Are common symbiosis genes required for endophytic rice-rhizobial interactions?

Abstract

The nitrogen-fixing bacteria, called rhizobia, are able to infect the roots of leguminous plants and induce the formation of root nodules. Within the root nodule, the bacteria can convert atmospheric nitrogen into ammonia, a biological form that can be directly used by the plant. Over the past decade, tremendous progress has been made in our understanding of the nodulation signaling pathway in legumes.1 It has become increasingly evident that the root nodule symbiosis has co-opted the signaling pathway that mediates the ancestral mycorrhizal symbiosis, a widespread mutualistic association between mycorrhizal fungi and the vast majority of land plants. [1][2][3][4][5] As such, most, if not all, legume genes required for root nodule symbiosis are already present in non-legumes, and these non-legume genes have been shown to function similarly to their legume counterparts. 2,6 These discoveries have reignited an old dream of plant biologists, i.e., to transfer the nitrogen-fixing symbiosis to cereals and other non-leguminous crops. 4,7 Despite being unable to induce nodulation, rhizobia have been shown to be able to infect and colonize the roots of nonlegumes such as rice (Oryza sativa). [8][9][10] This so-called "endophytic" interaction also can promote plant growth, even though the exact mechanisms for such positive responses are not well understood.9,11 Characterization of the infection and colonization processes of the rice-rhizobial association revealed that the bacteria primarily enter the plant tissue through root hairs and/ or crack entry located near the sites of newly emerging lateral roots.10 Similar infection strategies are also used by the rhizobia to enter the roots of the non-legume Paraponia and the water-tolerant legume Sesbania rostrata, both of which can nodulate with rhizobia.12-16 Intriguingly, similar to what occur in legumes, both root hair curling and infection thread-like structures were also Legume plants are able to establish root nodule symbioses with nitrogen-fixing bacteria, called rhizobia. recent studies revealed that the root nodule symbiosis has co-opted the signaling pathway that mediates the ancestral mycorrhizal symbiosis that occurs in most land plants. Despite being unable to induce nodulation, rhizobia have been shown to be able to infect and colonize the roots of non-legumes such as rice. one fascinating question is whether establishment of such associations requires the common symbiosis (Sym) genes that are essential for infection of plant cells by mycorrhizal fungi and rhizobia in legumes. here, we demonstrated that the common Sym genes are not required for endophytic colonization of rice roots by nitrogen-fixing rhizobia. Are common symbiosis genes required for endophytic rice-rhizobial interactions?Caiyan Chen 1 and hongyan Zhu Keywords: rice, rhizobia, nitrogen fixation, legume, common symbiosis genes observed on the rice roots inoculated with rhizobia. 10 However, in contrast to the root nodule symbiosis, in which bacteria colonize intracellularly, bacteria mainly resid...

Rhizosphere Microbiome Modulators: Contributions of Nitrogen Fixing Bacteria towards Sustainable Agriculture

“…or is a common characteristic. “In order to determine whether it is the plant that initiates the N 2 -fixation in its bacterial symbiont (as regards nodulation), a common SYM pathway rice mutant should be tested for its ability to form endophytic symbiosis with ORS571 [ 103 , 104 ]”. It is also essential to distinguish identified multitude of genes of Azorhizobium responsible for plant infection, tolerance to stress and nodulation in order to ascertain those involved in Azorhizobium - Sesbania mutualistic interaction and rhizobial-legume associations.…”

“…Oomycetes for instance use this pathway to gain access to plant and cause havoc [ 177 ]. Although, mutant strains of rice lacking SYM pathway reveal that certain endophytic microorganisms like R. leguminosarum are still able to infect plant roots [ 103 ] suggesting that this is not the only pathway available for microorganisms to gain entry into plant tissues. However, plants might be able to detect certain pathogens via the SYM pathway.…”

Role of root microbiota in plant productivity

“…However, rice mutants defective in the SYM pathway show that at least some endophytic bacteria such as Rhizobium leguminosarum bv. trifolii can still colonize plant roots (Chen and Zhu, 2013), suggesting that the SYM pathway is not the only pathway for microorganisms to colonize plants. Plants may also be able to detect specific pathogen using the SYM pathway.…”

Rice arbuscular mycorrhiza as a tool to study the molecular mechanisms of fungal symbiosis and a potential target to increase productivity

“…The rice blast fungus Magnaporthe oryzae infects the roots of Oscastor-1 and Osccamk-2 mutants similarly to those of wild type, indicating that the OsCSP genes are not required for this infection (Marcel et al 2010 ). Endophytic colonization of the rice mutants Oscastor-1 , Osccamk-2 , and Oscyclops (NC2794) by rhizobia also appears not to be affected under laboratory conditions (Chen and Zhu 2013 ). However, the OsCCaMK genotype affects the diversity of the bacterial community in rice roots in paddy and upland fields (Ikeda et al 2011 ).…”