An international collaboration in which the Institute of Sustainable Agriculture (IAS) of the CSIC participates has managed to identify the molecular basis that explains the defoliation (leaf loss) caused in cotton and olive trees by Verticillium dahliae, a fungus responsible for heavy yield losses in these crops and whose presence has spread throughout the Mediterranean basin.
As detailed by the IAS-CSIC, the study, published in the journal Nature Communications, allows for progress towards a more precise diagnosis of the infection, improved surveillance of the most dangerous strains, and guidance for the design of resistance strategies and field interventions.
Verticillium dahliae is a fungal pathogen that causes verticillium wilt or vascular wilting disease, a serious phytosanitary problem affecting hundreds of dicotyledonous plant species, including cotton, olive, and tomato. This soil fungus penetrates the plant through the roots and colonizes the xylem vessels, interfering with water transport.
Characteristic symptoms include wilting, growth reduction, chlorosis (yellowing of leaves due to lack of chlorophyll, essential for photosynthesis), and premature senescence, that is, a state in which the cell stops dividing but does not die and generates inflammation.
"Controlling Verticillium wilt is notoriously difficult," stated IAS-CSIC researcher Carmen Gómez-Lama Cabanás, who explained that "in cotton, olive, or pistachio, certain strains of V. dahliae can cause severe symptoms." "These strains, which scientists call pathotype D, are on the rise and pose a significant threat to cotton and olive plantations worldwide," she asserted.
Deciphering the most aggressive pathotype to design resistances
To date, the genetic basis responsible for the high virulence of pathotype D was unknown, and the scientific debate continued about what caused the difference in aggressiveness between defoliating and non-defoliating strains. In this work, the team has integrated comparative genomics, functional genetics, structural analysis, and phylogenomics to identify the molecular determinant of defoliation.
"We identified a small pathotype-specific genomic region that causes more severe effects, and we saw that it encodes two duplicated secreted effector genes," explained Gómez-Lama, recalling that an effector gene is a molecule produced by the pathogen - usually a protein - that alters the physiology of the host plant to favor infection.
According to her, "the simultaneous elimination of both copies of genes abolished pathogenicity and defoliation in cotton and olive, as well as in Nicotiana benthamiana and Arabidopsis thaliana, two widely used model plants for plant biology studies." Furthermore, "single deletions reduced virulence and genetic complementation restored disease symptoms," indicated the IAS-CSIC researcher.
Conversely, the introduction of these same genes into non-defoliating strains was sufficient to trigger defoliation. Although having this molecular basis does not imply immediate application in farm management, it does open the way to the development of more effective control and prevention strategies.
Specifically, the team advises plant breeding programs to prioritize the search for germplasm that has evolved to recognize effector protein D, as such material could contain resistance genes specifically targeted against this protein.
"After identifying the responsible protein, the next step is to better study its mechanism of action at the molecular level to understand how it contributes to the development of the disease and the defoliation process," pointed out Bart Thomma, Professor of Evolutionary Microbiology at the University of Cologne (Germany).
For his part, Luigi Faino, from Sapienza University of Rome (Italy) is confident that "if there is business collaboration, within ten years we could see the first genetically modified cotton plants for better resistance to this strain."