Gibberellins have been reported to play both positive and negative roles in arbuscular mycorrhizal (AM) symbioses. Despite extensive characterisation of the role of DELLAs in AM colonisation, studies of gibberellin function have largely been restricted to chemical interventions. Few studies have examined how disruption to gibberellin biosynthesis affects AM symbioses. To explore this further, we obtained Lotus japonicus LORE1 transposon insertion mutants in four key gibberellin biosynthetic genes: CPS, KS, KO, and KAO. Through a characterisation of their developmental phenotypes, we determined that for each gene there is a single homolog which has a major role in gibberellin biosynthesis. We name these genes CPS1, KS1, KO1, and KAO1. Mutations in these genes affect AM colonisation in the overall distribution of arbuscules, but not in total colonisation levels. These results are consistent with previous studies indicating that DELLAs control the number of cortical cell layers, and therefore regulate the number of cells able to accommodate arbuscules.

Arbuscular mycorrhizal (AM) fungi engage with land plants in a widespread, mutualistic endosymbiosis which provides their hosts with increased access to nutrients and enhanced biotic and abiotic stress resistance. The potential for reducing fertiliser use and improving crop resilience has resulted in rapidly increasing scientific interest. Microscopic quantification of the level of AM colonization is of fundamental importance to this research, however the methods for recording and processing these data are time-consuming and tedious. In order to streamline these processes, we have developed AMScorer, an easy-to-use Excel spreadsheet, which enables the user to record data rapidly during from microscopy-based assays, and instantly performs the subsequent data processing steps. In our hands, AMScorer has more than halved the time required for data collection compared to paper-based methods. Subsequently, we developed AMReader, a user-friendly R package, which enables easy visualization and statistical analyses of data from AMScorer. These tools require only limited skills in Excel and R, and can accelerate research into AM symbioses, help researchers with variable resources to conduct research, and facilitate the storage and sharing of data from AM colonization assays. They are available for download at https://github.com/EJarrattBarnham/AMReader, along with an extensive user manual.

Arbuscular mycorrhizal symbiosis is important for plant nutrition and has shaped the evolution and success of land plants throughout their evolution. Our understanding on the dynamics of colonization, as well as the signal transduction cascades that underpin the establishment, maintenance, and termination of this mutually beneficial symbiosis has been increasingly elucidated over decades of research. Recent work has revealed that the core signaling machinery for microbial accommodation is further regulated by signaling hubs that fine-tune the levels of symbiosis signaling. This book chapter reviews our knowledge on both the core microbial accommodation pathway of the symbiosis and the modes of regulation. We then finally explore how these regulatory layers might have been recruited in plant evolution. Understanding the signaling circuitry of symbiosis regulation helps pave the way for strategies for sustainable ways to enhance crop nutrition.

Many plants associate with arbuscular mycorrhizal fungi for nutrient acquisition, while legumes also associate with nitrogen-fixing rhizobial bacteria. Both associations rely on symbiosis signaling and here we show that cereals can perceive lipochitooligosaccharides (LCOs) for activation of symbiosis signaling, surprisingly including Nod factors produced by nitrogen-fixing bacteria. However, legumes show stringent perception of specifically decorated LCOs, that is absent in cereals. LCO perception in plants is activated by nutrient starvation, through transcriptional regulation of Nodulation Signaling Pathway (NSP)1 and NSP2. These transcription factors induce expression of an LCO receptor and act through the control of strigolactone biosynthesis and the karrikin-like receptor DWARF14-LIKE. We conclude that LCO production and perception is coordinately regulated by nutrient starvation to promote engagement with mycorrhizal fungi. Our work has implications for the use of both mycorrhizal and rhizobial associations for sustainable productivity in cereals.

When plants are starved of phosphate, many rely on an ancient symbiosis with arbuscular mycorrhizal fungi to secure a critical supply. A new study demonstrates a molecular basis for the regulation of symbiosis by phosphate starvation.

Plant cyclic nucleotide-gated channels (CNGCs) are tetrameric cation channels which may be activated by the cyclic nucleotides (cNMPs) adenosine 3′,5′-cyclic monophosphate (cAMP) and guanosine 3′,5′-cyclic monophosphate (cGMP). The genome of Arabidopsis thaliana encodes 20 CNGC subunits associated with aspects of development, stress response and immunity. Recently, it has been demonstrated that CNGC subunits form heterotetrameric complexes which behave differently from the homotetramers produced by their constituent subunits. These findings have widespread implications for future signalling research and may help explain how specificity can be achieved by CNGCs that are known to act in disparate pathways. Regulation of complex formation may involve cyclic nucleotide-gated channel-like proteins.