Discovery of a symbiotic signalling mechanism from maize.

Symbioses are fundamental to life on the Earth. One such example, known as arbuscular mycorrhizal (AM) symbiosis, evolved between fungi and plants to facilitate mineral uptake from the soil around 450 mya. This symbiotic arrangement is so widespread that it impacts on ecosystem productivity, and is considered of fundamental importance for crop productivity and sustainability.

Generating understanding of the processes that underpin AM symbiosis development and functioning are thus of universal relevance across the plant kingdom.

The establishment of functional symbioses relies on the fine-tuned orchestration of signals to achieve coordination of the interacting plant and AM fungus. Identification and analysis of mutant plants have formed the bedrock of our understanding of the molecular mechanisms that underpin the interspecies crosstalk allowing this symbiosis. Reciprocal recognition and plant and AM fungi in the rhizosphere are central to the outcome of the symbiosis and presently poorly understood.

The Paszkowski lab has previously discovered key genes from cereals that condition pre-symbiotic recognition [Gutjahr et al. (2015) Science 350:1521; Nadal et al. (2017) Nature Plants 3:17073] and most recently, has identified the independent of arbuscular mycorrhiza (ina) maize mutant. Maize ina plants show a complete loss of susceptibility to AM fungi, which can however be complemented by the addition of wild-type but not ina mutant root exudates.

This is a unique phenotype that has not been described before, suggesting that INA is part of a new pathway, indispensable for pre-symbiotic signalling in maize.

Positional cloning revealed that a deletion with three candidate genes co-segregated with the mutant phenotype. To discover the INA gene identity additional CRISPR/Cas9-edited maize alleles are presently produced and will be available at the end of 2020. It is the aim of this 18 months grant to use the new maize mutant material to discover the INA gene identity, employing our standard phenotyping protocols.

In addition it seeks to provide a first insight into the role INA plays by determining the transcriptome underpinning of the inability of ina to engage with AM fungi.

Knowledge of the nature of the INA gene will guide future research directions and will pave the way to discover a novel communication strategy for the most prevalent plant symbiosis on Earth.