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. <i>Phytophthora cinnamomi</i>, 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 <i>P. cinnamomi</i>, 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 <i>Persea americana NLR</i> genes have yet to be identified, though their discovery is crucial to understanding the molecular mechanisms underlying <i>P. americana-P. cinnamomi</i> interactions. In this study, a total of 161 <i>PaNLR</i> genes were identified in the <i>P. americana</i> West-Indian pure accession genome. These putative resistance genes were characterized using bioinformatic approaches and grouped into 13 distinct <i>PaNLR</i> gene clusters, with phylogenetic analysis revealing high sequence similarity within these clusters. Additionally, <i>PaNLR</i> expression levels were analyzed in both a partially resistant (Dusa®) and a susceptible (R0.12) avocado rootstock infected with <i>P. cinnamomi</i> using an RNA-sequencing approach. The results showed that the partially resistant rootstock has increased expression levels of 84 <i>PaNLRs</i> observed up to 24 h post-inoculation, while the susceptible rootstock only showed increased <i>PaNLR</i> 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 <i>P. cinnamomi</i> growth and combat disease caused by this pathogen. Furthermore, the identification of <i>PaNLRs</i> may be used to develop resistant rootstock selection tools, which can be employed in the avocado industry to accelerate rootstock screening programs.