Introduction

• Gousiekte

Gousiekte is rated one of the six most important plant poisonings in southern Africa. It is a disease of ruminants characterized by acute heart failure without premonitory signs 6-8 weeks after the initial ingestion of certain bacteriophilous Rubiaceae. Vangueria pygmaea is the most important of these plants, followed in descending order of importance by Fadogia homblei, Pavetta harborii, Vangueria thamnus, Pavetta schumanniana and Vangueria latifolia. The active principle of gousiekte plants has been isolated and identified as pavetamine.

• Bacterial endosymbiosis

Bacterial leaf symbiosis, characterized by the formation of leaf nodules in which bacterial symbionts are housed, is a rare and intimate interaction between plants and bacteria and has been known since 1902. The phenomenon has been reported in the monocot family Dioscoreaceae and in two eudicot families, i.e. Myrsinaceae and Rubiaceae. In the latter two families, the symbiotic cycle was shown to be closed. Rubiaceae have the largest number of species with bacterial leaf nodules, found in three, distantly related, genera, i.e. Pavetta, Psychotria and Sericanthe. Leaves of certain Fadogia and Vangueria species (formerly Pachystigma) from South Africa, do not have nodules, instead, they host their bacterial symbionts intercellularly in cavities between leaf mesophyl cells. It is hypothesized that this type of symbiosis is a precursor to the more specialized process of forming leaf nodules.

Scientific goals

The main objective is to achieve understanding in the relationship between plants, bacteria and gousiekte. This project can be broken down in six parts:

• Detection and identification of the endosymbionts in the six gousiekte-inducing species
• Screening other South African Fadogia, Pavetta and Vangueria for endosymbionts
• Reconstructing the phylogeny of the endosymbionts
• Reconstructing the phylogeny of the bacteriophilous Rubiaceae
• Investigating the bacterial life cycle in Pavetta schumanniana
• Determining whether the bacteria or the plants cause gousiekte

Completing these objectives will greatly improve our knowledge about the identity of the bacterial endosymbionts in non-nodulating representatives and about the evolution of bacterial leaf symbiosis in Rubiaceae.

This doctoral research project is supported with a financial grant from the Institute for the Promotion of Innovation by Science and Technology in Flanders (IWT). We work in close collaboration with Dr. Steven Dessein (National Botanical Garden of Belgium), Prof. Peter Vandamme (Laboratory of Microbiology, UGent), Prof. Els Prinsen (Laboratory of Plant Growth and Development, UAntwerpen) and Prof. Braam van Wyk (Curator of the H.G.W.J. Schweickerdt Herbarium, UPretoria).