A model for the development of the rhizobial and arbuscular mycorrhizal symbioses in legumes and its use to understand the roles of ethylene in the establishment of these two symbioses

Abstract

We propose a model depicting the development of nodulation and arbuscular mycorrhizae. Both processes are dissected into many steps, using Pisum sativum L. nodulation mutants as a guideline. For nodulation, we distinguish two main developmental programs, one epidermal and one cortical. Whereas Nod factors alone affect the cortical program, bacteria are required to trigger the epidermal events. We propose that the two programs of the rhizobial symbiosis evolved separately and that, over time, they came to function together. The distinction between these two programs does not exist for arbuscular mycorrhizae development despite events occurring in both root tissues. Mutations that affect both symbioses are restricted to the epidermal program. We propose here sites of action and potential roles for ethylene during the formation of the two symbioses with a specific hypothesis for nodule organogenesis. Assuming the epidermis does not make ethylene, the microsymbionts probably first encounter a regulatory level of ethylene at the epidermis – outermost cortical cell layer interface. Depending on the hormone concentrations there, infection will either progress or be blocked. In the former case, ethylene affects the cortex cytoskeleton, allowing reorganization that facilitates infection; in the latter case, ethylene acts on several enzymes that interfere with infection thread growth, causing it to abort. Throughout this review, the difficulty of generalizing the roles of ethylene is emphasized and numerous examples are given to demonstrate the diversity that exists in plants.Key words: AM, epidermis, evolution, pea, rhizobia, sym mutant.

Invasion of Lotus japonicus root hairless 1 by Mesorhizobium loti Involves the Nodulation Factor-Dependent Induction of Root Hairs

“…The cortical program, defined by the initiation of root cortical cell divisions and subsequent organogenesis of NP (Guinel and Geil, 2002), is induced efficiently by a NF-producing strain of M. loti on roots of L. japonicus double mutant lines belonging to all four root hair mutant classes. With one notable exception, namely Ljvrh1-1 har1-1 (see below), abundant organogenesis of NP occurs within the first few days after inoculation with M. loti.…”

Regulação do desenvolvimento de micorrizas arbusculares

“…Porém, deve-se considerar que uma das limitações da utilização do ethrel como fonte de ET para estudos de regulação de MAs é a liberação de fosfato, que inibe a formação da simbiose (Smith & Read, 1997). Em estudos semelhantes, decréscimos na colonização de raízes de ervilha e alho-porró por FMAs com o aumento da quantidade de ET aplicada, em baixas concentrações de P, também foram observados (Geil et al, 2001;Geil & Guinel, 2002;Guinel & Geil, 2002). Contudo, Ishii et al (1996), usando Poncirus trifoliata inoculada com Gigaspora ramisporophora, demonstraram que o ET pode tanto inibir quanto estimular a formação da MA, dependendo da concentração aplicada: em concentrações muito baixas de ET (0,05 μL L -1 ), há aumento da colonização das raízes de P. trifoliata por G. ramisporophora, enquanto em altas concentrações de ET (1,0 μL L -1 ) a taxa de colonização das raízes diminui.…”

Enhanced chickpea growth-promotion ability of a Mesorhizobium strain expressing an exogenous ACC deaminase gene

“…Rhizobial species producing ACC deaminase can promote nodulation of their host legumes (Ma et al 2003a(Ma et al , 2004Uchiumi et al 2004), since they decrease ethylene levels that are known to inhibit nodule formation in many leguminous plant species (Guinel and Geil 2002;Gage 2004).…”