Alteration of distribution of regions with high net proton extrusion in tomato roots infected with Meloidogyne incognita

Abstract

Regions of net proton extrusion activity were localized by use of the fluorescent dye, fluorescein (pK a ≈ 4·9), which can enter plant cells with a localized low apoplastic pH. After incubation in a fluorescein solution for 16 h, the root systems were surface screened with UV-irradiation. Uninfected root systems fluoresced brightly at apical root regions (2–3 cm from the root tip), whereas root systems infected with M. incognita showed only slight fluorescence. In the latter case fluorescence was restricted to uninfected root regions distal to infection sites and young growing galls near the root apex. Older galls displayed a sharp transition zone between the active and the less active proton-extruding root regions. The nematode-induced giant cells accumulated fluorescein from the apoplast. Dissipation of the proton-motive force by carbonylcyanide m-chlorophenylhydrazone strongly suppressed dye uptake by the apical root regions and giant cells. It is concluded that net proton extrusion activity of infected apical root regions is decreased compared to similar uninfected ones. This decreased proton extrusion may have major consequences for the uptake and mobilization of minerals. Giant cells have an increased net proton extrusion activity compared to other cell types at the infection site, suggesting that proton extrusion might be a driving force for solute uptake from the apoplast.