Pierce’s disease in plants is a vascular wilt disease caused by the Gram-negative proteobacterial pathogen Xylella fastidiosa. It is one of the ten most important bacteria in molecular plant pathology (Mansfield et al., 2012), in part because it has a wide host range, with >300 documented plant species affected, including economically important crops such as grape, almond, citrus, coffee, mulberry, peach and plum (European Food Safety Authority, 2020). In wine grapes (Vitis vinifera), X. fastidiosa subsp. fastidiosa is the causal agent of Pierce’s disease. The pathogen is transmitted predominantly by insect vectors, such as hemipteran leafhopper sharpshooters and spittlebugs that feed on xylem sap from leaf petioles and shoots (Redak et al., 2004). Humans can also spread it through infected plant material and tools. It is native to the Americas, but has recently been reported in Spain and is recognized as an A2 quarantine pest (Hopkins and Purcell, 2014). The disease is now considered a major threat to winegrowers worldwide.
Xylella fastidiosa subsp. fastidiosa largely infects the xylem conduits (Fig. 1), where it disrupts water transport, which, in turn, causes leaves to wilt, become chlorotic and scorched and, eventually, die. Once the pathogen can access the lumen, such as after initial inoculation by a sap-feeding insect, the bacterium secretes extracellular enzymes, such as polygalacturonases, lipases and esterases, via the function of a type II secretion system. These enzymes hydrolyse pit membranes and parenchymal cell walls adjacent to the xylem lumen, allowing the bacteria to spread between xylem vessels. Bacterial biofilm formation and host responses involving cell wall thickening, pectin gel accumulation and tylose formation then result in vessel occlusion and subsequent water and nutrient limitation in the shoot. Destruction of the pit membrane might also reduce cavitation resistance during drought. Given this understanding of X. fastidiosa infection in susceptible grapevines, anatomical characteristics of the xylem are hypothesized to influence the systemic spread of the pathogen. Fanton and Broderson (2021) previously observed clear differences in vessel length between genotypes contrasting in resistance. To improve our understanding of the relationship between Pierce’s disease and xylem structure, Fanton et al. (2024), in this issue of Annals of Botany, use high-resolution X-ray micro-computed tomography to investigate xylem anatomy in six varieties of wine grapes, including resistant wild species with Vitis arizonica parentage. Given that radial and tangential movement of the pathogen might also depend upon intervessel connections and associated pit membrane vulnerability to degradation by pathogen hydrolytic enzymes, comparisons were made of the three-dimensional structure of xylem. The findings revealed a highly conserved spatial distribution of xylem connections in the vascular networks of resistant and susceptible genotypes, with no evidence for reduced numbers of connections in resistant genotypes. Similar application of three-dimensional imaging of xylem networks from contrasting genotypes of citrus and olive indicated differences in vessel number and density correlated with patterns of resistance, indicating that xylem structure is a promising avenue for improving disease resistance (Walker et al., 2023). In their study with grape, however, Fanton et al. (2024) reported contrasting results, with no threshold value of vessel number and density observed that could accurately predict resistance to Pierce’s disease. Such findings highlight the importance of investigation of candidate traits for each specific pathosystem. The authors finally suggest that mechanisms governing the active spread of X. fastidiosa within the vessel network might outweigh the importance of the network properties, with host biochemical responses likely to be important in restricting pathogen movement.
Scanning electron micrograph of a xylem vessel from grape (Vitis vinifera) showing typical vascular occlusions caused by Xylella fastidiosa cell aggregates and excretion products. Bacteria are visible as oval-shaped cells along the right side of the vessel in the centre of the image, with carbohydrate excretions visible as tangled masses on the left side of the vessel. Scale bar: 1 µm. Image courtesy of Eduardo Alves (Universidade Federal de Lavras, Brazil).
Our understanding of the plant immune system has advanced considerably in the 21st century. Two tiers of immunity are recognized, each involving specific protein receptors for pathogen recognition (Ngou et al., 2022; Wang et al., 2022). The first, referred to as ‘pathogen-associated molecular pattern (PAMP)-triggered immunity’ (PTI), is induced by ‘cell-surface pattern recognition receptors’ to recognize conserved PAMPs. The downstream defence responses that are activated include cytosolic calcium influx, production of reactive oxygen species with associated callose deposition, activation of mitogen-activated protein kinase cascades and WRKY transcription factors, and upregulation of defence-related genes that encode defence-related hormones and antimicrobial compounds. Pathogens can also suppress PTI, with effector proteins secreted into plant cells, thus conferring effector-triggered susceptibility and disease. Recognition of pathogen effectors by plant resistance (R) protein receptors, however, can initiate a second tier of plant defence, termed effector-triggered immunity. Here, intracellular host Nod-like receptors (NLRs) can recognize specific pathogen effectors, triggering robust defence responses that can overlap with those of PTI, in addition to involving phytohormones, pathogenesis-related proteins, a hypersensitive response and systemic acquired resistance.
Investigation of the molecular mechanisms involved in the host immune response to Pierce’s disease is advancing with the application of high-throughput technologies for analysis of transcriptomic, proteomic and metabolomic responses to the pathogen. Zaini et al. (2018), for example, investigated the compatible interaction of V. vinifera to X. fastidiosa infection, with their multi-omic approach revealing considerable upregulation of genes involved in diverse components of the immune response. Although certain responses are likely to be associated with susceptibility, such as the observed cell-wall remodelling and lignification that can be associated with water limitation, or GABA induction, which might promote bacterial quorum sensing, the identification of upregulated genes involved in pattern recognition receptor-associated pathogen perception, pathogenesis-related proteins and phytoalexins indicates the potential for manipulation in susceptible genotypes via overexpression. Identification of potential susceptibility genes, such as nodulins, or those downregulated in the salicylic acid pathway during interaction with the pathogen are also candidates for gene editing approaches to enable recessive resistance. With the findings of Fanton et al. (2024) that xylem connection structure is not a major mode of resistance, grape improvement strategies can instead focus on molecular mechanisms of resistance, such as the two tiers mentioned above, or possibly, through incorporation of resistance alleles in wild relatives of grape.
In this context, Morales-Cruz et al. (2023) conducted a genome-wide association analysis on wild grapevine, V. arizonica. This species contains PdR1, which is the only plant locus to date known to segregate for resistance to X. fastidiosa (Riaz et al., 2006). Furthermore, recent findings have shown this locus harbours five genes encoding leucine-rich repeat receptor-like proteins, which are typically involved in PTI immune responses (Agüero et al., 2022). In their genome-wide association study, Morales-Cruz et al. (2023) identified further novel candidate genes within genomic regions associated with resistance, highlighting the potential in wild relatives for gene discovery and introgression into susceptible cultivars.
In summary, the article by Fanton et al. (2024) demonstrates that xylem vessel network connectivity in grapevine, as an anatomical trait that might influence radial and tangential movement of X. fastidiosa, does not explain resistance to Pierce’s disease. Continued investigation of traits in contrasting materials and wild relatives, together with further in-depth analysis of the genetics and molecular plant responses in the defence response in wild relatives and contrasting V. vinifera genotypes are necessary to provide appropriate tools for genetic improvement for resistance to this important disease.
FUNDING
The work was financially supported by the CNPq (grant number 308165/2021-7), INCT (grant number 465480/2014-4) and FAPDF (grant number 00193.00000778/2021-03).
ACKNOWLEDGEMENTS
I thank Eduardo Alves (Universidade Federal de Lavras, Brazil) for kindly providing electron microscopy images.
