<i><scp>WUSCHEL</scp>‐<scp>RELATED HOMEOBOX</scp>4 (<scp>WOX</scp>4)</i>‐like genes regulate cambial cell division activity and secondary growth in<i>Populus</i>trees

Abstract

Summary Plant secondary growth derives from the meristematic activity of the vascular cambium. In Arabidopsis thaliana , cell divisions in the cambium are regulated by the transcription factor WOX 4 , a key target of the CLAVATA 3 ( CLV 3)/ EMBRYO SURROUNDING REGION ( ESR )‐ RELATED 41 ( CLE 41) signaling pathway. However, function of the WOX 4 ‐like genes in plants that are dependent on a much more prolific secondary growth, such as trees, remains unclear. Here, we investigate the role of WOX 4 and CLE 41 homologs for stem secondary growth in Populus trees. In Populus , Ptt WOX 4 genes are specifically expressed in the cambial region during vegetative growth, but not after growth cessation and during dormancy, possibly involving a regulation by auxin. In Ptt WOX 4a/b RNA i trees, primary growth was not affected whereas the width of the vascular cambium was severely reduced and secondary growth was greatly diminished. Our data show that in Populus trees, Ptt WOX 4 genes control cell division activity in the vascular cambium, and hence growth in stem girth. This activity involves the positive regulation of Ptt WOX 4a/b through Ptt CLE 41‐ related genes. Finally, expression profiling suggests that the CLE 41 signaling pathway is an evolutionarily conserved program for the regulation of vascular cambium activity between angiosperm and gymnosperm tree species.