Transcriptome dynamics at the Arabidopsis graft junction reveal an inter-tissue recognition mechanism that activates vascular regeneration

35 36 The ability for cut tissues to join together and form a chimeric organism is a 37 remarkable property of many plants, however, grafting is poorly characterized at the 38 molecular level. To better understand this process we monitored genome-wide 39 temporal and spatial gene expression changes in grafted Arabidopsis thaliana 40 hypocotyls. Tissues above and below the graft rapidly developed an asymmetry such 41 that many genes were more highly expressed on one side than the other. This 42 asymmetry correlated with sugar responsive genes and we observed an accumulation 43 of starch above the graft that decreased along with asymmetry once the sugar44 transporting vascular tissues reconnected. Despite the initial starvation response 45 below the graft, many genes associated with vascular formation were rapidly 46 activated in grafted tissues but not in cut and separated tissues indicating that a 47 recognition mechanism activated that was independent of functional vascular 48 connections. Auxin which is transported cell-to-cell, had a rapidly elevated response 49 that was symmetric, suggesting that auxin was perceived by the root within hours of 50 tissue attachment to activate the vascular regeneration process. A subset of genes 51 were expressed only in grafted tissues, indicating that wound healing proceeded via 52 different mechanisms depending on the presence or absence of adjoining tissues. Such 53 a recognition process could have broader relevance for tissue regeneration, inter54 tissue communication and tissue fusion events. 55 56 INTRODUCTION 57 For millennia people have cut and rejoined plants through grafting. Generating such 58 chimeric organisms combines desirable characteristics from two plants, such as 59 disease resistance, dwarfing and high yields, or can propagate plants and avoid the 60 delays entailed by a juvenile state (1). Agriculturally, grafting is becoming more 61 relevant as a greater number of plants and species are grafted to increase productivity 62 and yield (2). However, our mechanistic understanding of grafting and the biological 63 processes involved, including wound healing, tissue fusion and vascular formation, 64 remain limited. 65 66 Plants have efficient mechanisms to heal wounds and cuts, in part through the 67 production of wound-induced pluripotent cells termed callus. The callus fills the gap 68 . CC-BY-NC-ND 4.0 International license not peer-reviewed) is the author/funder. It is made available under a The copyright holder for this preprint (which was . http://dx.doi.org/10.1101/198598 doi: bioRxiv preprint first posted online Oct. 5, 2017;