Innovation and appropriation in mycorrhizal and rhizobial Symbioses

Abstract

Most land plants benefit from endosymbiotic interactions with mycorrhizal fungi, including legumes and some non-legumes that also interact with endosymbiotic nitrogen-fixing bacteria to form nodules. In addition to these helpful interactions, plants are continuously exposed to would-be pathogenic microbes: discriminating between friends and foes is a major determinant of plant survival. Recent breakthroughs have revealed how some key signals from pathogens and symbionts are distinguished. Once this checkpoint has been passed and a compatible symbiont is recognized, the plant coordinates the sequential development of two types of specialized structures in the host. The first serve to mediate infection, and the second, which appear later, serve as sophisticated intracellular nutrient exchange interfaces. The overlap in both the signaling pathways and downstream infection components of these symbioses reflects their evolutionary relatedness and the common requirements of these two interactions. However, the different outputs of the symbioses, phosphate uptake vs. nitrogen fixation, require fundamentally different components and physical environments and necessitated the recruitment of different master regulators, NLPs and PHRs, for nodulation and mycorrhization, respectively.

CERK1, more than a co‐receptor in plant–microbe interactions

“…These chitooligosaccharides are recognized by plant LysM receptors. But instead of eliciting immune reactions, they stimulate the formation of mycorrhizal arbuscules in diverse plant species (Maillet et al ., 2011; Wang et al ., 2022).…”

The Rhizobial Type 3 Secretion System: The Dr. Jekyll and Mr. Hyde in the Rhizobium–Legume Symbiosis

“…On the one hand, rhizobia must overcome several checkpoints during root colonization and infection and nodule invasion, synthesizing the right molecular signals that will be recognized by the plant to assure the progression of symbiosis [ 1 , 2 , 5 , 6 , 7 , 8 ]. The nature and mode of action of these rhizobial molecular signals have been mainly studied in a limited number of model plants and model rhizobia, and many variations can be found when other symbiotic couples are analyzed [ 9 ]. On the other hand, rhizobia, since they are infecting and invading plant tissues, must suppress and overcome plant immune responses [ 10 , 11 ].…”

“…NF, which are N-acetyl-glucosamine oligosaccharides harboring different molecular decorations, when compatible, are recognized by plant LysM receptors, allowing root infection and colonization of the symbiotic cells within nodules [ 1 , 2 ]. Perception of NF by plant LysM receptors triggers a very complex regulatory cascade that has been detailed in several recent reviews [ 1 , 6 , 9 , 13 ]. Therefore, the two above-mentioned molecular interaction/recognition events (flavonoids–NodD and NF–LysM receptors) are key factors for the specificity of the rhizobium–legume interaction.…”

What role does the seed coat play during symbiotic seed germination in orchids: an experimental approach with Dendrobium officinale

“…In AM association, the gene expression and signalling pathways during root colonization have been well studied. The recognition of symbiotic signalling molecules allows host plants to quickly differentiate compatible symbionts from the greater population of microbes, while mycorrhizal factors secreted from AM fungi have been identified as key signals used to communicate with host plants, allowing the entry of the microbial partners into plant cells [16]. However, it is still unclear whether the molecular interactions, compounds and signalling pathways involved in AM are similar in OM or if there is another specific OM establishment mechanism [17].…”