The study, published in the journal Crop Health, focuses on polyamines, small positively charged molecules that all living cells need for growth and division. Plants deploy polyamines, particularly spermine, as signaling molecules and weapons during pathogen attack. When a fungus lands on a plant, the host ramps up spermine production at the infection site while simultaneously unleashing an oxidative burst of superoxide and hydrogen peroxide designed to kill the invader before it can establish a foothold. The researchers set out to determine how B. cinerea navigates this chemical minefield, and whether a specific transporter gene could be the linchpin connecting spermine handling, redox balance, and virulence.

The first clue came from simple but striking experiments. When the team exposed fungal spores to spermine, germination collapsed. The molecule also suppressed the formation of appressoria and infection cushions, the specialized structures the fungus builds to invade plant tissue, and sharply reduced lesion sizes on mung bean leaves. Remarkably, spermidine, a closely related polyamine that differs only in its chemical structure, had none of these effects. That functional specificity pointed to a dedicated transport or recognition system, and it marked spermine as a genuine battlefield molecule in the plant-fungus arms race rather than a general growth inhibitor.

That system, the researchers found, is BcTpo1, a member of the major facilitator superfamily of membrane transporters. Its yeast counterpart, Tpo1, is known to export excess polyamines and to help cells cope with oxidative stress, but its role in plant-pathogenic fungi had never been resolved. In B. cinerea, the gene was strongly upregulated when the fungus was treated with hydrogen peroxide, and it was also activated during the early hours of plant infection, precisely when the host oxidative burst peaks. Those expression patterns suggested BcTpo1 was doing something important under exactly the conditions a pathogen faces in the wild.

To find out what, the team deleted the gene. The resulting mutant, ΔBcTpo1, was pleiotropically impaired: its hyphae grew more slowly, it produced fewer aerial hyphae and spores, and it generated fewer sclerotia, the hard survival structures that let the fungus persist between seasons. Intriguingly, the mutant also turned visibly darker, and further analysis confirmed it was overproducing melanin, a pigment known to shield fungi from environmental stresses. Genes in the melanin biosynthesis pathway, including BcPKS1, BcTHR, BcMPS1, and BcCMR1, were all significantly upregulated. The researchers interpret this as an adaptive compensation: with its redox balance already disturbed, the fungus pumps out extra melanin to buffer the oxidative stress it can no longer manage efficiently.

The connection to spermine became clear when the researchers measured the molecule inside fungal cells. Under normal growth conditions, the mutant and the wild-type strain held similar amounts of intracellular spermine. But when hydrogen peroxide was added to mimic the plant oxidative burst, the mutant accumulated significantly less spermine than the wild type. This observation upends the conventional view of Tpo1 as a simple exporter. Instead, it suggests that BcTpo1 helps the fungus take up or retain spermine specifically during stress, maintaining the intracellular pool the pathogen needs to survive the attack. In yeast, polyamine excretion by Tpo1 is known to be modest, and the B. cinerea data now point toward a stress-responsive role in spermine homeostasis rather than straightforward efflux.

Why would losing spermine matter so much under oxidative stress? The answer lies in the fungus’s antioxidant arsenal. The mutant showed heightened sensitivity to hydrogen peroxide and accumulated more reactive oxygen species, as revealed by fluorescent staining of fungal hyphae. Crucially, its activities of the antioxidant enzymes superoxide dismutase and peroxidase were significantly reduced compared to the wild type. When the team supplied exogenous spermine to the mutant, both enzyme activities rebounded, intracellular reactive oxygen levels fell, and colony growth partially recovered. Quantitative gene expression analysis added nuance: in the wild type, spermine treatment downregulated antioxidant genes, consistent with a lowered stress load, whereas the mutant already ran those genes hot under baseline conditions and pushed them even higher with spermine supplementation. The effect appears to combine direct enzyme activation with indirect transcriptional regulation mediated in part by the stress regulators BcAp1 and BcSkn7.

Virulence assays tied everything together on living tissue. On mung bean leaves, the ΔBcTpo1 mutant produced dramatically smaller lesions than the wild-type strain, and a complemented strain carrying the restored gene regained full pathogenicity. Microscopy showed why: the mutant formed fewer and smaller infection cushions, the multicellular penetration structures that B. cinerea uses to breach plant surfaces, and reactive oxygen species accumulated heavily within those defective structures and in the surrounding hyphae. Adding spermine back reduced the oxidative load and promoted infection cushion formation, partially rescuing the virulence defect. BcTpo1 expression was also elevated throughout the infection window, reinforcing its role as a gene the fungus deploys precisely when it needs to withstand host defenses.

The implications reach beyond basic fungal biology. By linking polyamine transport, redox homeostasis, and pathogenicity in a single molecular axis, the study exposes a control point that plant scientists could exploit. A fungicide that disrupts BcTpo1, or a crop variety engineered to manipulate spermine levels at infection sites, could starve the pathogen of a resource it needs exactly at the moment of attack, when the plant’s own oxidative burst is doing its work. Because the transporter appears central to development as well as virulence, interference with it would likely hit the fungus at multiple life stages, making resistance evolution harder. The authors caution that several mechanistic questions remain, including the precise transport direction of BcTpo1 and whether other Tpo-family members compensate elsewhere in the genome. But as an entry point into a previously unrecognized connection between polyamine chemistry and fungal disease, the finding stands out. Gray mold has thrived for centuries by outmaneuvering plant defenses; this study shows that one small transporter sits at the heart of that strategy, and that it may now be within reach of the next generation of crop protection tools.

Subject of Research: Role of the MFS transporter BcTpo1 in spermine homeostasis, oxidative stress response, and virulence of Botrytis cinerea

Article Title: The MFS transporter BcTpo1 governs the oxidative stress response and infection of Botrytis cinerea

Article References: The MFS transporter BcTpo1 governs the oxidative stress response and infection of Botrytis cinerea. (n.d.). https://doi.org/10.1007/s44297-026-00074-7

Image Credits: AI Generated

DOI: 10.1007/s44297-026-00074-7

Keywords: Botrytis cinerea, gray mold, BcTpo1, spermine, polyamine transporter, oxidative stress, reactive oxygen species, virulence, infection cushions, antioxidant enzymes, melanin biosynthesis, crop protection