Secondary development in the stem: when Arabidopsis and trees are closer than it seems

- Many regulatory factors discovered in Arabidopsis are conserved in woody species.
- Thermospermine regulates xylem development in Arabidopsis and poplar.
- Thermospermine controls cell proliferation via bHLH transcription factors.
- Several LRR-RLK circuits mediate different aspects of secondary growth.

Secondary growth, the increase in girth of plant organs, is primarily driven by the vascular and cork cambium. In perennial dicotyledons and gymnosperms, it represents a major source of biomass accumulation in the form of wood. However, the molecular framework underlying secondary growth is largely based on studies in the annual herbaceous plant Arabidopsis thaliana. In this review, we will focus on a selection of major regulators of stem secondary growth, which have recently been shown to play a role in woody species. In particular, we will focus on thermospermine and its bivalent role in controlling xylem differentiation and cell proliferation and we will highlight the contributions of the different LRR-Receptor-Like Kinase signaling hubs.