How rhizobial symbionts invade plants: the Sinorhizobium-Medicago model

Abstract

Nitrogen-fixing rhizobial bacteria and leguminous plants have evolved complex signal exchange mechanisms that allow a specific bacterial species to induce its host plant to form invasion structures through which the bacteria can enter the plant root. Once the bacteria have been endocytosed within a host-membrane-bound compartment by root cells, the bacteria differentiate into a new form that can convert atmospheric nitrogen into ammonia. Bacterial differentiation and nitrogen fixation are dependent on the microaerobic environment and other support factors provided by the plant. In return, the plant receives nitrogen from the bacteria, which allows it to grow in the absence of an external nitrogen source. Here, we review recent discoveries about the mutual recognition process that allows the model rhizobial symbiont Sinorhizobium meliloti to invade and differentiate inside its host plant alfalfa (Medicago sativa) and the model host plant barrel medic (Medicago truncatula).

Figure 1. The initial signalling dialogue between Sinorhizobium meliloti and Medicago truncatula

a | The induction of rhizobial nod genes requires plant flavonoids,. The nod gene products produce Nod factor (NF), which is initially perceived by the M. truncatula MtNFP receptor,,. b | Root hair curling and cortical cell divisions require many M. truncatula gene products,: MtNFP; MtDMI1 (REF. 20); MtDMI2 (REF. 25); MtDMI3 (REFS 22,23); MtNSP1 (REF. 27); MtNSP2 (REF. 26); MtCRE1 (REFS –60); and MtNIN,. MtLYK3/HCL is required for colonized curled root hair (CCRH) formation, but not for the induction of cortical cell divisions (P. Smit and T. Bisseling, unpublished data). The required rhizobial genes are boxed in brown and the required plant genes are boxed in light green.

Figure 2. Downstream components of the Nod factor signal transduction system

A complete response of Medicago truncatula to Nod factor (NF) from Sinorhizobium meliloti requires multiple extracellular-domain-containing cell-surface receptors, including the LysM family receptors MtNFP and MtLYK3/HCL. DMI2 encodes a leucine-rich-repeat receptor kinase that is localized to the membrane and is required for tight root hair curling around the bacteria. Downstream of NF, DMI1 encodes a ligand-gated ion channel that localizes to the nuclear membrane. DMI3 encodes a Ca²⁺–calmodulin-dependent protein kinase that is required for the induction of cell division in the root cortex and for the transcriptional changes required for the establishment of the symbiosis. Two GRAS family transcriptional regulators, nodulation signalling pathway 1 (NSP1) and NSP2, are also required for Nod-factor-induced transcriptional changes. The response to NF also involves Ca²⁺ spiking.

Figure 3. Root hair invasion by Sinorhizobium meliloti

a | S. meliloti exoY, and Medicago trunculata MtLIN and MtNIN are required for infection thread initiation. b | S. meliloti exoH and M. trunculata MtNFP, MtLYK3/HCL (P. Smit and T. Bisseling, unpublished data), MtBIT1/ERN, MtNIN and MtCRE1 (REFS –60) are required for infection threads to extend to the base of the root hair cell. c | MtCRE1 (REFS –60), MtBIT1/ERN, MtRIT1 (REF. 62) and MtSLI are required for infection thread penetration into the underlying cell layers. The required rhizobial genes are boxed in brown and the required plant genes are boxed in light green.

Figure 4. Infection thread failure can be caused by plant or bacterial defects

Infection thread formation during invasion of Medicago truncatula or Medicago sativa by Sinorhizobium meliloti. a | An M. sativa root hair cell infected by S. meliloti wild-type bacteria, with a fully extended infection thread. b | An M. sativa root hair arrested at the colonized curled root hair (CCRH) stage during infection by an S. meliloti exoY mutant. c | A M. truncatula lin mutant arrested at the CCRH stage during infection by wild-type S. meliloti. d | An arrested CCRH, formed during infection of M. truncatula with an S. meliloti nodF nodL mutant. e | An aberrant, aborted infection thread formed by wild-type S. meliloti on M. truncatula partially depleted of MtNFP mRNA by RNA interference (RNAi). f | An aberrant infection thread formed by an S. meliloti nodF nodE mutant on M. truncatula partially depleted for MtLYK3 by RNAi. Parts a and b reprinted with permission from REF. © (2000) The American Society for Microbiology. Part c and e reprinted with permission from REF. © (2004) American Society of Plant Biologists. Parts d and f reprinted with permission from REF. © (2003) American Association for the Advancement of Science.

Figure 5. Endocytosis of bacteria and bacteroid differentiation

Bacterial endocytosis requires the Sinorhizobium meliloti hemA gene, the Medicago truncatula NIP gene and wild-type expression levels of the MtDMI2 (REF. 70) and MtHAP2-1 (REF. 71) genes. S. meliloti lpsB and bacA are required for bacterial survival within the symbiosome membrane. S. meliloti fixJ, M. truncatula MtSYM1 (REF. 64), MtDNF1, -4, -5 and -7 (REFS 62,63), and pea (Pisum sativum) PsSYM13 (REF. 95) are required for bacteroid differentiation. The S. meliloti nifHDK genes encode nitrogenase and are required for nitrogen fixation. The pea PsRUG4 gene encodes sucrose synthase and is required to support bacteroid nitrogen fixation,. The M. truncatula MtDNF3 and -6 genes are required for the maintenance of nitrogen fixation,. The required rhizobial genes are boxed in brown and the required plant genes are boxed in light green.