Plant hormones coordinate monolignol biosynthesis with seasonal changes in Populus tomentosa

Temperate woody plants suffer environmental stresses caused by seasonal changes. Seasonal changes affect tree cambium activity, resulting in the annual periodicity of growth. In the present study, we provide a model for plant hormone coordination of monolignol biosynthesis to cope with the seasonal changes in a poplar species. First, 27 core lignin-related genes were identified in Populus tomentosa, and co-expression in lignifying tissues was identified. Combining spatial and seasonal expression analysis and the seasonal dynamic changes in plant hormones of Populus tomentosa, we found that poplars reorganized plant hormone (cytokinin;3-indoleacetic acid, IAA; and abscisic acid, ABA) biosynthesis and transport pathways with a winter-down pattern, resulting in decreases of cytokinin, IAA, and ABA content in cambium cells. The same expression pattern was also found for lignin-related genes involved in monolignol precursors biosynthesis. The monolignol specific pathway genes PtoCAD1, 2, 8, PtoCCR1, 7 and PtoF5H2 showed contrasting winter-up expression patterns. We propose that 1) plant hormones, such as cytokinin, IAA, and ABA regulate the cambium activity and coordinate monolignol biosynthesis, especially the components upstream of this pathway, with seasonal changes in Populus tomentosa; 2) the inhibition of the biosynthesis of hydroxycinnamoyl-CoA esters and the accumulation of monolignol in winter are responsible for surviving the cold winter and provide materials for fast growth during the following spring.