The role of calcium signaling in Arabidopsis thaliana for haustoria development by the oomyceteHyaloperonospora arabidopsidis

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An ancient genetic program for root symbiosis is conserved among angiosperms to allow intracellular accommodation of nutrient-delivering fungi during the arbuscular mycorrhiza (AM) symbiosis. The origin of AM dates back to the earliest land plants and only exceptional plant lineages like the Brassicaceae have lost the AM symbiosis more recently, which correlates with the specific absence of a large cohort of symbiosis-specific genes in the Arabidopsis genome. However, vestigial symbiosis genes such as POLLUX are still present in Arabidopsis. POLLUX encodes a cation channel that in legumes is required for symbiosis-associated nuclear calcium oscillations (calcium-spiking) and is thus considered an essential component of the nuclear calcium-spiking machinery. Despite detailed analysis in different laboratories, pollux mutants of Arabidopsis lacked any discernable phenotypes so its role and the evolutionary forces leading to its specific functional retention in the genome was obscure. Legume pollux mutants are impaired in intracellular accommodation of fungal and bacterial symbionts. We recently discovered that in Arabidopsis, POLLUX is required for full reproductive success of Hyaloperonospora arabidopsidis and the development or maintenance of haustoria, the intracellular infection structures of this biotrophic oomycete. This exciting finding reveals previously suspected, but never actually detected, genetic commonalities between the host programs for the development or maintenance of symbiotic and pathogenic infection structures. We propose to investigate the role of calcium signatures in Arabidopsis for oomycetal haustoria development, which is implied by the involvement of POLLUX. We aim to clarify the predicted role of POLLUX in generating (nuclear) calcium signatures during infection in collaboration with the central calcium-imaging platform of FOR964. Furthermore, we will assess the relevance of candidate decoders of calcium signatures for haustorium development and the transcriptional reprogramming of the host during oomycetal infection. The expected results will help to further dissect the genetic program involved in the decoding of nuclear calcium signals. The analysis of POLLUX-specific transcriptional changes may help to provide answers to the puzzling question why Arabidopsis maintains a gene that has no detectable role but to improve the performance of a pathogen.