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

Abstract

The development of nitrogen-fixing nodules is induced on the roots of legume host plants by Rhizobium bacteria. We employed a novel strategy to probe the underlying mechanism of nodule morphogenesis in alfalfa roots using pTZS, a broad host range plasmid carrying a constitutive trans-zeatin secretion (tzs) gene from Agrobacterium tumefaciens T37. This plasmid suppressed the Nod-phenotype of Rhizobium nodulation mutants such that mutants harboring pTZS stimulated the formation of nodulelike structures. Alfalfa roots formed more or fewer of these nodules according to both the nitrogen content of the environment and the position along the root at which the pTZS+ bacteria were applied, which parallels the physiological and developmental regulation of true Rhizobium nodule formation. This plasmid also conferred on Escherichia coli cells the ability to induce root cortical cell mitoses. Both the pattern of induced cell divisions and the spatially restricted expression of an alfalfa nodule-specific marker gene (MsENOD2) in pTZS-induced nodules support the conclusion that localized cytokinin production produces a phenocopy of nodule morphogenesis.

Auxin transport inhibitors act through ethylene to regulate dichotomous branching of lateral root meristems in pine

“…ATIs might induce dichotomous branching by increasing the cytokinin : auxin ratio in root cells in a manner analogous to the ATI-induced formation of pseudo-nodules in legume roots (Hirsch et al, 1989 ;Cooper & Long, 1994), and the activity of ethylene could be explained by its ability to modify polar auxin transport (Suttle, 1988). Alternatively, ethylene might have a more direct role in controlling the organization of lateral root meristems in pine, and ATIs could act by stimulating ethylene production.…”

Flavonoids and Auxin Transport Inhibitors Rescue Symbiotic Nodulation in the Medicago truncatula Cytokinin Perception Mutant cre1

“…Several studies have implicated cytokinin as a central regulator in nodule development . The expression of a cytokinin synthesis gene in Sinorhizobium meliloti nodulation mutants was sufficient to induce cortical cell divisions and expression of nodulation genes in alfalfa (Cooper and Long, 1994). Application of cytokinin can induce cortical cell divisions and expression of nodulation genes in alfalfa Fang and Hirsch, 1998), white clover (Trifolium repens) (Mathesius et al, 2000), and Lotus japonicus (Heckmann et al, 2011), and pseudonodules on the actinorhizal plant Alnus glutinosa (Rodriguez-Barrueco and Bermudez de Castro, 1973).…”

Accumulation ofENOD2-Like Transcripts in Non-Nodulating Woody Papilionoid Legumes

“…After inoculation with rhizobia or treatment with ATIs or cytokinins, nodules and pseudonodules become visible on roots in 7 to 14 d, and ENOD2 transcripts can be detected in 4 to 10 d (Hirsch et al, 1989(Hirsch et al, , 1993(Hirsch et al, , 1997Govers et al, 1990;Dehio and de Bruijn, 1992;Scheres et al, 1992;Cooper and Long, 1994;Wu et al, 1996). In our experiment pseudonodules developed on the TIBA-treated roots of the nodulating controls, alfalfa and M. amurensis, and zeatin induced pseudonodules in alfalfa (Fig.…”

“…Sprent (1994) and others have questioned whether legumes considered non-nodulators lost the capacity to nodulate over time or never acquired it. In some non-nodulating, herbaceous legumes, perception of nod factors seems uncoupled from expression of ENOD genes (Cooper and Long, 1994;Hirsch et al, 1997). It is tempting to speculate that Japanese pagodatree and American yellowwood lack a component of the signal transduction pathway leading to localized cortical cell division and nodule organogenesis.…”