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 24h post-inoculation, while the
susceptible rootstock only showed increased PaNLR expression during the first 6h postinoculation. 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.
