A common graft formation pathway in Norway spruce and Arabidopsis thaliana identifies the PAT gene family as conserved regulators of tissue attachment and wound healing [RNAseq: Grafted-samples]
The widespread use of plant grafting has enabled different plants to join and combine their best properties to improve stress tolerance, growth and yields. Grafting is commonly performed between closely related eudicots or gymnosperms where mechanisms permit tissue fusion yet limit success as plants become unrelated. To investigate these aspects, we developed a micrografting method using young conifer tree seedlings that enabled divergent conifer members to successfully graft. Conifer grafts showed rapid connection of phloem and xylem at the junction, while a genome-wide transcriptome analysis of the Picea abies (Norway spruce) healing junction revealed differential expression of thousands of genes including those related to auxin response and cell wall biogenesis. We compared these genes to those induced during Arabidopsis thaliana graft healing and found a common activation of cambium, cell division, phloem and xylem-related genes. A gene regulatory network analysis revealed that PHYTOCHROME A SIGNAL TRANSDUCTION 1 (PAT1) acted as a central hub during Picea grafting and was also induced during Arabidopsis grafting. Arabidopsis mutants lacking PATs failed to attach tissues or successfully graft, while complementing Arabidopsis PAT mutants with the Picea abies PAT1 homolog could rescue tissue attachment and enhance callus formation. Together, our data demonstrate a competency for young tissues to graft to distantly related species and identifies the PAT gene family as conserved regulators of graft healing and tissue regeneration in eudicots and gymnosperms. Overall design: Graft junction libraries: Samples were collected from both ungrafted (intact) and grafted Picea abies above and below the graft junction at 0, 1, 3, 7,14 and 28 days after grafting. Approximately 1 mm of tissues from scions or rootstocks or 2 mm from intact plants.