A Supernodulation and Nitrate-Tolerant Symbiotic (nts) Soybean Mutant

Abstract

The nodulation characteristics of soybean (Glycine max) mutant nts382 are described. The mutant nodulated significantly more than the parent cultivar Bragg in the presence and absence of several combined nitrogen sources (KNO3, urea, NHsCI, and NH4NO3). The number of nodules on the tap root and on lateral roots was increased in the mutant line. In the presence of KNO3 and urea, nitrogenase activity was considerably higher in nts382 than in Bragg. Mutant medium (20). To a lesser extent, other sources of combined nitrogen have also been shown to inhibit nodulation (7,8,10, 24,27). The extent of this inhibition does vary considerably with the legume species, the form of combined nitrogen administered, and the experimental system. Some sources of combined nitrogen, for example urea or NH4', cause acidification of the growth medium (14), and the inhibitory effects of these nitrogen sources may be mediated indirectly through a reduction in pH rather than through the nitrogen status of the plant. To circumvent this complication, Vigue et al. (30) In the absence of externally supplied combined nitrogen, nodulation is tightly regulated with the number ofinfections greatly exceeding the final number of mature nodules (2). Interruption of invasion is related to the effectiveness of the host-Rhizobium association (21) as well as to other internal factors not directly related to the nitrogen status of the plant (23). Generally, ineffective strains of Rhizobium form more nodules, especially after the initial stages of nodule formation (21). Nutman also showed that excision of effective (but not ineffective) red clover nodules resulted in a transient increase in the number of nodules subsequently formed (22). In fact, removal of the nodule meristem of effective nodules was sufficient to stimulate subsequent nodule development. The effect of nodule meristem excision on nitrogenase activity was not considered, but the argument that an inhibitory factor emanating from the growing point, and not the bacterial tissue, of the nodule was supported by the finding that excision of the main root tip had the same effect as nodule meristem removal. Clear evidence for internal regulation or autoregulation independent of nitrogen fixation and in the early stages of nodule initiation was reported by Pierce and Bauer (23). Using the spot inoculation technique (26), they showed that inoculation of soybean roots with R. japonicum several hours prior to a second inoculation substantially reduced nodulation by the second inoculation. In a split-root system for soybeans, Kosslak and Bohlool (16) showed that autoregulation prior to nodule appearance and nitrogenase activity was not restricted to root tissue immediately adjacent to the inoculated area, since prior inoculation ofone side ofa split-root suppressed nodulation on the other side ( 16). Studies by Calvert et al. (4) characterized this rapid regulatory phenomenon (23) further and showed that suppression of nodulation due to prior inoculation is mediated through suppression of...

Tissue-specific down-regulation of LjAMT1;1 compromises nodule function and enhances nodulation in Lotus japonicus

“…The number of nodules is regulated in at least two ways: by aborting infection and by confining the nodulation zone. Defects in these mechanisms, caused by genetic mutation lead to hyper-or super-nodulation (Carroll et al 1985;Caetano-Anollès and Gresshoff 1991;Penmetsa and Cook 1997;Nishimura et al 2002;Krusell et al 2002;Oka-Kira et al 2005). On the other hand, a strict interdependence exists between the level of nitrogenase activity and the efficiency of carbon sources flux providing energy for the bacteroid-mediated N 2 reduction (Pathirana et al 1992;Schulze et al 1998).…”

Organic Farming for Crop Improvement and Sustainable Agriculture in the Era of Climate Change

“…One of the most successful "breeding for fixation" outcomes is the soybean program in Brazil (Alves et al, 2003), which as a result of continual evaluation of inoculated plants growing in now nitrogenous soils, has resulted in the development of lines that realize their full symbiotic potential. Mutant lines of soybean, pea, common bean and Lotus japonicus have also been generated that nodulate super abundantly (hyper-nodulation; Carroll et al, 1985). As single recessive gene controls the hyper-nodulation trait in soybean but despite potential utility, this gene has not been yet incorporated into commercial cultivars.…”

The Autoregulation Gene SUNN Mediates Changes in Root Organ Formation in Response to Nitrogen through Alteration of Shoot-to-Root Auxin Transport

“…Split-root experiments demonstrated that infection of the root by rhizobia induces a signal that moves to the shoot and activates NARK, and subsequently, a signal is sent back from the shoot to the root to inhibit further nodule development, at least temporarily (Kosslak and Bohlool, 1984). Interestingly, autoregulation mutants also show the nts phenotype (Carroll et al, 1985b;Sagan et al, 1995); that is, they still nodulate in the presence of nitrate concentrations that inhibit nodulation in the wild type, although various mutants differ in their degree of nitrate tolerance. Again, grafting studies showed that the nts phenotype is exerted by the shoot in soybean and M. truncatula (Day et al, 1989;Sagan et al, 1995;Jeudy et al, 2010).…”

“…Mutation of these autoregulation genes causes supernodulation, mostly even in the presence of high N concentrations, and were thus first termed nitrate tolerant symbiotic (nts) mutants in soybean (Glycine max; Carroll et al, 1985aCarroll et al, , 1985b.…”