Suppression of Nodule Development of One Side of a Split-Root System of Soybeans Caused by Prior Inoculation of the Other Side

Abstract

In a split-root system of soybeans (Glycine max L. Merr), inoculation of one half-side suppressed subsequent development of nodules on the opposite side. At zero time, the first side of the split-root system of soybeans received Rhizobium japonicum strain USDA 138 as the primary inoculum. At selected time intervals, the second side was inoculated with the secondary inoculum, a mixture of R. japonicum strain USDA 138 and strain USDA 110. In a short-day season, nodulation by the secondary inoculum was inhibited 100% when inoculation was delayed 10 days. Nodulation on the second side was significantly suppressed when the secondary inoculum was delayed for only 96 hours. In a long-day season, nodule suppression on the second side was highly significant, but not always 100%. Nodule suppression on the second side was not related to the appeaance of nodules or nitrogenase activity on the side of splitroots which were inoculated at zero time. When the experiments were done under different light intensities, nodule suppression was significantly more pronounced in the shaded treatments.The nitrogen-fixing symbiotic association formed between leguminous plants and soil bacteria of the genus Rhizobium has recently become an area of intense scientific research because of the economic and agricultural benefits derived from cropping systems using nodulated legumes. One of the specific goals of recent investigations has been to understand the mechanisms involved in the formation of the legume-Rhizobium association. Although both the host plant and the bacteria contribute to the specificity of the association (6), the mechanisms by which each partner exerts its influence remain poorly understood (1,4,13

The nutritional control of root development

“…The number of nodules on a root system is controlled by a mechanism called 'autoregulation', in which previously formed or forming nodules suppress the development of further nodules (Schultze and Kondorosi, 1998). Split-root experiments have established that autoregulation acts systemically and that the autoregulatory signal originates in the shoot (Kosslak and Bohlool, 1984). However in addition to autoregulation, or perhaps superimposed upon it, is a strongly suppressive effect of combined N (especially NO 3 − ) which legumes will utilise as a N source in preference to forming the N-fixing symbiosis (Carroll and Mathews, 1990).…”

Root-derived CLE glycopeptides control nodulation by direct binding to HAR1 receptor kinase

“…The initial step of autoregulation is thought to involve longdistance signalling from the root to the shoot [5][6][7] . This hypothesis was strongly supported by the findings from analyses of the Lotus japonicus hypernodulating mutant, hypernodulation aberrant root formation (har1) [8][9][10] , and orthologous mutants in other leguminous species 8,9,11,12 .…”

Partitioning of 14C‐labelled photosynthate to developing nodules and roots of soybean (Glycine max)

“…Caetano-AnoUes and Gresshoff (1990 postulated that early rhizobial infections induce the synthesis of a shoot-derived inhibitor (SDI), which then suppresses nodule development from later infections, Supernodulating soybean mutants apparently lack, or have much decreased, synthesis of SDI (Olsson et al, 1989. Francisco & .\kao, 1993, Francisco & Harper, 1995, Recent results of Francisco & Harper (1995) suggest that inhibitory compound(s), synthesized in the leaf, control nodulation phenotype independently of the root, Francisco & Akao (1993) proposed a late-acting nodulation control mechanism that is apparently unrelated to autoregulation and is observed in wild type and in supernodulating soybean, Kosslak & Bohlool (1984) used different levels of shading to show that nodule number per plant and the intensity of the regulatory response were directly related to the amount of photosynthetically active radiation available to soybean. These results suggested a role for photosynthate partitioning in regulation of root nodule development, similar to that reported for regulation of axillary bud and inflorescence development (reviewed by Wardlaw, 1990), Available experimental evidence suggests that at least two distinct phenomena are in\ olved in regulation of nodulation in legumes.…”

“…This control bean, proliferation of rhizobia within early nodules is mechanism, known as autoregulation or feedback apparently required to suppress the development of suppression (reviewed in Caetano-Anolles & Gress-late infections (George & Robert, 1991). hofr, 1991a, GresshofF& Caetano-Anolles, 1991 has Nutman (1948Nutman ( , 1949 proposed that nodule debeen demonstrated in common \egume/Rhizobium velopment in red clover was controlled by inhibitors symbioses (Nutman, 1948(Nutman, , 1949Pierce & Bauer, produced by early nodule and root meristems, and 1983; Kosslak & Bohlool, 1984;Sargent ff a/., 1987; showed that excision of early nodules and root tips George & Robert, 1991) and also in actinorhizal stimulated further nodulation (Nutman, 1952). associations (Dobritsa & Novik, 1992), AutoreguSimilar results were obtained with aJfaJfa (Caetanolation mechanisms might vary with plant species.…”

“…First, autoregulation of nodule initiation, which occurs early in the infection process, is apparently under physiological and genetic control, imposed by signal molecules that are normal products of bacterial (Takats, 1990;Caetano-Anolles & Bauer, 1988;re\iewed in Caetano-Anolles & Gresshoff, 1991(3) and plant (reviewed in GresshofT & Caetano-Anolles, 1991) genes. Second, regulation of maturation of nodule meristems into functional nodules might involve short distance and systemic interactions (CaetanoAnolles et al 1991 ;Francisco & Akao, 1993), as well as physiological controls imposed by the availability of carbon (Kosslak & Bohlool, 1984) and nitrogen (Atkins et al, 1989) for nodule development. The dependence of N^ fixation on the supply of carbohydrate from photosynthesis has been clearly demonstrated (Walsh, Vessey & Layzel, 1987).…”