcription factors play a crucial role in plant resistance to pathogen infection. In apples, transcription factors MdWRKY15, MdWRKY17, and MdWRKY100 enhance apple resistance to anthracnose by regulating SA accumulation. Here, we plotted the downstream regulatory patterns of AtwrKY33 and MdWRKYs involved in the MAPK cascade reaction, and presented some research results on MdWRKYs. At the end of the article, we summarized the research results on the regulatory mechanism of miRNA involvement in plant immunity. Clarifying the pathogenic process and molecular mechanism of the pathogen is of great significance for the comprehensive prevention and control of C. gloeosporioides. With the deepening of various studies, researchers will inevitably change their thinking on the prevention and control of C. gloeosporioides. Traditional chemical prevention and control methods, such as the extensive use of fungicides and insecticides, can achieve the effect of combating pathogens, but they also can cause serious harm to the environment and people. Breeding of resistant varieties is a fundamental means to solve the problems in preventing and controlling C. gloeosporioides. This article aimed to analyze the pathogenic mechanism of pathogens and their interaction with hosts, laying a theoretical foundation for screening potential candidate genes and breeding new varieties resistant to diseases.