Shade Inhibits Cambial Activity in Populus Stems by the SPL16/SPL23-Mediated Cytokinin Pathway

Trees in natural forests or plantations often encounter neighbor proximity signal that negatively impacts wood production. However, the molecular basis underlying shade-regulation of vascular cambial activity during stem radial growth remains unknown in woody species. Here, we revealed that high stand density and simulated shade (low R/FR ratio) suppress the division and differentiation of cambial cells in poplar stems. A genome-wide screen for Populus SQUAMOSA PROMOTER BINDING PROTEIN-LIKE (SPL) genes identified that SPL16 and SPL23 are preferentially expressed in the phloem and cambium, being downregulated by simulated shade. Knocking out SPL16/23 impaired cambial activity, whereas phloem-specific overexpression of SPL16 stimulated cambial proliferation and mitigated the shade-inhibition of cambial activity. Additionally, shade decreased bioactive cytokinin (CK) levels by suppressing the expression of CK biosynthesis genes IPT5a, IPT5b and LOG1b in poplar stems. Molecular and genetic studies reveled that SPL16/23 directly activate IPT5s/LOG1b expression to promote CK biosynthesis and cambial activity. Moreover, elevated miR156 expression in shade-treated stems regulated SPL16/23 at the post-transcriptional level, mediating shades effects on cambial activity. Collectively, our findings unravel that the miR156-SPL16/23-IPT5/LOG1-cytokinin pathway operates in the shade-mediated inhibition of cambial activity, providing potential targets for the genetic improvement of shade-tolerant trees.