Short-day photoperiod enhances graft success by increasing auxin accumulation

Plant grafting is a key horticultural practice used to promote agricultural performance by improving crop productivity and enhancing resistance to biotic and abiotic stresses. Successful graft union formation depends on tissue adhesion, callus formation, and vascular reconnection, which are regulated primarily by auxin signaling. Although environmental factors influence grafting union formation by modulating hormonal signaling, the role of photoperiod in graft union formation remains poorly understood. Here, we demonstrate that short-day (SD) photoperiods markedly enhance grafting success in Arabidopsis thaliana by promoting hypocotyl elongation and increasing auxin accumulation at the graft junction. DR5:GUS reporter assays revealed strong photoperiod-dependent differences in auxin responsiveness, with the highest auxin levels under SD conditions and substantially reduced levels under long-day (LD) and medium-day (MD) conditions. Exogenous indole-3-acetic acid (IAA) treatment restored the grafting success rate under LD and MD conditions, whereas inhibition of polar auxin transport by 2,3,5-triiodobenzoic acid (TIBA) significantly reduced grafting success under SD conditions. Notably, TIBA treatment did not lead to a further significant reduction under LD and MD conditions, where endogenous auxin levels were already limiting, confirming the necessity of localized auxin accumulation at the graft junction. Furthermore, transferring grafted seedlings from LD to SD conditions restored grafting success, highlighting the plasticity of light-responsive transcriptional pathways that regulate auxin biosynthesis. Together, these findings reveal that photoperiod enhances grafting success primarily by increasing endogenous auxin accumulation, which in turn enables efficient auxin transport and the activation of auxin-mediated regenerative programs at the graft junction. This work establishes photoperiod as a key environmental determinant of grafting efficiency and provides a framework for optimizing grafting outcomes through the modulation of light conditions and hormonal signaling.