Abstract
Avocado is an important agricultural food crop in many countries worldwide. Phytophthora cinnamomi, a hemibiotrophic oomycete, remains one of the most devastating pathogens within the avocado industry, as it is near impossible to eradicate from areas where the pathogen is present. A key aspect to Phytophthora root rot disease management is the use of avocado rootstocks partially resistant to P. cinnamomi, which demonstrates an increased immune response following infection. In plant species, Nucleotide binding-Leucine rich repeat (NLR) proteins form an integral part of pathogen recognition and Effector triggered immune responses (ETI). To date, a comprehensive set of Persea americana NLR genes have yet to be identified, though their discovery is crucial to understanding the molecular mechanisms underlying P. americana-P. cinnamomi interactions. In this study, a total of 161 PaNLR genes were identified in the P. americana West-Indian pure accession genome. These putative resistance genes were characterized using bioinformatic approaches and grouped into 13 distinct PaNLR gene clusters, with phylogenetic analysis revealing high sequence similarity within these clusters. Additionally, PaNLR expression levels were analyzed in both a partially resistant (Dusa®) and a susceptible (R0.12) avocado rootstock infected with P. cinnamomi using an RNA-sequencing approach. The results showed that the partially resistant rootstock has increased expression levels of 84 PaNLRs observed up to 24 h post-inoculation, while the susceptible rootstock only showed increased PaNLR expression during the first 6 h post-inoculation. Results of this study may indicate that the partially resistant avocado rootstock has a stronger, more prolonged ETI response which enables it to suppress P. cinnamomi growth and combat disease caused by this pathogen. Furthermore, the identification of PaNLRs may be used to develop resistant rootstock selection tools, which can be employed in the avocado industry to accelerate rootstock screening programs.
Highlights
Avocados (Persea americana Mill.) are an agriculturally important crop in many countries, including South Africa, Spain, and Mexico (Bulagi et al, 2016; Vargas-Canales et al, 2020)
We found significantly higher expression levels of 84 PaNLR genes in the partially resistant rootstock when compared to the susceptible rootstock after P. cinnamomi inoculation
Nucleotide binding-Leucine rich repeats (NLRs) gene sequences which did not include a Leucine rich repeat domain (LRR) domain sequence were removed from further analysis and considered as incomplete PaNLR genes
Summary
Avocados (Persea americana Mill.) are an agriculturally important crop in many countries, including South Africa, Spain, and Mexico (Bulagi et al, 2016; Vargas-Canales et al, 2020). Phytophthora root rot, caused by the hemibiotrophic oomycete, Phytophthora cinnamomi Rands, remains the largest threat to the avocado industry, in countries where the pathogen is present (Hardham and Blackman, 2018). The pathogen infects the fine feeder roots of avocado trees, leading to decreased water and nutrient transportation between cells (Coffey, 1987). Phytophthora cinnamomi can survive in soils over long periods of time through the production of chlamydospores and oospores, limiting the number of effective control methods for Phytophthora root rot (Dobrowolski et al, 2008; Belisle et al, 2019). Use of partially resistant rootstocks and organic mulching practices are methods currently employed by the avocado industry to control P. cinnamomi (Giblin et al, 2005). Continued screening for P. cinnamomi resistant rootstocks is of utmost importance and can be accelerated when host-pathogen interactions are understood
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