New research led by the Crop Science Centre has discovered an ancient protein domain and a new symbiotic gene in the signalling pathways facilitating arbuscular mycorrhizal association. Published in PNAS on 15th June, the study characterises a symbiotic gene in rice called arbuscular receptor-like kinase 2 (ARK 2), as well as unveiling an ancient protein domain that defines a new class of signalling proteins.
Arbuscular mycorrhizal association is the most prevalent symbiosis in plants. The mycorrhizal fungi help plants to capture nutrients from the soil. Scientists hope that by better understanding the pathways that enable plants to interact with these fungi, they can develop genetic strategies to enhance crop performance.
In the arbuscular mycorrhizal symbiosis, nutrient exchange occurs in tree-shaped structures formed inside root cells called arbuscules. Several plant proteins are known to function in cells hosting arbuscules, mostly nutrient transporters.
A previous study from the Cereal Symbiosis Laboratory, at the University of Cambridge, characterised arbuscular receptor-like kinase 1 (ARK1), which is the first known receptor-like kinase to regulate the symbiosis in arbusculated cells. Receptor-like kinases are cell-surface signalling proteins that normally have two modules: an extracellular domain, which perceives external signals and an intracellular kinase domain that initiates a cellular response.
In this new study, the authors performed a phylogenetic analysis to gain new insights into the evolutionary history of the receptor-like kinase subfamily that ARK1 belongs to. This revealed that a single gene called ARK duplicated early in the evolution of seed plants. This duplication generated ARK1 and ARK2. The authors functionally characterised ARK2 in rice using mutant lines.
Plants with ARK2 mutated had reduced levels of arbuscular mycorrhizal colonisation, demonstrating ARK2 to have a symbiotic function. Global analyses of gene expression further showed that a set of genes is co-regulated by ARK1 and ARK2, suggesting the two receptor-like kinases regulate the arbuscular mycorrhizal symbiosis in a novel signalling pathway.
Surprisingly, while analysing the sequences of the extracellular domains in this subfamily, the authors discovered a new protein domain. The domain, named SPARK, has a unique arrangement of cysteines, an amino acid that is often found to stabilise protein domains by forming sulphur bonds. The domain has no resemblance at the sequence level with other known protein domains. The presence of the SPARK domain in receptor-like kinases of a species of algae revealed it to be an ancient protein domain.
The lead author, Hector Montero, said: “The discoveries within this research kick-start the study of a new class of signalling proteins that had been overlooked and we believe these findings will draw attention from those interested not only in mycorrhizal functioning but also in the evolution of receptor-like kinases. It will be important for future studies to explore the details of the signalling pathway orchestrated by the receptors and the function of the SPARK domain.”