Grapevine can be severely affected by phytoplasmas, which are phytopathogenic Mollicutes invading the sieve elements of the host plant. The biochemical and molecular relationships between phytoplasmas and their hosts remain largely unexplored. Equally unknown is an interesting aspect of the pathogen-plant interaction called "recovery," which is a spontaneous remission of symptoms in previously symptomatic plants. Recovered plants develop resistance mechanisms correlated with ultrastructural and biochemical changes in the sieve elements.
Callose as well as
sugars are involved in several plant defense processes and signaling. In the present work we have examined the possible involvement of
callose, as well as
callose synthase,
sugar transporter, and cell wall
invertase genes, during the
infection and after "recovery" of grapevine from bois noir (BN). Ultrastructural investigation of leaf tissue showed that
callose accumulated in the sieve elements of diseased grapevine; moreover, two genes encoding for
callose synthase were up-regulated in the infected leaves. Regarding
sucrose, expression analysis showed that
sucrose transport and cleavage were severely affected by BN phytoplasma, which induced the establishment of a
carbohydrate sink in the source leaf, and was analogous to other obligate biotrophs that acquire most of their nutrients from the host plant. Interestingly, whereas in recovered plants the transcript level of
sucrose synthase was similar to healthy plants,
sucrose transporters as well as cell wall
invertase were expressed to a greater degree in recovered leaves than in healthy ones. Recovered plants seem to acquire structural and molecular changes leading to increases in
sucrose transport ability and defense signaling.