Abstract
Most legumes can engage in symbiosis with N-fixing bacteria called rhizobia. This symbiosis, called nodulation, evolved from the more widespread symbiosis that most land plants form with arbuscular mycorrhiza, which is reflected in a common requirement of certain genes for both these symbioses. One key nodulation gene, Nodule Inception (NIN), has been intensively studied. Mutants in NIN are unable to form nodules, which has made it difficult to identify downstream genes under the control of NIN. The analysis of data from our recent transcriptomics study revealed that some genes with an altered expression of nin during nodulation are upregulated in mycorrhizal roots. In addition, another study reported the decreased colonization of nin roots by arbuscular mycorrhiza. We therefore investigated a role for NIN in mycorrhiza formation. Our time course study, using two nin alleles with differing genetic backgrounds, suggests that that loss of NIN does not affect colonization of Medicago truncatula roots, either in the presence or absence of rhizobia. This, and recent phylogenetic analyses showing that the loss of NIN is correlated with loss of nodulation in the FaFaCuRo clade, but not with the ability to form mycorrhiza, argue against NIN being required for arbuscular mycorrhization in legumes.
Highlights
Arbuscular mycorrhization is a beneficial symbiosis formed between arbuscular mycorrhizal (AM) fungi and many terrestrial plants
We investigated a potential role for Nodule Inception (NIN) in nodulated and non-nodulated roots of M. truncatula
A comparison of genes that were differentially expressed in root hairs of rhizobia-inoculated seedlings to those genes induced by Rhizophagus irregularis revealed some overlap (Supplemental S1)
Summary
Arbuscular mycorrhization is a beneficial symbiosis formed between arbuscular mycorrhizal (AM) fungi and many terrestrial plants. This symbiosis promotes the uptake of phosphates and other nutrients by the plant host at the expense of host carbon, supplied to the fungal endosymbiont It entails the constant exchange of signals between the host and symbiont, which leads to the formation of nutrient exchange and fungus-accommodation structures called arbuscules within root cortical cells [2]. This process requires the differential activation of hundreds of genes which have been studied at the evolutionary and functional genetic levels using genomic approaches [3]. Plants 2020, 9, 71 hairs by rhizobia followed by the colonization of the cortical layers of the root [5] This symbiosis leads to the formation of special root outgrowths called nodules. We investigated a potential role for NIN in nodulated and non-nodulated roots of M. truncatula
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