Analysis of xyloglucan-modifying genes and enzymes at graft-junctions of tomato plants

Abstract

Grafting is a widely used technique for propagation of plants in both agriculture and horticulture worldwide and involves combining two or more plants into one chimeric plant, ultimately with a shared vascular system. Benefits of grafting such as pathogen resistance, increase in fruit yield and abiotic stress tolerance have long been known, but the biological mechanisms of how plants graft are still not fully understood. The process of graft development is interestingly similar to that of the host infection process of parasitic plants. This knowledge has contributed to viewing the parasitic plants as a blueprint for optimal graft development, as the parasites can bypass a taxonomic barrier that seem to exist for conventional grafting. Previous research has revealed interesting knowledge on the involvement of xyloglucan endo-transglycosylase/hydrolase (XTH) enzymes, a group of enzymes involved in loosening and tightening the plant cell wall through the modification of xyloglucan. Specifically, the gene Cr-XTH-1 in Cuscuta reflexa has been revealed to play a role in the infection process. The aim of this thesis was to investigate similarities between the graft formation in tomato plants and the infection of Cuscuta with respect to xyloglucan-associated activities. Graft success rates between the tomato lines Solanum lycopersicum cv. Moneymaker and M82, Solanum pennellii and two transgenic lines were assessed. The importance of a specific XTH gene in grafting was investigated using the two transgenic tomato lines, one over-expressing SlXTH1, the other suppressing SlXTH1 expression. Xyloglucan endo-transglycosylation activity (XET) was assessed using an in vivo activity assay on sectioned graft sites. The gene SlXTH1 was chosen along with its close homolog, SlXTH4, and a tomato XTH similar to Cr-XTH-1: SlXTH12, to investigate the gene expression of specific XTHs at the graft sites using RT-qPCR. The gene expression revealed no clear pattern between the three XTHs in relation to grafting. XET activity was observed to be graft specific based on this study design, with little difference between the genotypes. Several genotypes of tomato all showed XET activity at, or in cells with close proximity to the graft union, indicating that XTHs are involved in the grafting process. The findings of this thesis are proposed to be used as preliminary data and encourage further investigation of the effect of XTHs and XET activity at the graft interface.