COST OF RESISTANCE TO PATHOGENS FOR PLANTS

Resistance and tolerance may not be fully exchangeable, and it is predicted that both defense mechanisms would co-exist, with host fitness with maximum tolerance or maximum resistance or at intermediate level of both (Montes et al., 2020). The nature of cost to defense and their relationship to trade-off between resistance and tolerance is crucial (Best et al., 2008). The definition of tolerance is closely related to how infected plant reallocates resources to the different environment created by the pathogens. The less virulent pathogens will induce host resource allocation from growth to reproduction, a delay in host reproduction or both responses at the same time (Hochberg et al., 1992; Montes et al., 2020). Whereas when infected by a highly virulent pathogen, the host would accelerate reproduction to produce progeny before death (Montes et al., 2020).

Nutrients are required for growth in all organisms including plant pathogens. Phyllosphere, rhizosphere, phloem, xylem and cell organelles are nutrient niches in plants that are targeted by pathogens. Depending upon the nutrient availability, the pathogens adapt various acquisition strategies and inhabits niche (Fatima and Senthil-Kumar 2015). An adaptive hypothesis is prescribed to explain why parasites harm their hosts is known as the trade-off hypothesis, which states that increased parasite transmission come to the cost of shorter infection duration (Froissart et al., 2010). Virulence results in trade-offs amidst both between-host transmission and within -host multiplication, the basis of trade-off hypothesis (Sacristan and Garcia-Arenal 2008). The cost of infection varies with resistance strategy, pathogen load and plant age (Susi and Laine 2015). The hypersensitive response (HR) is associated with biotrophic resistance but does not limit necrotrophic pathogen growth. Hence necrotroph may place an ecological cost upon plant resistance to biotroph (Kliebenstein and Rowe 2008). The ecological cost in plant-pathogen interaction is the local cost of the HR; organismal cost of having machinery for HR and antagonism between salicylic acid (SA) and jasmonic acid (JA) signaling (Kliebenstein and Rowe 2008).  There is a substantial cost of resistance to pathogens for plants which results in trade-off between growth and immunity (Tsuda et al., 2009).

There are two modes of immunity in plants against the pathogens the pattern-triggered immunity (PTI) and the effector-triggered immunity (ETI). The PTI and ETI are triggered by recognition of pathogen- or microbe-associated molecular patterns (PAMPs/MAMPs) and pathogen effectors respectively. The PAMPs are recognized by cell surface-localized pattern recognition receptors and pathogen effector by intracellular nucleotide-binding domain leucine-rich repeat containing receptors (NB-LLRs) (Jones and Dangl 2006). The SA signaling is required for immunity against biotroph and JA/ethylene signaling is required to activate defense response against necrotrophs (Glazebrook 2005). Only infected hosts pay the cost of an induced defense (Boots and Best 2018). Constitutive defense prevents infection and induced defense shortens the infectious period (Boots and Best 2018). Induced responses are advantages because they are only used when required but are too costly to be maintained constantly, while constitutive responses are advantageous as they are always ready to act (Hamilton et al., 2008). In the constitutive defense, the response to pathogen attack works without delay but the costs of constitutive defenses are to be paid even in the absence of disease, whereas induced defenses incur the most substantial costs when they are used in response to infection (Shudo and Iwasa 2001; Boots and Best 2018).  Plant defense theory suggest that the inducible resistance has evolved to reduce the cost of constitutive defense expression (Gomez et al., 2007).

Upon pathogen or insect attack resources are allocated to defenses instead towards growth and reproduction, while the above- or below-ground interactions with beneficial organisms may also be disturbed (Vos et al., 2013). Growth defense trade-off occur in plants due to resource limitation to optimize plant fitness. The trade-off demands prioritization of allocation of resource towards either growth or defense, as both the processes are vital for plant survival (Huot et al., 2014). Phytohormone SA and JA have a role in regulation of induced defense and their associated fitness costs (Vos et al., 2013). Hormone controlled signaling pathway cross-communicate, proving plant defense regulatory system contribute to a reduction of fitness costs by repressing ineffective defenses. Moreover, the system can be highjacked by invading organisms that manipulate it for their own benefit (Vos et al., 2013). The phytohormone crosstalk regulates trade-off needed to achieve balance hormone crosstalk appears to be primary means for plant modulation of growth and defense (Huot et al., 2014). 

Pathogens exploit their host plant carefully to infect and avoid killing the host. They need host resources to reproduce and transmit infection to new hosts, thus pathogen face a tradeoff between prudent exploitation and rapid reproduction i.e. tradeoff between longevity and fecundity (Frank 1996). The evolution of virulence may determine emergence and re-emergence of pathogens, host switch and host range expansion as well as overcoming host resistance, which may compromise the success of control strategies for infectious disease of humans, domesticated animals and plants (Sacristan and Garcia-Arenal 2008). Genes providing resistance to pathogen impose a cost on the fitness of plants (Burdon and Thrall 2003). Virulence is a costly trait for pathogens which involves secretion of macromolecules, so that a trade-off between virulence and growth exists in phytopathogens (Peyraud et al., 2017). Laliberte et al.(2015) hypothesize a trade-off between the phosphorus acquisition efficiency and resistance to soil-borne pathogens. In strongly weathered and severely phosphorus-impoverished soils, ectomycorrhizal fungi are important for pathogen defense and the persistence of their hosts (Albornoz et al., 2017). Plant protection and fitness is crucial and therefore plant induce defense responses in a cost-effective manner.

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