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.

Differing Responses to Phytophthora cinnamomi Infection in Susceptible and Partially Resistant Persea americana (Mill.) Rootstocks: A Case for the Role of Receptor-Like Kinases and Apoplastic Proteases

“…In plants, the largest group of R proteins are that of cytoplasmic nucleotide-binding leucinerich repeat (NLR) proteins (McDowell and Woffenden, 2003). Notably, the full complement of avocado NLRs was recently described using some of the RNA-seq data presented in this study (Fick et al, 2022).…”

Comparative transcriptional analysis of Persea americana MYB, WRKY and AP2/ERF transcription factors following Phytophthora cinnamomi infection

“…The expression patterns for these three major TF families may indicate that the susceptible R0.12 is slowly shutting down biotrophic‐related immune responses at 12 and 24 hpi, while these responses remain activated in the partially resistant Dusa at 24 hpi. Furthermore, these expression patterns may explain the differences in callose deposition, PRR and NLR expression patterns previously observed within resistant and susceptible rootstocks following P. cinnamomi infection (Backer et al., 2022 ; Fick, Swart, Backer, et al., 2022 ; van den Berg et al., 2018 ). Because plant–pathogen interactions are extremely complex, the cause of these observations can only be hypothesized.…”

“…The increased expression of PaMYB and PaWRKY genes, which group closely with defence-related AtMYBs and WRKYs, at 120 hpi were unexpected as these defence-related TFs often activate the expression of many PRR and NLR genes. Previous studies reported increased expression levels of most PaPRRs and PaNLRs at 6, 12 and 24 hpi in Dusa and R0.12 (Backer et al, 2022;Fick, Swart, Backer, et al, 2022). Thus, it was expected that a higher number of defence-associated PaMYBs and PaWRKYs would be expressed at these early time points.…”

“…( 2022 ) and Fick, Swart, Backer, et al. ( 2022 ). PaAP2/ERF with homologues associated with fatty acid accumulation in Hass avocado was also downregulated at early infection time points in both rootstocks.…”

The Ups and Downs of Plant NLR Expression During Pathogen Infection

“…For example, 22 NLRs were upregulated in Raphanus sativus resistant to Plasmodiophora brassicae , but not in a susceptible genotype during P. brassicae infection ( Wang et al, 2022a ). Furthermore, significant differences in NLR expression were observed between a partially resistant and susceptible Persea americana rootstocks infected with Phytophthora cinnamomi , especially after 6 h post-inoculation ( Fick et al, 2022 ). The expression of NLR genes is regulated by three main mechanisms: (1) epigenetic mechanisms, (2) cis elements and TFs, and (3) post-transcriptional modifications ( Figure 2 ; Bezerra-Neto et al, 2020 ).…”