A Positive Role for Rhizobitoxine inRhizobium-legume Symbiosis

Ethylene (C2H4), the chemically simplest plant hormone, is among the best-characterized plant growth regulators. It participates in a variety of stress responses and developmental processes. Genetic studies in Arabidopsis have defined a number of genes in the ethylene signal transduction pathway. Isolation of two of these genes has revealed that plants sense this gas through a combination of proteins that resemble both prokaryotic and eukaryotic signaling proteins. Ethylene signaling components are likely conserved for responses as diverse as cell elongation, cell fate patterning in the root epidermis, and fruit ripening. Genetic manipulation of these genes will provide agriculture with new tools to prevent or modify ethylene responses in a variety of plants.

Development of the Rhizobium-legume symbiosis is controlled by the host plant, although the underlying mechanisms have remained obscure. A mutant in the annual legume Medicago truncatula exhibits an increase of more than an order of magnitude in the number of persistent rhizobial infections. Physiological and genetic analyses indicate that this same mutation confers insensitivity to the plant hormone ethylene for multiple aspects of plant development, including nodulation. These data support the hypothesis that ethylene is a component of the signaling pathway controlling rhizobial infection of legumes.

Ag(I), applied foliarly as AgNO(3), effectively blocked the ability of exogenously applied ethylene to elicit the classical "triple" response in intact etiolated peas (Pisum sativumcv. Alaska); stimulate leaf, flower, and fruit abscission in cotton (Gossypium hirsutumcv. Stoneville 213); and induce senescence of orchids (Hybrid white Cattleya, Louise Georgeianna). This property of Ag(I) surpasses that of the well known ethylene antagonist, CO(2), and its persistence, specificity, and lack of phytotoxicity at effective concentrations should prove useful in defining further the role of ethylene in plant growth.