The auxin-PLETHORA 5 module regulates wood fibre development in Populus tomentosa

Auxin, as a vital phytohormone, is enriched in the vascular cambium, playing a crucial role in regulating wood formation in trees. While auxin's influence on cambial stem cells is well established, the molecular mechanisms underlying the auxin-directed development of cambial derivatives, such as wood fibres, remain elusive in forest trees. Here we identified a transcription factor, AINTEGUMENTA-like 5 (AIL5)/PLETHORA 5 (PLT5) from Populus tomentosa, that is specifically activated by auxin signalling within the vascular cambium. PLT5 regulated both cell expansion and cell wall thickening in wood fibres. Genetic analysis demonstrated that PLT5 is essential for mediating the action of auxin signalling on wood fibre development. Remarkably, PLT5 specifically inhibits the onset of fibre cell wall thickening by directly repressing SECONDARY WALL-ASSOCIATED NAC DOMAIN 1 (SND1) genes. Our findings reveal a sophisticated auxin-PLT5 signalling pathway that finely tunes the development of wood fibres by controlling cell wall thickening.