SUGAR METABOLISM AND PATHOGENICITY OF SPIROPLASMA CITRI
Spiroplasma citri is a plant-pathogenic mollicute, phylogenetically related to Gram-positive bacteria. Spiroplasma cells are restricted to the phloem sieve elements and are transmitted by leafhopper vectors. Recent research has allowed depicting a unique scenario in S. citri pathogenicity, where sugar metabolism plays a major role. In vitro S. citri uses fructose, glucose, and trehalose, which are imported through the Phosphoenolpyruvate phosphoTransferase System (PTS). When both fructose and glucose are present, fructose is used preferentially. The fructose-PTS permease is made of one single polypeptide IIABCFru, whereas the glucose permease is split into two polypeptides IICBGlc and IIAGlc encoded by two separated genes ptsG and crr. The glucose IICBGlc and trehalose IIBCTre permeases function with a single IIAGlc domain, enabling the spiroplasma to rapidly adapt from glucose to trehalose and vice versa. Upregulation of expression of the PTS permease genes by the relevant sugar also illustrates the adaptive capacity of the spiroplasma. S. citri mutants unable to import glucose and trehalose are still highly pathogenic. In contrast, mutants unable to import fructose induce very mild and delayed symptoms, indicating that fructose and glucose play distinct roles in spiroplasmal pathogenicity. While fructose is used as a substrate by the spiroplasma to grow, glucose accumulates in source leaves of the host plant, leading to physiological disorders and down-regulation of photosynthesis genes. Fructose utilization by the spiroplasmas is postulated to deprive companion cells of fructose, thereby impairing sucrose loading in the sieve elements. Alternatively, preferential use of fructose is hypothesized to increase invertase activity, leading to glucose accumulation, and inhibition of photosynthesis. Both model mechanisms may contribute to S. citri pathogenicity.