RAPID, PLATEAU-LIKE INCREASES IN INTRACELLULAR FREE CALCIUM ARE ASSOCIATED WITH NOD-FACTOR-INDUCED ROOT-HAIR DEFORMATION

Rhizobia excrete variously substituted lipo-oligosaccharide Nod factor s into the legume rhizosphere. Homologous legumes respond to these sig nals through deformation of the root hairs and the development of nodu lation foci in the root cortex, Cellular events in root hairs from the susceptible zone of nearly mature root hairs were studied in root seg ments 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, i n plateau-like increases in intracellular free calcium ([Ca2+](i)) in 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 an d epidermal cells of the nonhost V. unguiculata, and also induced root -hair deformation, The chitin tetramer, N-N'-N ''-N'''-tetracetylchito tetraose, which represents the backbone of Nod factors, induced neithe r root-hair deformation nor changes in [Ca2+](i) in V. unguiculata, Ro ot hairs and epidermal cells of the nonlegume nonhost Arabidopsis thal iana showed neither [Ca2+](i) increases nor root-hair deformation in r esponse to both factors.