Phloem injury and repair following potato leafhopper feeding on alfalfa stems

We examined phloem injury and repair over an 8-day period following probing (feeding with piercing-sucking mouthparts inserted in plant) by the potato leafhopper, Empoasca fabae (Harris) (Homoptera: Cicadellidae), in stems of alfalfa, Medicago sativa L. A videomicrography technique was used to apply standard-duration probing by potato leafhoppers on alfalfa stems. Leafhopper-induced plant responses, observed using transmission electron microscopy and fluorescence microscopy, included phloem cell wall loosening and collapse, increased cytoplasmic density and dissolution of necrotic phloem cells, enlargement of nuclei and nucleoli in surviving cells, increased mitotic activity, thickening of phloem cell walls, formation of short and irregularly shaped wound sieve elements (often with side-wall sieve plates), accelerated chloroplast maturation and formation of abnormally large starch grains, and generation of phloem transfer cells from companion cells. Aniline blue staining of callose in sieve elements revealed that wound sieve tubes circumvented damaged phloem by 8 days after leafhopper-induced injury. These new sieve tubes often developed in the interfascicular area adjacent to the wounded bundles.Key words: plant wound response, wound phloem, sieve element, transfer cell, leafhopper feeding, Empoasca fabae, Medicago sativa.