SUGAR METABOLISM AND PATHOGENICITY OF SPIROPLASMA CITRI

SUMMARY Spiroplasma citri is a plant-pathogenic mollicute, phy- logenetically related to Gram-positive bacteria. Spiro- plasma cells are restricted to the phloem sieve elements and are transmitted by leafhopper vectors. Recent re- search 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 IIABC Fru , whereas the glucose permease is split into two polypeptides IICB Glc and IIA Glc encoded by two separated genes ptsG and crr. The glucose IICB Glc and trehalose IIBC Tre permeases function with a single IIA Glc domain, enabling the spiroplasma to rapid- ly adapt from glucose to trehalose and vice versa. Up- regulation 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 glu- cose and trehalose are still highly pathogenic. In con- trast, 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-regu- lation 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 fruc- tose is hypothesized to increase invertase activity, leading to glucose accumulation, and inhibition of photosynthe- sis. Both model mechanisms may contribute to S. citri pathogenicity.