Laser microdissection of grapevine leaf phloem infected by stolbur reveals site‐specific gene responses associated to sucrose transport and metabolism

ABSTRACT Bois Noir is an emergent disease of grapevine that has been associated to a phytoplasma belonging to the XII‐A stolbur group. In plants, phytoplasmas have been found mainly in phloem sieve elements, from where they spread moving through the pores of plates, accumulating especially in source leaves. To examine the expression of grapevine genes involved in sucrose transport and metabolism, phloem tissue, including sieve element/companion cell complexes and some parenchyma cells, was isolated from healthy and infected leaves by means of laser microdissection pressure catapulting (LMPC). Site‐specific expression analysis dramatically increased sensitivity, allowing us to identify specific process components almost completely masked in whole‐leaf analysis. Our findings showed decreased phloem loading through inhibition of sucrose transport and increased sucrose cleavage activity, which are metabolic changes strongly suggesting the establishment of a phytoplasma‐induced switch from carbohydrate source to sink. The analysis focused at the infection site also showed a differential regulation and specificity of two pathogenesis‐related thaumatin‐like genes (TL4 and TL5) of the PR‐5 family. Bois Noir is an emergent disease of grapevine that has been associated to a phytoplasma belonging to the XII‐A stolbur group. Since the molecular interactions between the pathogen and the plant host are still largely unclear, phloem tissue was isolated from healthy and infected leaves by means of Laser Microdissection and expression of plant genes involved in sucrose transport and metabolism examined. Site‐specific expression analysis revealed inhibition of sucrose transport and possibly phloem loading, together with increased sucrose cleavage activity. These metabolic changes strongly suggest the establishment of a phytoplasma‐induced switch from carbohydrate source to sink.