Interaction between cucumber plants and the broad mite, Polyphagotarsonemus latus: From damage to defense gene expression

The broad mite (BM), Polyphagotarsonemus latus (Banks) (Acari: Tarsonemidae), is a minute polyphagous mite that has severe effects on the host plant. Mechanisms of BM injury and subsequent plant responses are unclear. In this study we characterized the morphological and molecular reactions of cucumber plants (Cucumis sativus L, Cucurbitaceae) to BM infestation. Infested plants showed growth inhibition and a decrease in leaf number and leaf area. There was also an increase in the firmness of the infested leaves, as measured by a texture analyzer. Broad mite feeds on the epidermis, but structural and ultrastructural studies revealed aberrations in the whole leaf tissue. Severe infestation led to a complete loss of epidermis and an increase in mesophyll cell size and number. In transmission electron microscope (TEM) images, the entire epidermal tissue appeared to have collapsed, and the mesophyll cell walls appeared thick and distorted. In the infested leaves, a Northern blot analysis revealed the induction of genes related to the jasmonic acid (JA)/ethylene and salicylic acid (SA) pathways, such as lipoxygenase (LOX) and β-1,3 glucanase (BGL2), and an induction of oxidative stress-responsive transcripts, such as peroxidase (PRX). Transcript levels of ACC oxidase (ACO) that participate in ethylene biosynthesis, remained relatively constant. This work reveals that BM feeding causes dramatic morphological, structural, and ultrastructural changes, along with an induction of genes involved in defense pathways. Further studies are needed to evaluate whether the observed changes in leaf structure and ultrastructure affect the mite, and how the induction of the defense pathways affects the susceptibility of plants to BM infestations.