Plant Vascular Cell Division Is Maintained by an Interaction between PXY and Ethylene Signalling

<div><p>The procambium and cambium are meristematic tissues from which vascular tissue is derived. Vascular initials differentiate into phloem towards the outside of the stem and xylem towards the inside. A small peptide derived from <em>CLV-3/ESR1-LIKE 41</em> (<em>CLE41</em>) is thought to promote cell divisions in vascular meristems by signalling through the PHLOEM INTERCALLATED WITH XYLEM (PXY) receptor kinase. <em>pxy</em> mutants, however, display only small reductions in vascular cell number, suggesting a mechanism exists that allows plants to compensate for the absence of PXY. Consistent with this idea, we identify a large number of genes specifically upregulated in <em>pxy</em> mutants, including several AP2/ERF transcription factors. These transcription factors are required for normal cell division in the cambium and procambium. These same transcription factors are also upregulated by ethylene and in ethylene-overproducing <em>eto1</em> mutants. <em>eto1</em> mutants also exhibit an increase in vascular cell division that is dependent upon the function of at least 2 of these <em>ERF</em> genes. Furthermore, blocking ethylene signalling using a variety of ethylene insensitive mutants such as <em>ein2</em> enhances the cell division defect of <em>pxy</em>. Our results suggest that these factors define a novel pathway that acts in parallel to PXY/CLE41 to regulate cell division in developing vascular tissue. We propose a model whereby vascular cell division is regulated both by PXY signalling and ethylene/ERF signalling. Under normal circumstances, however, PXY signalling acts to repress the ethylene/ERF pathway.</p>