Auxin transport inhibitors act through ethylene to regulate dichotomous branching of lateral root meristems in pine

Many soil fungi colonize the roots of pines to form symbiotic organs known as ectomycorrhizas. Dichotomous branching of short lateral roots and the fo rmation of coralloid organs are diagnostic of ectomycorrhizas in many pine species, although the regulation of these changes in root morphology is not well understood. We used axenic root cultures of six pine species to exami ne the role of auxin, cytokinin, ethylene and nutrients in the regulation o f root architecture. Surprisingly, extensive dichotomous and coralloid bran ching of lateral roots occurred spontaneously in Pinus taeda, P. halepensis and P. muricata. In P. sylvestris, P. ponderosa and P. nigra, treatment wi th auxin transport inhibitors (ATIs), the ethylene precursor 1-aminocyclopr opane-1-carboxylic acid (ACC) or the ethylene-releasing compound 2-chloroet hylphosphonic acid (CEPA or ethephon) induced extensive dichotomous branchi ng and coralloid organ formation. Formation of both spontaneous and ATI-ind uced coralloid structures was blocked by treatment with an ethylene synthes is inhibitor L-alpha-(2-aminoethoxyvinyl)glycine; this inhibition was rever sed by either ACC or CEPA. In addition, the induction of this unique morpho genetic pattern in pine root cultures was regulated by nutrient levels. The morphology and anatomical organization of the chemically induced dichotomo us and coralloid structures, as well as the regulation of their formation b y nutrient levels, show a striking similarity to those of ectomycorrhizas.