The auxin-PLETHORA 5 module regulates wood fibre development in poplar in a non-cell-autonomous manner

Abstract Auxin, as a vital phytohormone, is enriched in the vascular cambium, playing a crucial role in regulating wood formation in trees. Despite its significance, the molecular mechanisms underlying the influence of auxin on wood development remain elusive. In this study, we report a transcription factor, PLETHORA 5 (PLT5), whose expression was specifically activated by auxin signalling in the vascular cambium. PLT5 was found to regulate cell expansion and lignification of fibres in poplar. Genetic experiments confirmed the noncell-autonomous regulation of auxin signalling from the vascular cambium and revealed the necessity of PLT5 protein mobility to mediate this process. Remarkably, PLT5 proteins specifically inhibit the initiation of fibre cell wall thickening by directly repressing SND1 genes. This study unveils a sophisticated model wherein the auxin-PLT5 signalling cascade intricately regulates wood fibre development in poplar by fine-tuning the thickening of fibre cell walls.