The experimental design was elegant in its simplicity. Detached composite leaves from greenhouse-grown plants of each variety were either left untreated as healthy controls, mock inoculated with water, or inoculated with droplets containing one million zoospores per millilitre of a fluorescently labelled P. infestans strain. Samples were collected at 48 hours post inoculation both inside the inoculated area and in surrounding tissue, allowing the team to distinguish local responses from more systemic ones. In total, 36 samples were sequenced using Illumina paired-end technology, with reads mapped to the reference potato genome at rates of 85 to 92 percent. Visual symptoms told the first part of the story: Sarpo Shona developed dark necrotic spots by 24 hours after inoculation, while Duke of York did not react until 48 hours, and by nine days the susceptible variety’s leaves were extensively yellowed and colonised. Callose staining revealed that Duke of York deposited substantially less of this defensive wall material at 24 hours than either resistant variety, with Sarpo Mira showing the strongest accumulation.

Principal component analysis of the thousand most variable genes showed that the first principal component, explaining 51 percent of the variation, cleanly separated healthy leaves from all inoculated conditions, while the second component, explaining 26 percent, separated the two resistant Sarpo varieties from Duke of York across every condition. This was a striking finding: even in completely healthy, uninfected leaves, resistant and susceptible potatoes occupied distinct transcriptional states. The largest number of differentially expressed genes appeared when comparing healthy controls with mock-inoculated leaves after 48 hours of incubation, but the infection response itself revealed an asymmetry that would prove central to the study’s conclusions. Sarpo Mira showed the smallest transcriptional response to infection, Sarpo Shona an intermediate one, and Duke of York the largest, with more than twice as many genes downregulated in the susceptible variety as in either resistant one.

Gene ontology enrichment analysis sharpened this picture considerably. In Duke of York, genes downregulated inside the infected area were enriched for 16 biological process terms, including abscisic acid biosynthesis and signalling, jasmonic acid signalling, and several defence-related categories, none of which were enriched in the resistant varieties. This pattern suggests that P. infestans actively suppresses hormone signalling and defence pathways in the susceptible host, effectively disarming the plant’s response machinery. Downregulation of specific WRKY transcription factors was observed only in Duke of York, consistent with pathogen-mediated suppression of transcriptional regulation. Meanwhile, all three varieties upregulated lignin biosynthesis genes inside the infected area, but the response was markedly stronger in the Sarpo varieties, which activated 19 and 20 lignin-related genes respectively compared with 15 in Duke of York. Crucially, seven of these upregulated lignin genes were also induced outside the infected area in each Sarpo variety, compared with only two in the susceptible potato, hinting at a broader systemic reinforcement of cell walls.

The most provocative finding emerged when the researchers compared gene expression across all conditions between resistant and susceptible varieties. A total of 685 genes had consistently higher expression and 431 genes consistently lower expression in both Sarpo varieties than in Duke of York, regardless of whether the plants were healthy, mock treated, or infected. Among the top upregulated genes were a glycosyl hydrolase, a beta-1,3-glucanase capable of degrading pathogen cell walls, an R gene encoding a leucine-rich repeat and NB-ARC domain resistance protein, a protein kinase, and a raffinose synthase. The single most highly upregulated gene across all conditions, with a log2 fold change exceeding 15, was a basic chitinase. These are not emergency responses; they are standing garrisons. The resistant varieties maintain elevated expression of a broad arsenal of defence genes even in the absence of any threat, a state the authors describe as stressed-like expression patterns existing before the onset of stress.

Wall-associated kinase receptors emerged as a particularly compelling component of this pre-armed state. WAKs are cell surface receptors that recognise pectin fragments released from the plant cell wall during the early stages of pathogen attack, functioning as damage-sensing sentinels that trigger pattern-triggered immunity. Ten WAK genes were consistently differentially expressed in one or both Sarpo varieties compared with Duke of York, with seven upregulated in Sarpo Mira and six in Sarpo Shona. Most of these were not themselves induced by infection, meaning the resistant varieties simply carry more of these receptors at all times. Combined with the upregulation of specific pectin lyase-like genes that generate oligogalacturonide fragments acting as damage-associated molecular patterns, the resistant potatoes appear to operate both a louder alarm and more sensitive detectors, enabling faster activation of pattern-triggered immunity when the pathogen breaches the wall.

Resistance genes told a similar story of abundance rather than activation. Ninety-one R genes showed consistent differential expression in one or both Sarpo varieties relative to Duke of York, and almost none of these were further induced during infection. Several, including a cluster on potato chromosome 4 containing multiple known resistance genes, were expressed at consistently higher levels in the resistant varieties. Notably, the three major qualitative resistance genes of Sarpo Mira, R3A, R3B and R8, were not differentially expressed at a log2 fold change of one or greater during infection in any variety, suggesting that the variety’s celebrated resistance cannot be explained by these five genes alone. High basal expression of many R proteins likely provides more intracellular receptors poised to detect P. infestans effectors, allowing a faster hypersensitive response without the growth penalties associated with constitutive R protein activation.

Cell wall composition itself appears to differ between the varieties before infection. Lignin-related genes were expressed at higher levels in healthy Sarpo leaves, implying a physically tougher wall, a pattern previously observed in Camelina sativa resistant to Sclerotinia sclerotiorum. Cutin transport genes were also more highly expressed in healthy resistant leaves, suggesting a thicker cuticular barrier against pathogen penetration. In contrast, Duke of York upregulated numerous xyloglucan endotransglucosylase/hydrolase genes during infection, enzymes that remodel hemicellulose and alter wall extensibility, while the resistant varieties tended to downregulate them. The susceptible variety also downregulated seven lignin-related genes, including five phenylalanine ammonia lyases, the gateway enzymes of the phenylpropanoid pathway that feeds monolignol biosynthesis. Perhaps most tellingly, both Sarpo varieties strongly and consistently downregulated three target of rapamycin genes, the master growth-promoting kinase that suppresses defence, indicating that these varieties actively prioritise immunity over growth, accepting a metabolic trade-off in exchange for protection.

The study’s authors acknowledge limitations, including the absence of pathogen load quantification, the use of a single 48-hour time point, and the need to validate cell wall composition biochemically and candidate genes functionally, potentially through genome editing. Nevertheless, the findings provide a transcriptome-wide blueprint for quantitative late blight resistance that could transform breeding strategies. Rather than pyramiding single R genes that P. infestans has repeatedly overcome in its evolutionary arms race, breeders may be able to select for the basal expression states that make varieties like Sarpo Mira and Sarpo Shona permanently vigilant: higher standing levels of wall-associated kinases, R proteins, beta-1,3-glucanases, lignin and cutin machinery, and phosphorylation components, coupled with suppressed growth signalling. As climate change threatens to expand the disease burden of this devastating oomycete, the insight that durable resistance begins before infection may prove one of the most valuable lessons the humble potato has to offer.

Subject of Research: Basal expression of defence mechanisms underlying quantitative resistance to potato late blight

Article Title: Keep up your guard: basal expression of defence mechanisms distinguishes blight resistant from blight susceptible potato varieties

Article References: Ponce, O. P., Dunne, N., Cremona, L., Edbali, O., Woellwarth, T., Bosanquet, J., Luna, E., Prashar, A., & Compton, L. (2026). Keep up your guard: basal expression of defence mechanisms distinguishes blight resistant from blight susceptible potato varieties. BMC Agriculture, 2(1), Article 21. https://doi.org/10.1186/s44399-026-00041-1

Image Credits: AI Generated

DOI: 10.1186/s44399-026-00041-1

Keywords: potato, late blight, Phytophthora infestans, transcriptomics, quantitative resistance, Sarpo Mira, Sarpo Shona, Duke of York, wall-associated kinase, R genes, cell wall, plant immunity