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Rapid, Plateau-like Increases in Intracellular Free Calcium Are Associated with Nod-Factor-Induced Root-Hair Deformation

¹School of Botany, La Trobe University, Melbourne Victoria 3083, Australia; ²School of Zoology, La Trobe University, Melbourne Victoria 3083, Australia; ³Department of Pharmaceutical Biology & Pharmacology, Victorian College of Pharmacy, Monash University, Parkville Campus, 381 Royal Parade, Melbourne Victoria 3052, Australia; ⁴Laboratoire de Biologie Moléculaire des Plantes Supérieures, Université de Genève, 1 chemin de l'Impératrice, 1292 Chambésy-Genève, Switzerland; ⁵New address: School of Biological & Chemical Sciences, Deakin University, Geelong campus, Geelong, Victoria 3217, Australia

Rhizobia excrete variously substituted lipo-oligosaccha-ride Nod factors into the legume rhizosphere. Homologous legumes respond to these signals through deformation of the root hairs and the development of nodulation foci in the root cortex. Cellular events in root hairs from the susceptible zone of nearly mature root hairs were studied in root segments loaded with the calcium indicators Fura-2 or Fluo-3. Application of 10^-9 M Nod factors of the broad-host-range Rhizobium sp. NGR234 to the homologous legume Vigna unguiculata resulted, within seconds, in plateau-like increases in intracellular free calcium ([Ca²⁺]_i) in the root hairs and root epidermal cells. Nod factors of R. meliloti at 10^-9 M caused equally rapid increases in [Ca²⁺]_i in the root hairs and epidermal cells of the nonhost V. unguiculata, and also induced root-hair deformation. The chitin tetramer, N-N′-N″-N′″-tetracetylchitotetraose, which represents the backbone of Nod factors, induced neither root-hair deformation nor changes in [Ca²⁺]_i in V. unguiculata. Root hairs and epidermal cells of the nonlegume non-host Arabidopsis thaliana showed neither [Ca²⁺]_i increases nor root-hair deformation in response to both factors.