Gene expression and enzymatic activity of invertases and sucrose synthase in Spiroplasma citri or stolbur phytoplasma infected plants

Plant pathogenic Mollicutes induce multiple symptoms: yellowing, floral abnormalities or growth aberrations. Mollicutes are also responsible for impaired partitioning of photoassimilates in plants: accumulation of soluble sugar in source organs and depletion of sugar in sink organs. However, the molecular mechanisms of pathogenicity are poorly understood. For Spiroplasma citri, sugar metabolism play an important role in pathogenicity: a mutant unable to import fructose is non-pathogenic, whereas a mutant unable to import glucose is still pathogenic. These data suggest that key enzymes of sugar metabolism could be deregulated. In plants, four main enzymes hydrolyse sucrose into glucose+fructose, and are susceptible to provide sugar to the mollicutes situated in the phloem sieve tubes: the vacuolar, cell-wall and neutral invertases, and the sucrose synthase. These enzymes are already known to be induced in response to biotic or abiotic stress. So, gene expression and enzymatic activity of these enzymes were studied in leaves of healthy or infected plants. Sugar concentration was also determined. Results showed that in contrast to gene expression, the enzymatic activities showed variation in the three pathosystems studied.