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). The annual gross production value of avocados in South Africa increased by 14.2% in 2018–2019 to a total of R1.42 billion, when compared to 2017–2018. 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). A decline in tree health is observed which ultimately leads to plant death. Phytophthora cinnamomi can survive in soils over long periods of time through the production of chlamydospores and oospores, thus limiting the number of effective control methods for Phytophthora root rot (Dobrowolski et al., 2008; Belisle et al., 2019). Phosphite trunk injections, 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). However, research has shown that P. cinnamomi has the potential to become less sensitive toward phosphite trunk injections (Dobrowolski et al., 2008). Continued screening for P. cinnamomi resistant rootstocks is thus of utmost importance and can be accelerated when host-pathogen interactions are understood.

Plant immune responses influence host-pathogen interactions and involve a myriad of proteins which activate complex, multilayered signaling pathways in response to pathogen attack (Dangl and Jones, 2001; Naveed et al., 2020). These can be categorized into two main responses; the Pathogen associated molecular pattern (PAMP) triggered immune response (PTI) and the Effector triggered immune response (ETI; Davis and Hahlbrock, 1987; Jones and Dangl, 2006). The recognition of PAMPs by membrane-bound Pattern recognition receptors (PRRs) activate an innate immune response, which is lower in amplitude when compared to the ETI response, and forms part of the plant’s first line of defense against pathogens (Matzinger, 2007). Pathogens, in turn, produce effector proteins which are secreted into plant cells to interfere with this process. These effector molecules may then be recognized by intracellular proteins, such as Resistance (R) proteins, either directly or indirectly (Monteiro and Nishimura, 2018). Upon effector recognition, R proteins are activated and trigger ETI—a high amplitude, robust immune response. The primary mode of action of ETI is to activate localized cell death caused by the Hypersensitive response (HR), aimed at arresting pathogen growth (Cui et al., 2015).

Resistance proteins are classified into five diverse groups based on protein structure and domains (Bezerra-Neto et al., 2020). The largest group consists of proteins with Nucleotide binding and Leucine rich repeat domains (LLRs), referred to as Nucleotide binding-Leucine rich repeats (NLRs; McDowell and Woffenden, 2003). Other groups include Receptor-like proteins (RLPs), Receptor-like kinases (RLKs), and Transmembrane Coiled-coil proteins (TM-CCs). The NLR group can be further sub-divided into two classes, based on the NLR’s N-terminus domain. The first class has a Coiled-coil (CC) domain, while the second class has a Toll/interleukin-1 receptor (TIR) structure domain. These NLRs are termed CNLs and TNLs, respectively. The CNL class also includes NLR proteins with both a CC domain and a RPW8 domain (resistance to powdery mildew), termed CC_R-NLRs or C_RNLs (Zhong and Cheng, 2016). CNLs are more abundant in the genomes of tree species when compared to TNLs, although the opposite is seen in Arabidopsis (Neale et al., 2017). Certain angiosperm and conifer species also do not follow this pattern due to TNL duplications, resulting in increased TNL:CNL ratios. Tandem duplications and NLR gene family expansions may have increased fitness levels of tree species that need long-term defense strategies against pathogens (Tobias and Guest, 2014). As a result, these duplicated gene sequences are mostly found in gene clusters within plant genomes (Meyers et al., 2003). Head-to-head NLR genes may express proteins which interact to form homo– or heterodimers, often vital for proper NLR function (Liang et al., 2019). These NLR protein dimers greatly increase the pathogen recognition potential of different NLR protein complexes (Van Wersch and Li, 2019).

A few NLR proteins are constantly expressed at low basal levels which allow plants to “scan” for invading pathogens (Meyers et al., 2002). Most of the genes coding for NLR proteins, however, show differential expression patterns after pathogen attack. This is influenced by the species of pathogen, excreted effector proteins and the plant’s genotype (Christie et al., 2016; Andam et al., 2020). In Eucalyptus grandis challenged by Leptocybe invasa and Chrysoporthe austroafricana, 218 and 343 NLRs were differentially expressed, respectively (Christie et al., 2016). RGA1, a TNL protein in the tree species Salix viminalis, showed higher expression in the resistant host when compared to its susceptible counterpart after Melamspora larici-epitea infection (Martin et al., 2016). Higher RGA1 expression allows for earlier ETI activation which ultimately leads to enhanced disease resistance. The level and timing of NLR expression is crucial, as this ultimately governs whether a plant would be successful in countering pathogen attack (Umadevi and Anandaraj, 2017). Transgenic plants with higher NLR gene expression demonstrated increased resistance to plant pathogens, even when these plants were transformed with non-native NLR genes. Expression of ZmNB25, a NLR first identified in maize, increased the resistance levels of Arabidopsis and rice toward Pseudomonas syringae pv. tomato DC3000 and Bipolaris maydis, respectively (Xu et al., 2018). Understanding how the expression of NLRs change during pathogen infection, and subsequently influence disease resistance, is vital to understanding complex plant-pathogen interactions.

To date, 49 complete putative NLR genes have been identified in avocado using microarray and RNA-sequencing analysis (Van den Berg et al., 2018; Pérez-Torres et al., 2021). In the study done by Pérez-Torres et al. (2021), Hass avocado stems were infected with Fusarium kuroshium, which causes Fusarium dieback disease in avocado. However, only four NLR genes were differentially expressed after F. kuroshium infection. Additionally, only a single avocado NLR gene has been implicated in the defense against P. cinnamomi infection in an avocado rootstock (Van den Berg et al., 2018). This NLR, functionally annotated as RPP13-like protein 4, showed increased expression after P. cinnamomi infection in a partially resistant rootstock. The use of RNA-seq and microarray data to identify NLR genes is limited by the fact that these genes need to be expressed to enable detection and identification. Avocado NLR gene identification has further been hampered by the lack of a high-quality genome assembly. Three avocado genomes, the Mexican landrace cultivar (Persea americana var. drymifolia), the Hass fruiting cultivar (Rendón-Anaya et al., 2019) and the West-Indian pure accession (WI) rootstock (Avocado Genome Consortium, Article in preparation) have only recently been sequenced, providing an opportunity to identify significantly more NLR genes within the avocado genome.

The discovery of NLR genes within the avocado genome could provide novel insight into the interactions of this plant with various pathogens. The current study set out to identify avocado NLR genes using the available genome sequences, and subsequently assess their expression during P. cinnamomi infection of partially resistant and susceptible rootstocks. We identified 161 putative PaNLR genes in the WI rootstock avocado genome, based on amino acid sequences characteristic of conserved NLR domains. Furthermore, we analyzed the expression of the candidate PaNLR genes in a partially resistant and susceptible rootstock following P. cinnamomi inoculation using an RNA-seq approach. 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. This knowledge may benefit future rootstock screening programs aimed at increasing resistance levels toward P. cinnamomi. The PaNLR gene sequences identified in this study serve as an invaluable resource which can be used to pinpoint proteins that play a role in defense responses against other avocado pathogens.

Nucleotide binding-Leucine rich repeat proteins play a crucial role in plant immune responses by recognizing effector molecules produced by invading pathogens. Following effector recognition, NLR proteins activate ETI through complex signaling pathways, which leads to pathogen resistance (Monteiro and Nishimura, 2018). NLR proteins have been studied extensively in many other crops including S. bulbocastanum, Zea mays, Oryza sativa, and Triticum monococcum (Collins et al., 1998; Mago et al., 1999; Bozkurt et al., 2007; Lokossou et al., 2010). The results of these studies ultimately led to the breeding of crops with increased resistance toward various pathogens (Farnham and Baulcombe, 2006; Wang et al., 2019). Thus far, a comprehensive set of P. americana NLR genes have not been identified, and moreover, P. americana NLR gene expression has never been studied during P. cinnamomi infection. Thus, a large knowledge gap remains in understanding ETI activation during P. cinnamomi infection in avocado rootstocks (Van den Berg et al., 2018). The knowledge of avocado NLR functionality is vital to understanding resistance toward P. cinnamomi in avocado rootstocks and can be used by the avocado industry for molecular breeding purposes.

Using the P. americana WI genome, we identified 161 putative complete PaNLR gene sequences. No PaNLR gene sequences could be identified in the Mexican and Hass genome assemblies, since these genomes are highly fragmented (Rendón-Anaya et al., 2019; Talavera et al., 2019). Of the 161 complete PaNLR sequences, 102 were classified as CNL proteins, two as C_RNL proteins, 56 as NL proteins, and one as a TNL protein (Figure 1). The 56 gene sequences encoding NL proteins were found to be expressed during P. cinnamomi infection, indicating that these proteins may play a role in this host-pathogen interaction, even though they lack CC and TIR domains. NLR sequences lacking these motifs are also expressed in other plant species, including E. grandis, Malus x domestica, and Vitis vinifera, further suggesting that these NLR proteins are still functional (Arya et al., 2014; Christie et al., 2016; Goyal et al., 2020). A higher CNL:TNL ratio was also observed in E. grandis, M. x domestica, and V. vinifera woody species, making it unsurprising to observe a higher CNL:TNL ratio in P. americana. However, it was not expected that only one TNL sequence would be identified. This could be a result of the genome assembly and annotation programs used not identifying full length gene sequences, thus producing truncated protein sequences as a result. BLASTp analysis was performed on the entire set of P. americana protein sequences and no additional PaNLR sequences could be identified. Furthermore, an independent study found only one TNL gene being expressed in Hass avocado stems during F. kuroshium infection (Pérez-Torres et al., 2021). This validated that no TNL motifs were missed during PaNLR identification using the RGAugury program and WI genome.

Putative protein functional annotation revealed that more than 50% of the identified PaNLR genes encode RGA2-like proteins (Figure 1). This type of NLR protein was first identified in S. bulbocastanum and is encoded for by Rpi-blb1 (Van der Vossen et al., 2003). RGA2 proteins elicit an immune response and confer resistance toward Phytophthora infestans in potato and tomato plants, after recognizing ipiO RxLR proteins (Champouret et al., 2009). Recently, two P. cinnamomi RxLR proteins with high sequence similarity to P. infestans ipiO RxLRs were identified by Joubert et al. (2021). One of these RxLRs, PcinRxLR34a, was significantly upregulated in P. cinnamomi during infection of the susceptible rootstock R0.12, when compared to expression in mycelia. This suggests that this RxLR plays a role during pathogen infection. Future research should focus on identifying whether P. americana RGA2 proteins recognize these P. cinnamomi RxLRs.

RPP13-like protein 4 type proteins were the second largest group of PaNLR proteins identified in P. americana. RPP13-like protein 4 and RPP8 has been shown to confer resistance toward Peronospora parasitica and Hyaloperonospora arabidopsidis, respectively, in Arabidopsis thaliana (Bittner-Eddy et al., 1999; Mohr et al., 2010). Peronospora parasitica, H. arabidopsidis, and P. cinnamomi are oomycetes, suggesting that these pathogens may express Avirulence (Avr) proteins with similar structure and function (Cooke et al., 2000). This indicates that RPP13-like protein 4 and RPP8 in avocado may recognize P. cinnamomi effectors and play a role in rootstock resistance toward P. cinnamomi. The same assumption can be made regarding RPS2, which confers partial resistance toward Phytophthora sojae, a close relative to P. cinnamomi, in Glycine max (Mideros et al., 2007).

RPM1-like NLR proteins were also identified in avocado. Homologs of RPM1-like NLR genes in A. thaliana are responsible for recognizing P. syringae effectors during infection (Boyes et al., 1998). Pseudomonas syringae has been isolated from avocados; however, no symptoms of infection were observed (Scortichini et al., 2003). This might also explain why so few (5.6% of NLRs) RPM1-like genes were identified in avocado. Furthermore, since P. syringae infection does not present a threat to the avocado industry, NLR genes which confer resistance toward this pathogen would likely be of limited use in avocado screening programs. Lastly, two PaNLRs were annotated as RGA4-like proteins; in O. sativa, RGA4 proteins form heterodimers with RGA5 proteins, which recognize Magnaporthe oryzae infection (Césari et al., 2013). RGA5 proteins act as a receptor for M. oryzae Avr proteins and as a repressor of RGA4. Once RGA5 recognizes Avr proteins, RGA4 is released and activates cell death responses. Thus, in the absence of RGA5 proteins, RGA4 activates cell death in an Avr-independent manner (Césari et al., 2014). Since no P. americana proteins were identified as RGA5 proteins, it remains unclear whether the RGA4 proteins would respond to P. cinnamomi Avr proteins in avocado.

Gene cluster analysis was performed to identify possible duplication events of P. americana NLRs (Meyers et al., 2003). If NLR genes within a cluster were shown to be functionally important for rootstock resistance, NLR gene clusters can be targeted during molecular screening strategies. In total, 13 PaNLR gene clusters were identified in the P. americana genome (Table 1). Of these, four clusters were identified on chromosome 2 with one containing nine PaNLR gene sequences. No clusters were observed on chromosomes 4, 5, 8, 9, 10, and 12 (Figure 1). Eight clusters only contained RGA2 protein sequences, indicating that these genes may have originated from gene duplication events as described by Meyers et al. (1998) and López et al. (2003). Retained NLRs following duplication indicate functional relevance, suggesting that these RGA2 NLRs may play an important role in avocado defense responses. In Phaseolus vulgaris, RGA2 gene clusters were identified as Quantitative trait loci (QTL), which confer resistance toward Colletotrichum lagenarium (López et al., 2003). Further investigation focusing on functional significance will help identify whether the PaNLR gene clusters in P. americana can be used as QTL molecular markers during rootstock breeding programs. Ultimately, these clusters serve as a reservoir for NLR diversity since duplicated genes are free to mutate, which may lead to novel NLRs being able to recognize novel effector proteins from pathogens (Innes et al., 2008).

Phylogenetic analysis revealed high similarity between PaNLRs within gene clusters, further indicating that PaNLR gene clusters may have originated from gene duplication events (Shao et al., 2014). During phylogenetic tree construction, 161 PaNLR Nucleotide binding domain protein sequences were used together with protein sequences from C. micranthum f. kanehirae and S. bulbocastanum (Song et al., 2003; Chaw et al., 2019). Sequences from C. micranthum f. kanehirae were used since this species is the closest relative to P. americana (both species form part of the Lauraceae family) in which NLRs have been identified (Wu et al., 2017). A RGA2 sequence from S. bulbocastanum was also included, since no RGA2 proteins were identified in C. micranthum f. kanehirae (Chaw et al., 2019). Phylogenetic analysis revealed that NB domain sequences within a PaNLR gene cluster grouped together, indicating high sequence similarity within these clusters (Figure 2). Moreover, few PaNLRs formed a clade with NLR sequences from C. micranthum f. kanehirae, indicating large NLR diversification within P. americana species. These observations might be the result of different pathogens shaping the PaNLR arsenal during the coevolutionary arms race between hosts and pathogens (Anderson et al., 2010).

Once putative PaNLR genes were identified in the WI genome, their expression was analyzed using dual transcriptomic data from partially resistant (Dusa®) and susceptible (R0.12) rootstocks inoculated with P. cinnamomi. Of the 161 PaNLRs identified in this study, 16 PaNLRs were not expressed in either rootstock at any timepoint. Many NLRs have tissue-specific expression levels in other plants, making these results unsurprising (Munch et al., 2018). Since this study investigated PaNLR expression in root tissues, it is expected that these 16 PaNLRs might play a role in recognizing pathogens which infect other avocado tissues. Interestingly, PaNLR genes within a gene cluster did not show similar expression patterns after P. cinnamomi inoculation. This was also observed in E. grandis when infected with C. austroafricana and L. invasa. The authors attributed this to expressed NLR genes being functionally relevant, and not the result of being located within active transcription zones by coincidence (Christie et al., 2016). Thus, we can hypothesize that PaNLRs in gene clusters being expressed following P. cinnamomi inoculation, do indeed have functional significance in activating defense responses against the invading pathogen.

During the first 6 h of infection, more than 60 PaNLR genes showed a significant increase in expression in either rootstock, with a similar pattern of expression activation for 54 of the same PaNLR genes in both rootstocks (Figure 4). This indicates that both rootstocks have similar responses with regards to PaNLR expression during the first 6 h of P. cinnamomi infection. PaNLR genes with the largest increase in expression at 6 hpi, were mainly RGA2 type proteins (PC01g020030, PC03g007960, and PC11g007690). RGA2 proteins activate the HR, and higher RGA2 transcript levels were associated with increased P. infestans resistance in S. bulbocastanum (Bradeen et al., 2009). This upregulation of RGA2 in both avocado rootstocks would likely result in a strong HR, which may limit P. cinnamomi growth.

PaNLR gene expression levels in Dusa® was higher when compared to R0.12, at both 12 and 24 hpi. Very few PaNLR genes showed differential expression patterns at 12 and 24 hpi in R0.12, which might indicate a decrease in ETI activation compared to Dusa®. Thus, the expression analysis revealed that Dusa® rootstocks overall have a stronger, more prolonged response to P. cinnamomi inoculation when compared to R0.12 rootstocks. NLR expression in susceptible varieties of S. viminalis, C. arietinum L. and Brassica oleracea do not show such stark differences in the expression when compared to resistant varieties, when infected with Melampsora larici-epitea, Ascochyta rabiei, and Fusarium oxysporum f. sp. conglutinans, respectively (Martin et al., 2016; Andam et al., 2020; Liu et al., 2020). It was thus expected that a greater portion of PaNLRs would show increased expression in R0.12 at these timepoints. These results might be due to either the pathogen interfering with PaNLR expression, or the pathogen suppressing host responses in R0.12. For example, W boxes, which are cis-regulatory elements recognized by WRKY transcriptions factors, are often overrepresented in plant defense-related gene promoters including NLR promoter sequences (Mohr et al., 2010). In A. thaliana, WRKY expression was downregulated by Avr3a-type effectors from Phytophthora parasitica (Li et al., 2019). This would subsequently lead to decreased NLR expression. It would be interesting to see whether P. cinnamomi uses similar tactics to influence ETI in P. americana. Thus, investigating which cis-regulatory elements are shared between PaNLR genes would be of interest in future research. Moreover, P. cinnamomi RxLRs were shown to have increased expression levels at 12 and 24 hpi in R0.12 (Joubert et al., 2021). Since some RxLRs suppress programed cell death, P. cinnamomi RxLRs could influence PaNLR expression and contribute to the results observed for R0.12 (Dalio et al., 2018). This data will help understand which PaNLR proteins might be important for recognizing P. cinnamomi effectors during infection and limiting P. cinnamomi growth. However, it must be noted that further studies, including protein–protein interaction studies, are needed to concretely state which individual PaNLR proteins recognize P. cinnamomi effectors.

A previous study, also done on R0.12 and Dusa® rootstocks, showed that R0.12 had significantly higher P. cinnamomi pathogen loads when compared to Dusa®, at all tested time-points (Engelbrecht et al., 2013). The increased PaNLR expression in Dusa®, especially RGA2 PaNLRs, at 12 and 24 hpi is likely to increase the amplitude of ETI activation and the HR, assuming successful P. cinnamomi Avr detection. Studies have shown that overexpression of NLR genes leads to higher levels of resistance and subsequent decreased disease symptoms. In Nicotiana benthamiana plants, overexpression of the Vitis amurensis NLR gene, VaRGA1, resulted in increased resistance toward P. parasitica (Li et al., 2017). Two RGA2 NLRs (PC11g001210 and PC11g001240) showed much higher expression in Dusa® when compared to R0.12, in all samples (Figure 5). As described earlier, the RxLR in P. cinnamomi with high similarity to a RGA2 protein counterpart, ipiO1, showed increased expression in R0.12 at 12 hpi, when compared to mycelia control samples (Joubert et al., 2021). Since R0.12 RGA2 proteins are not upregulated at this timepoint, it may suggest that fewer of these RxLR effectors are recognized, resulting in a compromised HR. However, high expression of RGA2 NLRs in Dusa® at all timepoints could result in increased ETI and might lead to decreased pathogen growth rates and/or decreased zoospore germination. However, in R0.12, pathogen load could be higher due to decreased ETI. These differences might be why Dusa® is able to survive P. cinnamomi attack for longer periods of time and show less disease symptoms.

This study is the first to identify and classify putative PaNLR genes using the P. americana WI genome. Phylogenetic analysis revealed that many PaNLRs found within NLR gene clusters may have originated from gene duplication events. Up to 94 PaNLR genes showed expression differences in response to P. cinnamomi attack, indicating a possible role in P. cinnamomi recognition and ETI activation. Furthermore, PaNLRs showed sustained, increased expression in a partially resistant rootstock (Dusa®) after inoculation, which could explain how this rootstock is able to suppress P. cinnamomi growth. This research paves the way toward understanding P. americana-P. cinnamomi interactions on a molecular level. Future studies should focus on investigating protein–protein interactions between PaNLRs and P. cinnamomi Avr proteins, and how P. cinnamomi is able to suppress PaNLR expression in R0.12 rootstocks. Furthermore, future studies should also include functionally characterizing the identified PaNLRs and investigating their role in defense responses against other P. americana pathogens. However, the lack of an efficient transformation system for P. americana greatly limits functional studies, and the results of this study highlights the need for the development of an improved system.

The datasets presented in this study can be found in online repositories. The names of the repository/repositories and accession number(s) can be found at: https://www.ncbi.nlm.nih.gov/genbank/, PRJNA675400.

AF analyzed all data and drafted the manuscript. RB performed the early analysis of RNA sequencing data and data curation. AB completed the assembly of the WI genome. JE designed and performed the experiments. VS and NB provided supervision of the study and revised the manuscript. All authors contributed to the article and approved the submitted version.

Project funding was generously provided by the Hans Merensky Foundation.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.