Nitrate-induced ethylene biosynthesis and the control of nodulation in alfalfa

Abstract

We previously reported that inhibition of ethylene biosynthesis with aminoethoxyvinylglycine (AVG) eliminated the inhibitory effect of NO 3 -on nodulation of alfalfa (Medicago sativa L. cv. Aragon) plants grown aeroponically. In this work, the effect of Ag + , as an inhibitor of ethylene action, has been studied in plants growing aeroponically or in darkened tubes with vermiculite, and low-nitrate or high-nitrate solution. Vermiculite-grown plants developed up to 3 times as many nodules as did those growing aeroponically. Nodule formation was mirrored by dry-matter accumulation. High (10 mol m -3 ) NO 3 -applied from planting inhibited nodulation to an equal extent (c. 50%) in the two growth conditions. In contrast, Ag + treatment increased nodule formation at all NO 3 -concentrations assayed under the two growth conditions, with the stimulation being higher in plants grown aeroponically. Finally, no effect of Ag + (10 mmol m -3 ) on plant growth was observed in either of the growth conditions. The effectiveness of NO 3 -as a nodulation inhibitor and enhancer of ethylene biosynthesis in roots of alfalfa was also studied. Within 24 h after inoculation, 10 mol m -3 NO 3 -exerted most of its inhibitory effect on nodulation. At the same time, both 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase activity and ethylene evolution rates markedly increased in inoculated and uninoculated alfalfa roots treated with NO 3 -. Support for a role of endogenous ethylene in the control of nodule formation in legumes is discussed.Key-words: Medicago sativa; endogenous ethylene; growth system; nitrate; nodulation control. INTRODUCTIONNodule formation in legumes is tightly controlled by an internal mechanism called autoregulation, but the establishment and function of the symbiosis is also regulated by fixed nitrogen (mainly nitrate) in the soil (Streeter 1988;Carroll & Mathews 1990;Gresshoff 1993). Basically, autoregulation involves early infection events that systemically suppress the development into nodules of subsequent infected cell-division foci (Caetano-Anollés & Gresshoff 1990); the result is restriction of nodulation to the root crown. The shoot is believed to be the source of the autoregulation inhibitor (Delves et al. 1986). Supernodulating mutants, which lack this autoregulatory response and show prolific nodulation, have been generated for several legume species. It is feasible that such a regulatory mechanism operates in other nodulated legumes, although variation in the mode of action may also exist (see Postma, Jacobsen & Fenstra 1988;Gremaud & Harper 1989;Akao & Kouchi 1992;Sagan & Gresshoff 1996). For example, nodule detachment experiments in alfalfa and soybean revealed both additional regulatory mechanisms controlling nodule number and different stages at which autoregulation could be exerted (CaetanoAnollés & Gresshoff 1991).Nitrate inhibition of nodulation is a common phenomenon among legume species; however, considerable variation in the degree of regulation exists even between cultivars or accessions with...

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

“…Furthermore, if nitrogen fixation in the bacteroids is impaired by the exceedingly low O 2 levels, the plant which utilizes rhizobial nitrogen fixation as a unique source of nitrogen loses that nitrogen source. It is possible that the nitrate mechanism that is also known to regulate nodulation (Caba et al 1998;Schmidt et al 1999;Ferguson et al 2010) might be deregulated under waterlogging conditions as well.…”

Differential sensitivity of nodulation to ethylene in soybean cv. Bragg and a supernodulating mutant

“…The decrease in nodule mass with Ag + treatment observed here is not well understood. We know, however, that Ag + delays nodule organogenesis (Caba et al, 1998), and that nodules of Ag + -treated plants appeared smaller and more juvenile than the rest at harvest. A similar argument could be presented for the effect of low inoculum dose on nodule mass (Table 5).…”

“…nigra ; Zaat et al, 1989) and soybean (Sugamuna et al, 1995 ;Ligero et al, 1999). Ethylene inhibitors significantly increased nodulation in alfalfa (Ligero et al, 1991 ;Caba et al, 1998) and in soybean (Ligero et al, 1999), even in the presence of inhibitory levels of nitrate, and restored nodulation in some pea nodulation mutants (Fearn & LaRue, 1991). It has also been suggested that endogenous ethylene provides positional information controlling the site where nodule primordia are formed (Heidstra et al, 1997).…”

“…Solutions were always balanced for cations and anions with KCl. The culture tubes were wrapped with opaque paper foil (three layers) and the plants were grown in a growth chamber as described before (Caba et al, 1998), except that a 26\20mC day\night temperature cycle was used. Four days after planting (DAP) the translucent polypropylene closures of the tubes were removed and replaced with sterile cotton plugs surrounding the stems.…”

“…In separate plants (7 plants per treatment) acetylene reduction activity (ARA) was measured on entire plants upon harvest, according to Ligero et al (1991). Another set of plants was used to determine root ethylene evolution (10 plants per treatment) and ACC oxidase activity (8 plants) 48 h after treatment application, according to Caba et al (1998). Experiments (in all cases with a randomized block design) were conducted in triplicate.…”