Pathogen can severely effect plant health. Plant defense system play a pivotal role in protecting plant against the insect-pests and pathogens. Plant possess resistance related structural and/or morphological traits as well as an array of secondary metabolites. Plant resistance can be categorized in three categories i.e. antibiosis, antixenosis (often referred to as non-preference) and tolerance (Kogan and Ortman 1978; Koch et al., 2016). Antibiosis plant trait has an adverse effect on pest’s biology through increase in mortality, reduced growth, longevity and fecundity (Koch et al., 2016). Plants have physical traits conferring antixenosis and antibiosis such as trichomes, slippery surface and fortified tissues (Tadesse et al., 2022). Antibiosis and antixenosis coexist in many resistant plants with enormous diversity of plant biochemicals such as repellent, deterrent, toxic and antinutritive properties (Mithofer and Boland 2012; Stout 2013). Chemical factor in antibiosis for example are: chemicals present in plants such as Benzoxazinoids are phytoanticipins present in Poaceae like 2,4-dihydroxy-7-methoxy-2H-1,4- benzoxazin-3(4H)-one (DIMBOA), that functions as a defense regulatory signal in maize innate immunity, and has biocidal activity (Ahmad et al., 2011); the sesquiterpenoid phytoalexins such as gossypol, capsidiol and zealexin are present in cotton, tobacco and maize plants respectively (Tian et al., 2019); sinigrin is a glucosinolate found in plants of Brassicaceae family and may possess antifungal, antibacterial and antioxidant properties (Shofran et al., 1998; Mazumdar et al., 2016); cucurbitacin present in Cucurbitaceae family is extremely bitter tetracyclic triterpenoid and protects leaves and fruits from birds arthropods and pathogens (Bar-Nun and Mayer 1990; Bruno et al., 2022); salicylic acid is a plant hormone and has a role in induction of plant defense response against biotic and abiotic stress (Durner et al., 1997, War et al., 2011). Antixenosis resistance is associated with the production of volatile organic compounds (VOCs) such as (E)-ß-farnesene, (Z)-3-hexenal, (E)-2-hexenal, (Z)-ocimene, linalool, benzyl acetate, methyl salicylate etc. (Skoczek et al., 2016; Gebretsadik et al., 2022). The VOCs released by plants can repel herbivores or attract natural enemies of herbivores (War et al., 2012).
Virulent pathogen can cause infection resulting in damage to the plant which reduces plant host fitness (Read 1994). The two major mechanisms of plant defense against pathogens are resistance (plants ability to control colonization and multiplication) and tolerance (plants ability to reduce infection to prevent or to limit/reduce the adverse effect of attack) and maximum virulence evolves for intermediate infection rate, at which coevolved levels of resistance and tolerance are both high (Carval and Ferriere 2010; Pagan and Garcia-Arenal 2018). Resistance can be subdivided into constitutive and induced as well as direct /indirect subcategories (Stout 2013). Overall categories of direct plant defense were antinutritional (limiting food supply, reducing nutrient value, reducing preference, disrupting physical structures and inhibiting chemicals pathway of the attacking insect) and toxicity (chemicals include secondary metabolite, protein inhibitors of insect digestive enzymes, proteases, lectins, amino acid deaminases and oxidases) (Chen 2008).
In plant-pathogen systems, both defense strategies resistance and tolerance generally coexist. Tolerance is not an alternative to plant resistance there could be inter- and intraspecific tradeoff between defensive strategies (Strauss and Agrawal 1999). Plant endures severe disease without severe losses in yield or quality. Tolerance fits into different schematic classification of disease resistance such as a) escape, b) exclusion, c) host-parasite interactions following infection, which led to different levels of disease and d) tolerance or endurance of a given level of disease (Schafer 1971).
Resistance reduces risk of infection and/or replication rate of pathogen in the host whereas, tolerance does not (Pagan and Garcia-Arenal 2018). A more tolerant host genotype will suffer less loss in fitness per unit increase of pathogen population present within the host (called the pathogen burden) as compared to less tolerant host genotype (Mikaberidze and McDonald 2020). A significant genetic correlation suggested that plants that were large and healthy had high tolerance when infected (Carr et al., 2006). Natural selection will favor those individuals whose pattern of allocation yields more fitness (Agnew et al., 2000). Genetic resistance often fails because a resistance-breaking (RB) pathogen genotype increase in frequency (Garcia-Arenal and McDonald 2003). Pathogens that pose high risk of breaking down resistance genes have a mixed reproduction system, a high potential for gene flow, large effective population sizes and high mutation rates. While the lowest risk pathogens are those that have strict asexual reproduction, low potential gene flow, small effective population sizes and low mutation rates (McDonald and Linde 2002).
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