PUCK: A Novel Regulator of Arabidopsis Secondary Growth

Abstract

Secondary growth is thickening of the plant stem, hypocotyl and root, providing structural support and increased nutrient transport to the growing plant. The main driver of secondary growth is the vascular cambium, a bifacial stem cell population which gives rise to xylem and phloem via organised cell division and differentiation.
The TRACHEARY ELEMENT DIFFERENTIATION INHIBITORY FACTOR (TDIF) peptide ligand diffuses into the cambium and signals to the PHLOEM INTERCALATED WITH XYLEM (PXY) receptor-like kinase to promote cell division, vascular organisation and repress xylem identity. However, several aspects of TDIF and PXY biosynthesis, turnover and signalling are not fully understood, in part because all the factors involved have not yet been identified. CLE41 (CLAVATA3/EMBRYO SURROUNDING REGION 41) is the precursor of TDIF. 35S::CLE41 lines, characterised by ectopic cambium, were used in a
mutagenesis screen to identify novel TDIF-PXY signalling components. This screen identified PUCK, which contains WD40 repeat domains functioning in protein-protein
interactions, as a suppressor of 35S::CLE41. Here, GUS transcriptional reporter analysis revealed that PUCK is expressed in the phloem and xylem parenchyma. Histochemical
staining identified how puck affects the differentiated vascular tissue. pxy puck mutants showed a significantly increased xylem width, but no increase in xylem vessels,
suggesting PUCK does not regulate xylem differentiation. pxy puck mutants showed a significantly decreased phloem width compared to wild type, but a significantly wider
phloem than pxy single mutants. This suggests that PUCK may repress phloem differentiation within the TDIF-PXY pathway. It is hypothesised that PUCK may be a scaffold protein involved in influencing phloem production. Lignified xylem vessels make up woody tissue, therefore, understanding secondary growth could allow manipulation of wood formation to increase forest productivity. Secondary growth also provides stability to plant stems and the phloem allows reallocation of resources, manipulation of which could be useful for increasing climate resilience.