<i>PHLOEM EARLY DOFs (PEARs)</i> are induced within clubroot galls as a consequence of cambial stimulation and vascular reprogramming during <i>Plasmodiophora brassicae</i> infection in <i>Arabidopsis thaliana</i>

Clubroot galls represent complex cellular structures produced by Plasmodiophora brassicae, a soil-borne protist, which infects below-ground parts of Brassicaceae plants, including Arabidopsis thaliana. This obligate biotroph hijacks the host machinery to manipulate vascular development and patterning. The galls display a dramatic cellular reprogramming where phloem proliferation is enhanced while xylogenesis is suppressed. Here, we show that at least three PHLOEM EARLY DOF (PEAR) genes-PEAR1, DOF6, and HIGH CAMBIAL ACTIVITY2 (HCA2)-are induced within proliferating phloem during gall development. Mutation of these genes, however, does not decrease pathogen-driven phloem proliferation. To find upstream cues governing PEAR activities in galls, we compared auxin-cytokinin signalling responses within hypocotyls of mock- and P. brassicae-inoculated plants using fluorescent reporter lines and found strong auxin signalling responses within galls. We then modified auxin homeostasis in galls through chemical perturbation of its biosynthesis, transport, and signalling, by employing synthetic inhibitors. These compounds drastically changed vascular development and patterning; however, they did not inhibit the induction of DOF6 promoter activity upon P. brassicae infection. Our results indicate that since PEAR genes promote cell proliferation in the phloem lineage, their induction reflects the altered proliferative status within the gall, rather than a direct impact of P. brassicae infection to overdrive phloem production.