NEW MODEL OF THE CELLULAR REACTION OF GIBBERELLA PULICARIS TO PHYTOANTICIPINS AND PHYTOALEXINS OF POTATO
K-M WELTRING and K LOSER
Institut für Botanik, Westf. Wilhelms-Universität, Schlossgarten 3, D-48149 Münster, Germany
Backround and objectives
Gibberella pulicaris is a causal agent of potato dry rot. The fungus enters the tubers via wounds, where it is confronted with phytoanticipins such as the saponins chaconine and solanine, and phytoalexins such as the sesquiterpenes rishitin and lubimin. When colonizing the tubers the fungus has to cope with these defence compounds using detoxification or other tolerance mechanisms. G. pulicaris is able to metabolize rishitin and lubimin  as well as the saponins  to less toxic compounds. To learn more about the cellular reaction of G. pulicaris to rishitin, apart from detoxification of this compound, we set out to identify genes specifically expressed in response to rishitin exposure.
Results and conclusions
A genomic and a cDNA library were differentially screened with cDNA prepared from mRNA isolated from rishitin induced and non-induced mycelium. Nine rishitin-induced (rin) genes were isolated which showed an increased expression in response to the phytoalexin. The sequence of five genes revealed homology to known sequences such as fimbrin and ATP citrate-lyase. Especially interesting was the identification of a polyubiquitin gene and a HMG14/17 gene. Genes of the high-mobility-group classes 14 and 17 seem to be involved in increasing the efficiency of transcription by binding to nucleosomes in active chromatin. Polyubiquitin has been found in yeast and other organisms to be upregulated in response to environmental stress. Therefore the induction of these two genes might indicate a switch to stress metabolism in response to rishitin.
Another homology has been identified to multi-facilitator-superfamiliy (MFS) proteins of the multidrug-resistance type found in yeast. These trans-membrane proteins are proton gradient-driven sym- or antiporters which transport toxins out of the cell, thus possibly representing an important factor in resistance to xenobiotics. The induction of a MFS-MDR type gene in response to rishitin could serve as an explanation for the non-degradative tolerance of fungi to phytoalexins . Based on the deduced function of the identified genes and the results obtained for the saponin metabolism by G. pulicaris , a new model of the cellular reaction of the fungus to defence compounds of potato is proposed.
1. Desjardins AE, Gardner HW, Weltring K-M, 1992. Journal of Industrial Microbiology 9, 201-211.
2. Weltring K-M, Wessels J, Geyer R, 1997. Phytochemistry 46, 1005-1009.
3. Denny TP, Matthews PS, VanEtten HD, 1987. Physiological and Molecular Plant Pathology 30, 93-107.