Abstract
The symbiosis receptor kinase (SymRK) is required for morphological changes of legume root hairs triggered by rhizobial infection. How protein turnover of SymRK is regulated and how the nodulation factor signals are transduced downstream of SymRK are not known. In this report, a SymRK-interacting E3 ubiquitin ligase (SIE3) was shown to bind and ubiquitinate SymRK. The SIE3-SymRK interaction and the ubiquitination of SymRK were shown to occur in vitro and in planta. SIE3 represents a new class of plant-specific E3 ligases that contain a unique pattern of the conserved CTLH (for C-terminal to LisH), CRA (for CT11-RanBPM), and RING (for Really Interesting New Gene) domains. Expression of SIE3 was detected in all tested tissues of Lotus japonicus plants, and its transcript level in roots was enhanced by rhizobial infection. The SIE3 protein was localized to multiple subcellular locations including the nuclei and plasma membrane, where the SIE3-SymRK interaction took place. Overexpression of SIE3 promoted nodulation in transgenic hairy roots, whereas downregulation of SIE3 transcripts by RNA interference inhibited infection thread development and nodule organogenesis. These results suggest that SIE3 represents a new class of E3 ubiquitin ligase, acts as a regulator of SymRK, and is involved in rhizobial infection and nodulation in L. japonicus.
Highlights
The symbiosis receptor kinase (SymRK) is required for morphological changes of legume root hairs triggered by rhizobial infection
We designed primers according to the reconstituted full-length coding region and amplified its complementary DNA (cDNA) using RNA isolated from L. japonicus roots inoculated with Mesorhizobium loti
SymRK-interacting E3 ubiquitin ligase (SIE3) interacted with itself (Supplemental Fig. S2B), which is consistent with the homodimer model
Summary
The symbiosis receptor kinase (SymRK) is required for morphological changes of legume root hairs triggered by rhizobial infection. Overexpression of SIE3 promoted nodulation in transgenic hairy roots, whereas downregulation of SIE3 transcripts by RNA interference inhibited infection thread development and nodule organogenesis These results suggest that SIE3 represents a new class of E3 ubiquitin ligase, acts as a regulator of SymRK, and is involved in rhizobial infection and nodulation in L. japonicus. NFs can trigger a series of host responses, including root hair deformation, cellular alkalinization, membrane potential depolarization, ion flux changes, early nodulin gene expression, and nodule primordial formation (Ehrhardt et al, 1996; Felle et al, 1999; D’Haeze and Holsters, 2002; Wais et al, 2002). These responses allow the rhizobia to enter the root epidermal cells. The SymRK protein contains a signal peptide sequence for protein sorting, an extracellular region for interacting with unidentified signal molecules, a transmembrane segment for anchoring to the plasma membrane, and an intracellular protein kinase domain (Endre et al, 2002; Stracke et al, 2002; Mitra et al, 2004; Capoen et al, 2005)
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