The metabolism of carbohydrates in roots of cassava (Manihot esculenta Crantz) infected with frogskin disease

Cassava roots with symptoms of frogs kin disease (CFSD) characteristically possess longitudinal "lips" and a peel that has a cork-like aspect. Yields, comprising bulked and starchy roots, are low. Previous studies reported that the phytoplasma 16 SrIII-L is associated with CFSD. However, the plant- pathogen interaction and the pathogenic effects on the host are unknown. The study's objective was therefore to di scover the changes occurring in the metabolism of carbohydrates in cassava roots, both in the presence and absence of symptoms. Samples were take n from healthy and diseased roots of two cassava varieties: Vale ncia and Señorita. The diseased roots were selected for different levels of severity of att ack. Parameters were dry matter content and concentrations of organic acids, re ducing sugars, and total sugars, using HPLC liquid chromatography. Malic acid was found to be more highly concentrated in diseased roots, correlating with the amounts of reducing and total sugars, particularly fructose ( R 2 = 95%). Because of the small quantity of stored starch, the percentage of dry matte r in diseased roots is less than that found in healthy roots. That is, in the diseased roots, starch probably converts into fructose and glucose during the Krebs cycle. These findings contribute towards understanding the pathogenic effects of phytoplasma 16 SrIII-L in cassava roots attacked by CFSD. They are also consistent with reports on phytoplasmas attacking other plant species. (Texte intégral