Nodules Initiated by Rhizobium meliloti Exopolysaccharide Mutants Lack a Discrete, Persistent Nodule Meristem

Abstract

Infection of alfalfa with Rhizobium meliloti exo mutants deficient in exopolysaccharide results in abnormal root nodules that are devoid of bacteria and fail to fix nitrogen. Here we report further characterization of these abnormal nodules. Tightly curled root hairs or shepherd's crooks were found after inoculation with Rm 1021-derived exo mutants, but curling was delayed compared with wild-type Rm 1021. Infection threads were initiated in curled root hairs by mutants as well as by wild-type R. meliloti, but the exo mutant-induced threads aborted within the peripheral cells of the developing nodule. Also, nodules elicited by Rm 1021-derived exo mutants were more likely to develop on secondary roots than on the primary root. In contrast with wild-type R. meliloti-induced nodules, the exo mutant-induced nodules lacked a well defined apical meristem, presumably due to the abortion of the infection threads. The relationship of these findings to the physiology of nodule development is discussed.

Infection and Invasion of Roots by Symbiotic, Nitrogen-Fixing Rhizobia during Nodulation of Temperate Legumes

“…Those that do initiate are bloated and grossly misshapen and usually cannot extend beyond the basal part of the root hair (Fig. 4) (31,121,190). Infection threads also cannot initiate normally in vetch root hairs inoculated with R. leguminosarum bv.…”

Morphogenetic Rescue of Rhizobium meliloti Nodulation Mutants by trans-Zeatin Secretion

“…As shown in Figures 2 and 3, pTZSinduced nodules consisted of an outer nodule cortex, one to severa1 diffuse "meristem-like" regions (yellowish regions in Figures 2D and 2F) consisting of small dividing cells (asterisks in Figure 3), an enlarged central region of tightly packed parenchyma cells (see Esau, 1977;Van de Wiel et al, 1990a including both highly vacuolated cells and cells that stained intensely with acidic toluidine blue (Figures 3 6 and 3C), and a bifurcating vascular system differentiating from the root stele ( Figures 2E and 36). In some cases, as shown in Figure 2F, branched nodules were induced by pTZS+ bacteria, indicating that the meristem-like regions in pTZS nodules can develop into persistent meristems, unlike the case with Exo-nodules (Yang et al, 1992). The similarities between the responses of alfalfa roots to pTZS+ bacteria and to Rhizobium were further examined using in situ hybridization experiments to test for the pTZSinduced expression of alfalfa MsENOD2.…”

“…Furthermore, nod gene expression is correlated with the biosynthesis of modified lipooligosaccharide Nod factors that are both necessary and sufficient to elicit the early host responses (Lerouge et al, 1990;Truchet et al, 1991). Genes involved in the synthesis of surface exopolysaccharides (exo genes) are also required for the invasion of nodule tissues by R. meliloti but are not required for the initiation of infection or for nodule morphogenesis (Finan et al, 1985;Leigh et al, 1985;Yang et al, 1992).…”

A Gene That Encodes a Proline-Rich Nodulin with Limited Homology to PsENOD12 Is Expressed in the Invasion Zone of Rhizobium meliloti-Induced Alfalfa Root Nodules

“…RNA was isolated from nodules elicited by wild-type or mutant R. meliloti and also from pseudonodules formed in response to NPA treatment. The R. rneliloti exo mutantinduced nodules are ineffective, free of bacteria, and lack a persistent nodule meristem (Yang et al, 1992). Infection threads abort in the peripheral cells of nodules induced by R. meliloti exo mutants except in the nodules induced by the exoH mutants (Leigh et al, 1987), where infection threads penetrate into the interior of the nodule in about 10 to 15% of the nodules formed.…”

“…We also wanted to find a gene that could serve as an early genetic marker for studies on signal exchange between R. meliloti and its host. Alfalfa offers the advantage that it has been well studied with regard to points of nodule arrest following inoculation with various invasion-defective R. meliloti mutants, especially the exopolysaccharide ( e m ) mutants (Finan et al, 1985;Leigh et al, 1987;Yang et al, 1992). It also can be transformed using Agrobacterium-derived vectors (Deak et al, 1986).…”