Susceptibility of carrot cultivars to Mycocentrospora acerina and the structure of cell wall polysaccharides

Abstract

The role of cell wall polysaccharides in pre-formed resistance to necrotrophic fungi is poorly understood. The relationships between cell wall polysaccharides, especially the middle lamella pectin, of four carrot cultivars and their susceptibility to Mycocentrospora acerina , a cold storage pathogen, were investigated. Amounts of cell wall material and composition of the pectic fraction (galacturonic acid, neutral sugars) were not correlated with cultivar resistance. However, amounts of acid-insoluble pectin (protopectin) increased with the resistance of the cultivar. Maceration in vivo of carrot tissues from different cultivars by M. acerina enzyme preparations, mainly polygalacturonases, correlated with their susceptibility to the pathogen. In the same way, the solubilization rate in vitro by M. acerina enzymes of pectic material isolated from the most susceptible cultivar was 1·8 times higher than from the least susceptible cultivar. Hydrolysis of purified middle lamella pectin by endopolygalacturonase confirmed this correlation. Although the degrees of methoxylation of carrot cultivar pectins were similar, their behaviour towards endopolygalacturonase was different, indicating a variation in the distribution of the methoxyl groups along the pectin chain. The results showed a similar composition of cell wall polysaccharides in the different cultivars but the solubility and esterification characteristics of pectins suggest that pectic substances have an important role in constitutive resistance of carrot to M. acerina .