Study reveals physico-chemical nature of xylem barriers that confer bacterial wilt resistance

A new work led by Núria Sánchez Coll, CSIC researcher at the Centre for Research in Agricultural Genomics (CRAG), elucidates how tomato plant varieties resistant to the bacterial wilt pathogen Ralstonia solanacearum have the ability to restrict bacterial movement in the plant. The study, recently published in the journal New Phytologist, analyses the composition and formation of the xylem barriers that confer resistance to R. solanacearum, a soil bacterium with devastating effects on many solanaceous crops such as tomato, potato, pepper and eggplant. Results have allowed researchers to engineer resistance to R. solanacearum in commercial susceptible varieties of tomato plants.

The agro-economic impact of R. solanacearum, the pathogen responsible for the bacterial wilt disease, preoccupies farmers all over the world due to the large number of species it affects, its broad geographical distribution, and its persistence in soil and water. This pathogen enters the plant through the roots and colonizes the xylem vessels that carry water and nutrients to the stems and leaves, spreading systemically and eventually killing the plant. Tomato plant varieties resistant to bacterial wilt are able to synthesize reinforcement coatings that confine R. solanacearum into infected vessels, preventing bacterial spread to healthy tissues. Despite being a key factor of resistance, the composition and formation of these barriers had not been studied in detail until now.

Wall reinforcements to confine the infection

In order to understand how bacterial wilt resistance works, researchers compared a susceptible commercial tomato plant variety with a highly resistant tomato cultivar which, despite producing very small fruits unfit for consumption, contributes a reliable source of resistance in breeding programmes. After infecting both varieties with R. solanacearum, histological, live-imaging and spectroscopic analysis revealed the formation of vascular coatings containing ligno-suberin and related phenolic compounds (such as HCAAs) in resistant plants. Such structural wall reinforcements, which were not present in susceptible plants, provide a physico-chemical barrier that confines the bacteria into the xylem and makes its vessels resilient to pathogenic degradation.

"In our previous work, we identified the bottlenecks through which resistant tomato is able to limit R. solanacearum spread, uncovering that the xylem tissue is a major battleground for the interaction between vascular wilt pathogens and their hosts, where the outcome of the infection is at stake. Thanks to the collaboration with our colleagues at the Institute of Materials Science of Barcelona (ICMAB, CSIC), the Institute of Natural Resources and Agrobiology of Sevilla (IRNAS, CSIC) and Universitat de Girona, now we have been able to identify the intense structural and metabolic modifications the xylem vasculature of resistant plants undergoes in response to pathogens, preventing the bacterial colonization of the surrounding tissues and cells", points out Núria Sánchez Coll, CSIC researcher at CRAG in charge of this study.

Engineering tomato resistance to bacterial wilt

In line with the observed accumulation of ligno-suberin and related compounds in vascular coatings, further analysis showed that the genes involved in the synthesis pathways of these molecules were overexpressed in resistant plants infected with R. solanacearum. Based on these results, researchers set to determine whether overexpressing such genes in susceptible tomato plants would increase their resistance to bacterial wilt.

Our experiments demonstrate that overexpressing genes of the ligno-suberin pathway in a commercial susceptible variety of tomato provides a very effective resistance mechanism against R. solanacearum, drastically restricting bacterial spread and blocking the onset of disease."

Álvaro Luis Jiménez, PhD researcher at CRAG

"Interestingly, the accumulation of suberin has also been reported in response to drought, and the synthesis of ligno-suberin compounds is well-conserved across the plant kingdom. Therefore, engineering these pathways could have a double impact both on bacterial and drought resistance, improving plant performance in the field under adverse conditions", concludes Sánchez Coll.