LAUSR

Colonization of the xylem of sugarcane by Leifsonia xyli subsp. xyli (Lxx) results in the occlusion of the vessels by a gum-like compound and compromises the elongation of the stalk leading to stunted plants. However, no study has been performed in the apical tissue where the elongation of the stalks initiates at the intercalary meristem (IM). Microscopic and histochemical analyses were performed in plants with lower and higher bacterial titers and revealed that in both cases Lxx is present in this tissue and colonizes the forming xylem vessels in a similar way as observed in developed internodes. In both cases it was observed adhering to the secondary walls, but only in plants with higher titers a mild degradation of the walls and a granular material filling the vessels were observed. The mixed composition of lipids, proteins and pectin indicates that the filling is not a bacterial extracellular polymeric substance. Plants with higher bacterial populations also presented lower starch content in the ground parenchyma at the node elements, possibly resulting from the reported downregulation of photosynthesis, and increased accumulation of phenolics. Their second and third IMs presented fewer cells and reduced expression of genes related to the cell cycle and to the synthesis of ABA in the apical tissue. These results indicate that increased Lxx colonization affects the development of the intercalary meristem which ultimately would reduce the length of the internodes, resulting in the main symptom of the disease.