BIL1-mediated MP phosphorylation integrates PXY and cytokinin signalling in secondary growth

The Dynamics of Cambial Stem Cell Activity

Stem cell populations in meristematic tissues at distinct locations in the plant body provide the potency of continuous plant growth. Primary meristems, at the apices of the plant body, contribute mainly to the elongation of the main plant axes, whereas secondary meristems in lateral positions are responsible for the thickening of these axes. The stem cells of the vascular cambium-a secondary lateral meristem-produce the secondary phloem (bast) and secondary xylem (wood). The sites of primary and secondary growth are spatially separated, and mobile signals are expected to coordinate growth rates between apical and lateral stem cell populations. Although the underlying mechanisms have not yet been uncovered, it seems likely that hormones, peptides, and mechanical cues orchestrate primary and secondary growth. In this review, we highlight the current knowledge and recent discoveries of how cambial stem cell activity is regulated, with a focus on mobile signals and the response of cambial activity to environmental and stress factors.

Auxin-mediated Aux/IAA-ARF-HB signaling cascade regulates secondary xylem development in Populus

Wood development is strictly regulated by various phytohormones and auxin plays a central regulatory role in this process. However, how the auxin signaling is transducted in developing secondary xylem during wood formation in tree species remains unclear. Here, we identified an Aux/INDOLE-3-ACETIC ACID 9 (IAA9)-AUXIN RESPONSE FACTOR 5 (ARF5) module in Populus tomentosa as a key mediator of auxin signaling to control early developing xylem development. PtoIAA9, a canonical Aux/IAA gene, is predominantly expressed in vascular cambium and developing secondary xylem and induced by exogenous auxin. Overexpression of PtoIAA9m encoding a stabilized IAA9 protein significantly represses secondary xylem development in transgenic poplar. We further showed that PtoIAA9 interacts with PtoARF5 homologs via the C-terminal III/IV domains. The truncated PtoARF5.1 protein without the III/IV domains rescued defective phenotypes caused by PtoIAA9m. Expression analysis showed that the PtoIAA9-PtoARF5 module regulated the expression of genes associated with secondary vascular development in PtoIAA9m- and PtoARF5.1-overexpressing plants. Furthermore, PtoARF5.1 could bind to the promoters of two Class III homeodomain-leucine zipper (HD-ZIP III) genes, PtoHB7 and PtoHB8, to modulate secondary xylem formation. Taken together, our results suggest that the Aux/IAA9-ARF5 module is required for auxin signaling to regulate wood formation via orchestrating the expression of HD-ZIP III transcription factors in poplar.

Plant Vascular Tissues-Connecting Tissue Comes in All Shapes

For centuries, humans have grown and used structures based on vascular tissues in plants. One could imagine that life would have developed differently without wood as a resource for building material, paper, heating energy, or fuel and without edible tubers as a food source. In this review, we will summarise the status of research on Arabidopsis thaliana vascular development and subsequently focus on how this knowledge has been applied and expanded in research on the wood of trees and storage organs of crop plants. We will conclude with an outlook on interesting open questions and exciting new research opportunities in this growing and important field.