Abstract
Chemical treatments are used widely in agricultural and natural settings to protect plants from diseases; however, they may exert an important selection pressure on plant pathogens, promoting the development of tolerant isolates through adaptive evolution. Phosphite is used to manage diseases caused by Phytophthora species which include a large number of the most economically damaging plant pathogens worldwide. Phosphite controls the growth of Phytophthora species in planta without killing it; as a result, isolates can develop tolerance to phosphite after prolonged exposure. We investigated the inter- and intra-specific variability in phosphite tolerance of eleven Phytophthora species, including P. ramorum, an internationally important, highly regulated pathogen. Phytophthora ramorum is a good model system because it is comprised of multiple genetically homogeneous lineages. Seven species were found to be consistently sensitive to phosphite based on the low Effective Concentration (EC) 50 values of all isolates tested (amount of phosphite required to inhibit mycelial growth by 50% relative to growth in the absence of phosphite). However, P. ramorum, P. lateralis, P. crassamura and P. cambivora showed intraspecific variability in sensitivity to phosphite, with at least one isolate showing significantly higher tolerance than the other isolates. Within the three P. ramorum evolutionarily divergent lineages tested, NA1 was the most susceptible to phosphite, the NA1 and EU1 lineages showed intralineage variability and the NA2 lineage showed a decreased sensitivity to phosphite overall as all isolates were relatively tolerant. This finding is relevant because NA1 is dominant in the wild and can be controlled using phosphite, while the EU1 lineage has recently been identified in the wild and is phosphite-tolerant, making the treatment approach potentially less effective. Phytophthora ramorum, P. lateralis and P. crassamura are either selfing, homothallic species, or are known to reproduce exclusively clonally, indicating tolerance to phosphite can emerge even in the absence of sexual recombination.
Highlights
Adaptive evolution by invasive plant pathogens, including the development of tolerance to chemical treatments, presents an ongoing threat to plant health, influencing agricultural sustainability and food security [1]
Interspecific variability in tolerance to phosphite was observed among the Phytophthora species tested, with P. cinnamomi, P. nemorosa and P. multivora showing higher sensitivity to phosphite and P. ramorum lineages NA2 and EU1 showing lower sensitivity to phosphite (Fig 1)
Phytophthora multivora, P. nemorosa and P. cinnamomi were effectively unable to grow on the two highest phosphite concentrations (200 and 500 μg/mL phosphite) and they had the lowest EC50 values on average (Fig 2)
Summary
Adaptive evolution by invasive plant pathogens, including the development of tolerance to chemical treatments, presents an ongoing threat to plant health, influencing agricultural sustainability and food security [1]. The development of resistance to chemicals (e.g. chemicals present in nature) can occur on an evolutionary time scale and is correlated to: a) the intensity and duration of exposure, b) size of the population exposed, c) its generation time, and, d) the innate constitutive ability of a species to survive in presence of genomic structural variation associated with differential gene expression. When they are the target of chemical treatments, some pathogen species may progressively evolve tolerance through evolutionary processes of varying lengths. If selection pressure is constant the tolerance trait may become fixed in a species and be a common phenotype of all individuals [2]
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