PagKNAT2/6b regulates tension wood formation and gravitropism by targeting cytokinin metabolism.

Tension wood (TW) is a specialised xylem tissue associated with gravitropism in angiosperm trees. However, few regulators of TW formation have been identified. The molecular mechanisms underpinning TW formation remain elusive. Here, we report that a Populus KNOTTED-like homeobox gene, PagKNAT2/6b, is involved in TW formation and gravity response. Transgenic poplar plants overexpressing PagKNAT2/6b displayed more sensitive gravitropism than controls, as indicated by increased stem curvature. Microscopic examination revealed greater abundance of fibre cells with a gelatinous cell wall layer (G-layer) and asymmetric growth of secondary xylem in PagKNAT2/6b overexpression lines. Conversely, PagKNAT2/6b dominant repression plants exhibited decreased TW formation and reduced response to gravity stimulation. Moreover, sensitivity to gravity stimulation showed a negative relationship with development stage. Expression of genes related to growth and senescence was affected in PagKNAT2/6b transgenic plants. More importantly, transcription activation and electrophoretic mobility shift assays suggested that PagKNAT2/6b promotes the expression of cytokinin metabolism genes. Consistently, cytokinin content was increased in PagKNAT2/6b overexpression plants. Therefore, PagKNAT2/6b is involved in gravitropism and TW formation, likely via modulation of cytokinin metabolism.