Shoot Control of Supernodulation in a Number of Mutant Soybeans, Glycine max (L.) Merr

Abstract

Seven supernodulation mutants of soybean (Glycine max) derived from the cultivar Bragg were investigated for shoot control of the supernodulation response. Reciprocal grafts showed that supernodulation in all lines tested was obtained only if the genotype of the shoot was homozygous for the mutant allele(s). In addition two lines of Korean origin (selected in the field for increased nitrate tolerance) were included in the grafting studies. Under our experimental conditions nitrate tolerance could not be demonstrated in the Korean cultivars. Grafts gave near-control levels of nodulation, suggesting that the genetic basis of nitrate tolerance and increased nodulation in the Korean lines is different from that seen in the induced mutants of Bragg. The data suggest that, contrary to earlier reports with pea (Pisum sativum) mutants, no root control of supernodulation could be demonstrated. Furthermore the results point to a possible novel regulatory system governing nodule numbers and patterning which involves an intertissue transfer of a chemical signal.

Rhizobial and Actinorhizal Symbioses: What Are the Shared Features?

“…Grafting experiments with legumes have shown that there is a shoot factor involved in autoregulation that is active even across species boundaries (Delves et al, 1987a(Delves et al, , 1987b. Thus, it is likely that the translocatable signal formed in roots affects the production of a translocatable shoot factor that controls nodule number (Caetano-Anollés and Gresshoff, 1991).…”

Shoot versus Root Signal Involvement in Nodulation and Vegetative Growth in Wild-Type and Hypernodulating Soybean Genotypes

“…The biochemical nature, the biosynthetic site(s), and the pathway by which the signals are transported and function are unknown. Wedgegrafting studies with most legume nodulation mutants and their respective wild-type parents have demonstrated that it is the shoot that determines the root nodulation phenotype, which suggests that the shoot is the source of the proposed SDI (Delves et al, 1986(Delves et al, , 1987Gremaud and Harper, 1989;Francisco and Akao, 1993;Hamaguchi et al, 1993). Recently, by wedge-and approach-grafting with different shoots (with or without cotyledons and with or without primary leaves) and by nodulation of rooted leaf cuttings, Francisco and Harper (199513) clearly demonstrated that the leaf autoregulates the number of nodules in soybean plants, which confirmed a report by Delves et al (1992).…”

“…One must keep in mind that much of our knowledge of legume nodule autoregulation is based on the comparative studies of wild-type and hypernodulating or supernodulating mutants (Delves et al, 1986(Delves et al, , 1987(Delves et al, , 1992CaetanoAnolles andGresshoff, 1990, 1991a;Francisco and Akao, 1993;Francisco and Harper, 1995b). It is possible that the hypernodulating or supernodulating mutants have been chemically altered in a biosynthetic step associated with the production of a "common" hormonal compound rather than the proposed nodulation-specific signal.…”

“…Rather, the nodule number in Williams 82 can be affected by available infection sites (root size) at the time of inoculation, so that a delayed inoculation will result in more nodules (Francisco and Harper, 1995a). It appears certain that new nodule primordia are arrested during early nodule ontogeny by previously formed SCDs in the root through a shootmediated feedback process (Gresshoff and CaetanoAnolles, 1992), and signal communication must occur in the root-shoot interactions (Delves et al, 1986(Delves et al, , 1987; reviewed by Gresshoff and Caetano-Anolles, 1992;Francisco and Harper, 199513). Our knowledge about the proposed plant-translocatable signals (the putative SDI and its more speculative rootderived precursor) is limited.…”

Photoassimilate partitioning in hypernodulation mutant of soybean (Glycine max(L.) Merr.) NOD1-3 and its parent williams in relation to nitrate inhibition of nodule growth

“…It is assumed that in these hypernodulating mutants there is a lack of or decrease in the autoregulatory control of nodulation which restricts the nodule number followed by the nodule primordial initiation after infection with Bradyrhizobia. Several reciprocal grafting experiments (Delves et al 1987;Barbera and Harper 1993) and a hypernodulation trait in a rooted single-leaf of mutant lines revealed that the shoot organs especially mature leaves, and not the roots or apical shoots, regulate the nodule number.…”