Introduction
Horticultural crops typically include fruits, vegetables, and ornamental plants. They are not only an excellent source of daily nutrition for people, but also have important economic value. Common fruits and vegetables can provide a variety of nutrients that the human body needs, including fiber, vitamins, organic acids, and mineral elements [1]. Like other plants, horticultural crops are infected by various bacterial, oomycete, and fungal pathogens that cause enormous economic losses and can have great impacts on human society. The Irish potato famine in the 19th century caused by the oomycete Phytophthora infestans is considered to be a historical tragedy [2]. According to nutrient-acquisition strategies, horticultural crop pathogens can be categorized as biotroph, hemibiotroph, or necrotroph [3]. Biotrophic pathogens infect living plant cells and feed on them. In contrast, necrotrophic pathogens acquire nutrients from dead cells after killing plant tissues. Hemibiotrophic pathogens exhibit two forms of nutrition acquisition, the early biotrophic stage and the late necrotrophic stage. Some typical pathogens of horticultural crops include hemibiotrophic bacteria Pseudomonas syringae and Ralstonia solanacearum, necrotrophic bacteria Xanthomonas spp., biotrophic oomycetes Phytophthora spp., biotrophic fungus Cladosporium fulvum, hemibiotrophic fungus Fusarium oxysporum, and necrotrophic fungi Botrytis cinerea, Alternaria alternata, and Penicillium spp. [4,5].
Effectors are unified agents used by bacterial, oomycete and fungal pathogens to facilitate infection. They are broadly defined as biological molecules secreted by pathogens, including proteins, small RNA, and toxic metabolites [6]. Effectors play dual roles during the complicated plant–pathogen interaction and trigger a series of molecular events. In the early stages of infection, plants recognize pathogen-associated molecular patterns (PAMPs) and activate PAMP-triggered immunity (PTI). At the same time, pathogens can secrete effectors to suppress PTI, and plants recognize effectors thereby inducing effector-triggered immunity (ETI) which may lead to hypersensitive response (HR) in the host [7]. The gene-for-gene model suggests that there is a one-to-one correspondence between an avirulence gene (Avr) of the pathogen and a resistance gene (R) of the plant [8]. In this model, effectors are often referred to as Avr proteins that can be recognized by R proteins (immunity receptors), triggering defense responses leading to host resistance [9]. Avr effectors identified from C. fulvum, the causal agent of tomato leaf mold, are a classic example of the gene-for-gene model [10].
Functions of effectors are diverse, for example, preventing recognition of pathogens by plants, suppressing host immune responses, and manipulating host-cell physiology to help colonization. Most effectors are secreted biomacromolecules and have an effect on the apoplastic area or in the cells of host. Different localization is closely associated with specific functions [11]. To date, numerous effectors have been identified from horticultural crop pathogens (Table 1), most of which are proteinaceous effectors. Modes of action of some type-III-secreted effectors (T3SEs) in bacteria, RXLR and CRN (crinkling and necrosis) motif effectors in oomycetes, and LysM (lysin motifs) domain effectors in fungi have been well characterized in various horticultural host plants, especially in tomato and potato [11]. In addition, these findings on effectors and recognition receptors are valuable for the breeding of resistant cultivars [12,13,14,15].
Though a number of reviews have successfully documented the advances in effector biology [3,16,17,18,19], most of them have been mainly focused on pathogens of model plants (such as Arabidopsis or tobacco) or cereal crops (such as rice or wheat). The crucial roles of effectors in interactions between horticultural crops and pathogens have rarely been summarized. This review focuses on proteinaceous and non-proteinaceous effectors in horticultural crop pathogens, with emphasis on the functions and action mechanisms of proteinaceous effectors from bacterial, oomycete, and fungal pathogens, respectively. Moreover, resistance responses in horticultural crops from recognition to fightback are also discussed.
References
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Journal: Horticulturae Authors: Liu et al
Introduction Horticultural crops typically include fruits, vegetables, and ornamental plants. They are not only an excellent source of daily nutrition for people, but also have important economic value. Common fruits and vegetables can provide a variety of nutrients that the human body needs, including fiber, vitamins, organic acids, and mineral elements [1]. Like other plants, horticultural crops are infected by various bacterial, oomycete, and fungal pathogens that cause enormous economic losses and can have great impacts on human society. The Irish potato famine in the 19th century caused by the oomycete Phytophthora infestans is considered to be a historical tragedy [2]. According to nutrient-acquisition strategies, horticultural crop pathogens can be categorized as biotroph, hemibiotroph, or necrotroph [3]. Biotrophic pathogens infect living plant cells and feed on them. In contrast, necrotrophic pathogens acquire nutrients from dead cells after killing plant tissues. Hemibiotrophic pathogens exhibit two forms of nutrition acquisition, the early biotrophic stage and the late necrotrophic stage. Some typical pathogens of horticultural crops include hemibiotrophic bacteria Pseudomonas syringae and Ralstonia solanacearum, necrotrophic bacteria Xanthomonas spp., biotrophic oomycetes Phytophthora spp., biotrophic fungus Cladosporium fulvum, hemibiotrophic fungus Fusarium oxysporum, and necrotrophic fungi Botrytis cinerea, Alternaria alternata, and Penicillium spp. [4,5]. Effectors are unified agents used by bacterial, oomycete and fungal pathogens to facilitate infection. They are broadly defined as biological molecules secreted by pathogens, including proteins, small RNA, and toxic metabolites [6]. Effectors play dual roles during the complicated plant–pathogen interaction and trigger a series of molecular events. In the early stages of infection, plants recognize pathogen-associated molecular patterns (PAMPs) and activate PAMP-triggered immunity (PTI). At the same time, pathogens can secrete effectors to suppress PTI, and plants recognize effectors thereby inducing effector-triggered immunity (ETI) which may lead to hypersensitive response (HR) in the host [7]. The gene-for-gene model suggests that there is a one-to-one correspondence between an avirulence gene (Avr) of the pathogen and a resistance gene (R) of the plant [8]. In this model, effectors are often referred to as Avr proteins that can be recognized by R proteins (immunity receptors), triggering defense responses leading to host resistance [9]. Avr effectors identified from C. fulvum, the causal agent of tomato leaf mold, are a classic example of the gene-for-gene model [10]. Functions of effectors are diverse, for example, preventing recognition of pathogens by plants, suppressing host immune responses, and manipulating host-cell physiology to help colonization. Most effectors are secreted biomacromolecules and have an effect on the apoplastic area or in the cells of host. Different localization is closely associated with specific functions [11]. To date, numerous effectors have been identified from horticultural crop pathogens (Table 1), most of which are proteinaceous effectors. Modes of action of some type-III-secreted effectors (T3SEs) in bacteria, RXLR and CRN (crinkling and necrosis) motif effectors in oomycetes, and LysM (lysin motifs) domain effectors in fungi have been well characterized in various horticultural host plants, especially in tomato and potato [11]. In addition, these findings on effectors and recognition receptors are valuable for the breeding of resistant cultivars [12,13,14,15]. Though a number of reviews have successfully documented the advances in effector biology [3,16,17,18,19], most of them have been mainly focused on pathogens of model plants (such as Arabidopsis or tobacco) or cereal crops (such as rice or wheat). The crucial roles of effectors in interactions between horticultural crops and pathogens have rarely been summarized. This review focuses on proteinaceous and non-proteinaceous effectors in horticultural crop pathogens, with emphasis on the functions and action mechanisms of proteinaceous effectors from bacterial, oomycete, and fungal pathogens, respectively. Moreover, resistance responses in horticultural crops from recognition to fightback are also discussed.
References