Resumen de Respostes de defensa desencadenades per ralstonia solanacearum a la vasculatura del xilema de l'arrel del tomàquet aprofundint en la via de síntesi de la suberina
Ralstonia solanacearum is one of the world’s most devastating bacterial pathogens of plants. This soil-borne bacterium is the causative agent of bacterial wilt on more than 450 plant species, and it causes severe devastation on Solanaceous crops like tomato. R. solanacearum gains access to the root system through wounds and lateral root emerging sites and then moves to the root vasculature, where it multiplies in the xylem and later spreads vertically within the xylem sap and horizontally between vessels and to the surrounding tissues. To date, the most reliable management strategy to control R. solanacearum has been the use of genetic resistance. In tomato, among the best resistance sources is the resistant cultivar Hawaii 7996 (H7996), which is commonly used as a rootstock in commercial tomato cultivation. This resistant cultivar has evolved effective defense mechanisms to prevent vessel colonization or movement between vessels once vascular colonization has occurred.
Recent reports show that the root xylem vascular cylinder acts as a predominant tissue for mounting an efficient defense response against vascular invaders, restricting onward movement of the pathogen to the aerial tissue. However, the mechanisms regulating this form of resistance remain elusive. Unravelling the mechanisms through which the plants block or slow down pathogen progression at the root xylem vasculature can be vital in the development of resistant cultivars by biotechnological interventions. Hence, in this thesis we attempted to shed light on the defense responses acting at the root xylem vasculature that effectively restrict colonization by the vascular bacterial pathogen R. solanacerum in tomato.
Comparative histopathological studies in resistant and susceptible tomato lines indicated that a ferulate vascular reinforcement, culminating into a ferulo-suberin zone, may act as a strong physico-chemical barrier against R. solanacearum invasion (Chapter 1). This phenolic- aliphatic barrier reinforces the walls of xylem vascular tissue in H7996 and may restrict movement of the bacterium from the xylem vessel lumen to the surrounding xylem parenchyma cells and nearby vessels and inter-cellular spaces. Compositional changes in walls were studied using spectroscopy, which showed strong accumulation of suberin- compatible metabolites specifically in roots of resistant H7996, upon infection of R. solanacearum (Chapter 2). Differences in the polymerization state of lignin in roots were observed after infection between resistant and susceptible tomato. H7996 contained a higher proportion of G-type lignin, which is more resistant to degradation, whereas susceptible Marmande contained more S-type lignin, which is more prone to degradation. Expression of genes from the suberin fatty acid biosynthetic pathway was significantly upregulated in the taproot xylem vascular tissue of H7996 infected plants compared to the mock controls or susceptible tomato (Chapter 3). This indicates that upregulation of these genes is a specific response of resistant H7996 plants that takes place in suberizing vasculature upon R. solanacearum infection. Further, induction of ProSlFHT::GUS was observed in taproot xylem vasculature of infected H7996 plants, as well as in tissues known to deposit suberin such as epidermis, exodermis and tissues undergoing wound healing.
Implications of overexpressing genes from pathway of suberin and the associated soluble phenolics synthesis, were evaluated in susceptible tomato background. Overexpression of suberin feruloyl transferase (FHT), which catalyzes the formation feruloyl esters showed limited restriction against R. solanacearum. In contrast, overexpression of tyramine N-hydroxycinnamoyltransferase (THT 1-3), responsible for synthesis of hydroxycinnamic acid amides (HCAA) such as feruloyl tyramine resulted in an increase of resistance against R. solanacearum with disease progressing remarkably slower in this line compared to wild type plants. Accumulation of such aminated phenolics may act as a chemical barrier acting as phytoalexin but could also act as physical barrier if cross-linked to the poly-aromatic domain of suberin.
