Partially Resistant Avocado Rootstock Dusa® Shows Prolonged Upregulation of Nucleotide Binding-Leucine Rich Repeat Genes in Response to Phytophthora cinnamomi Infection

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.