Different Polyphenolic Parenchyma Cell and Phloem Axial Resin Duct-Like Structure Formation Rates in Cupressus sempervirens Clones Infected with Seiridium cardinale.

ABSTRACT

The aim of this study was the characterization of constitutive and induced defense mechanisms in the bark tissues of Cupressus sempervirens before and after infection with the bark fungus Seiridium cardinale, which is responsible for cypress canker disease. The time-course development of polyphenolic parenchyma (PP) cells and phloem axial resin duct (PARD)-like structures in the phloem was investigated in two C. sempervirens clones, one resistant and one susceptible to the disease, through anatomical and histological observations carried out by light microscope during a 19-day trial. PP cells were constitutively more abundant in the canker-resistant clone (R clone) compared with the susceptible clone (S clone), whereas PARD-like structures were not present in the bark of untreated plants of both clones. PP cells increased in both clones as a response to infection, but in the R clone, they were more abundant 5 and 12 days after inoculation. After inoculation, PARD-like structures appeared in the phloem after 5 days in the R clone and only after 12 days in the S clone. Even the number of cells surrounding the PARD-like structures was higher in the R clone 5 and 12 days after inoculation compared with the S clone. These observations demonstrate a faster phloem response of the R clone in the early phase of the infection. This may slow down initial growth of the fungus, contributing to the resistance mechanism.