CERBERUS and NSP1 of Lotus japonicus are Common Symbiosis Genes that Modulate Arbuscular Mycorrhiza Development

Abstract

Arbuscular mycorrhizal symbiosis (AMS) and root nodule symbiosis (RNS) are mutualistic plant-microbe interactions that confer nutritional benefits to both partners. Leguminous plants possess a common genetic system for intracellular symbiosis with AM fungi and with rhizobia. Here we show that CERBERUS and NSP1, which respectively encode an E3 ubiquitin ligase and a GRAS transcriptional regulator and which have previously only been implicated in RNS, are involved in AM fungal infection in Lotus japonicus. Hyphal elongation along the longitudinal axis of the root was reduced in the cerberus mutant, giving rise to a lower colonization level. Knockout of NSP1 decreased the frequency of plants colonized by AM fungi or rhizobia. CERBERUS and NSP1 showed different patterns of expression in response to infection with symbiotic microbes. A low constitutive level of CERBERUS expression was observed in the root and an increased level of NSP1 expression was detected in arbuscule-containing cells. Induction of AM marker gene was triggered in both cerberus and nsp1 mutants by infection with symbiotic microbes; however, the mutants showed a weaker induction of marker gene expression than the wild type, mirroring their lower level of colonization. The common symbiosis genes are believed to act in an early signaling pathway for recognition of symbionts and for triggering early symbiotic responses. Our quantitative analysis of symbiotic phenotypes revealed developmental defects of the novel common symbiosis mutants in both symbioses, which demonstrates that common symbiosis mechanisms also contribute to a range of functions at later or different stages of symbiont infection.

Recent developments in arbuscular mycorrhizal signaling

“…Both NSP1 and NSP2 regulate expression of strigolactone biosynthetic genes and nsp1nsp2 double mutants have reduced levels of strigolactones and of AM colonization in both M. truncatula and rice [47]. Addition of strigolactone partially complemented AM colonization of the nsp1 mutant, indicating a possible alternative role for NSP1 in addition to regulation of strigolactone biosynthesis (Figure 1h) [48]. Phylogenetic analyses identified signatures of selection constraints on NSP1 from plant species colonized by AM fungi [46].…”

Activation of Symbiosis Signaling by Arbuscular Mycorrhizal Fungi in Legumes and Rice

“…NSP1 and NSP2 are essential for nodulation, and RAM1 is essential for mycorrhization Oldroyd and Long, 2003;Gobbato et al, 2012). While recent work has shown some role for NSP1 and NSP2 during mycorrhizal colonization (Maillet et al, 2011;Lauressergues et al, 2012;Delaux et al, 2013;Takeda et al, 2013), the mycorrhizal defects in these mutants are very minor compared with the nodulation phenotypes. To assess whether the gene induction observed was dependent on these GRAS proteins, we assessed induction in the mutants.…”

PUB1 Interacts with the Receptor Kinase DMI2 and Negatively Regulates Rhizobial and Arbuscular Mycorrhizal Symbioses through Its Ubiquitination Activity in Medicago truncatula

“…S6A). Recently, in M. truncatula and L. japonicus, use of a lower inoculum has provided evidence of a quantitative AM phenotype in several mutants where no visible phenotype was detected with a high inoculum (Maillet et al, 2011, Takeda et al, 2013Delaux et al, 2013;Zhang et al, 2015a,b). By using a low inoculum, we found a significant 2-fold increase in fungal colonization in pub1-1 at 6 wpi ( Fig.…”