Potato resistance to soft-rot erwinias is related to cell wall pectin esterification

Abstract

Resistance to soft-rot erwinias of two siblings (4708 and 4680) from a backcross between a Solanum brevidens × S. tuberosum subsp. tuberosum somatic hybrid and cv. Katahdin and of three commercial potato cultivars (Record, Estima and Katahdin) was examined. Tuber cortex was generally more resistant than the medulla when inoculated with 103 cells ml-1 of Erwinia carotovora subsp. atroseptica (strain SCRI 1039). The extent of rotting in all genotypes was similar for each of the two tissues whether under aerobic or anaerobic conditions. The only exception was 4680 in which lesions were considerably smaller. A similar resistance pattern was obtained with stem tissue of the different potatoes, and with E. carotovora subsp. carotovora and E. chrysanthemi. The percentage esterification of pectin in cell wall extracts was generally greater in the tuber cortex than in the medullary tissue and was 2·4 and 1·6 times greater in the medulla (69·4%) and cortex (94·1%) of 4680 respectively than in corresponding tissues of the other genotypes which were all similar. Pectin esterification in stem cell walls was twice as great in 4680 (69·3%) than in the other potatoes. Levels of Ca2+ in tubers were greater in the cortex than in the medulla but were not different between genotypes, including 4680. Pectate lyase (PL) activity of SCRI 1039 was low with cell wall extracts of 4708 and 4680 as substrate but increased in the presence of pectin methyl esterase (PME), especially with higher concentrations of substrate from 4680. Discs of tuber cortex were also more resistant to maceration by PL than discs from the medulla, while medullary and cortical tissues of 4680 were respectively 2·1 and 1·7 times more resistant to maceration than the corresponding tissues of the other genotypes. Maceration was generally more rapid if PME was also present. The results suggest that there is an association between tuber resistance to erwinias and pectin esterification which, as a preformed resistance characteristic, would be operative under anaerobic conditions in which disease usually develops.