Ethylene inhibits nodulation in various legumes. In order to investigate strategies employed by Rhizobium to regulate nodulation, the 1-aminocyclopropane-1-carboxylate (ACC) deaminase gene was isolated and characterized from one of the ACC deaminase-producing rhizobia, Rhizobium leguminosarum bv. viciae 128C53K. ACC deaminase degrades ACC, the immediate precursor of ethylene in higher plants. Through the action of this enzyme, ACC deaminase-containing bacteria can reduce ethylene biosynthesis in plants. Insertion mutants with mutations in the rhizobial ACC deaminase gene (acdS) and its regulatory gene, a leucine-responsive regulatory protein-like gene (lrpL), were constructed and tested to determine their abilities to nodulate Pisum sativum L. cv. Sparkle (pea). Both mutants, neither of which synthesized ACC deaminase, showed decreased nodulation efficiency compared to that of the parental strain. Our results suggest that ACC deaminase in R. leguminosarum bv. viciae 128C53K enhances the nodulation of P. sativum L. cv. Sparkle, likely by modulating ethylene levels in the plant roots during the early stages of nodule development. ACC deaminase might be the second described strategy utilized by Rhizobium to promote nodulation by adjusting ethylene levels in legumes.Gram-negative soil bacteria that belong to the family Rhizobiaceae are well-known for their ability to infect the root tissues of their compatible host legumes and induce the formation of nitrogen-fixing nodules (34). For more than a decade, the phytohormone ethylene has been known to inhibit nodulation in various legumes (16,18,22,26). Decreased levels of nodulation have been observed after application of exogenous ethylene or 1-aminocyclopropane-1-carboxylic acid (ACC) prior to or at the same time as the addition of rhizobia (18,22); conversely, nodulation can be promoted when plants are treated with ethylene inhibitors or antagonists (18,22,26,38).The fate of rhizobial infection in the root hairs of legumes has been proposed to be regulated by the levels of ethylene in the underlying plant cortex (13); a low level of ethylene, allowing proper disposition of the cytoskeleton, is probably required for successful entry of the infection thread in the outermost layer of cortical cells, whereas higher levels of the hormone induce abortion of the infection thread by inducing cross-linking of its matrix glycoproteins. This hypothesis is substantiated by numerous types of evidence. For example, sickle, an ethylene-insensitive mutant of barrel medic (Medicago truncatula), has very high persistence of infection threads and a hypernodulation phenotype (23); on the other hand, in brz, a potential ethylene-oversensitive mutant of Pisum sativum, the number of aborted infection threads is much higher than the number in wild-type plants (15).On an evolutionary basis, it would have been beneficial for the rhizobia to develop mechanisms by which the levels of plant endogenous ethylene are reduced. One rhizobial species, Bradyrhizobium elkanii, appears to have done this ...

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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.

Nodules in the family Leguminosae come in many different shapes and forms, depending partly on the host that bears them. This review focuses on the peripheral tissues that surround the infected tissues of four anatomically distinct classes of nodules: nodules with indeterminate meristems (from the Trifolieae and Fabeae tribes); desmodioid nodules with determinate meristems (from the Phaseoleae and Loteae tribes); aeschynomenoid nodules (Arachis hypogaea L., peanut); and lupinoid nodules (Lupinus sp. L., lupine). I have especially stressed the importance of the three-dimensionality of the organ, because not all nodules display a radial symmetry as it is often assumed. The goal of this review is to provide a strong base of nodule structure so that forthcoming molecular studies can integrate this information into their approach.

The dry weight (0.1%) and water potential ‐7 kPa) of root‐cap mucilage from 3‐d‐old axenically grown maize seedlings have been determined. The results suggest strong gelling properties and weak water‐holding capacity for the mucilage. Root tips from seedlings grown under low or high water stress were fixed by freeze‐substitution. Micrographs showed that in both conditions, mucilage was secreted into the periplasmic space and extruded through the cell wall, though in dry conditions, the mucilage was tightly pressed against the root‐cap surface. Histochemical and structural evidence is presented indicating chemical changes in the composition of the mucilage upon extrusion and a sharp increase in its hydration at increasing distance from the secretory cells. The possible functions of the root‐cap mucilage in the rhizosphere are examined in light of these findings.