AUTHOR=Reyes-Rivera Jorge , Rodríguez-Alonso Gustavo , Petrone Emilio , Vasco Alejandra , Vergara-Silva Francisco , Shishkova Svetlana , Terrazas Teresa 

TITLE=Expression of the KNOTTED HOMEOBOX Genes in the Cactaceae Cambial Zone Suggests Their Involvement in Wood Development

JOURNAL=Frontiers in Plant Science

VOLUME=8

YEAR=2017

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2017.00218

DOI=10.3389/fpls.2017.00218

ISSN=1664-462X

ABSTRACT=<p>The vascular cambium is a lateral meristem that produces secondary xylem (i.e., wood) and phloem. Different Cactaceae species develop different types of secondary xylem; however, little is known about the mechanisms underlying wood formation in the Cactaceae. The <italic>KNOTTED HOMEOBOX (KNOX)</italic> gene family encodes transcription factors that regulate plant development. The role of class I <italic>KNOX</italic> genes in the regulation of the shoot apical meristem, inflorescence architecture, and secondary growth is established in a few model species, while the functions of class II <italic>KNOX</italic> genes are less well understood, although the <italic>Arabidopsis thaliana</italic> class II KNOX protein KNAT7 is known to regulate secondary cell wall biosynthesis. To explore the involvement of the <italic>KNOX</italic> genes in the enormous variability of wood in Cactaceae, we identified orthologous genes expressed in species with fibrous (<italic>Pereskia lychnidiflora</italic> and <italic>Pilosocereus alensis</italic>), non-fibrous (<italic>Ariocarpus retusus</italic>), and dimorphic (<italic>Ferocactus pilosus</italic>) wood. Both class I and class II <italic>KNOX</italic> genes were expressed in the cactus cambial zone, including one or two class I paralogs of <italic>KNAT1</italic>, as well as one or two class II paralogs of <italic>KNAT3</italic>-<italic>KNAT4</italic>-<italic>KNAT5</italic>. While the <italic>KNOX</italic> gene <italic>SHOOTMERISTEMLESS</italic> (<italic>STM)</italic> and its ortholog <italic>ARK1</italic> are expressed during secondary growth in the <italic>Arabidopsis</italic> and <italic>Populus</italic> stem, respectively, we did not find <italic>STM</italic> orthologs in the Cactaceae cambial zone, which suggests possible differences in the vascular cambium genetic regulatory network in these species. Importantly, while two class II <italic>KNOX</italic> paralogs from the <italic>KNAT7</italic> clade were expressed in the cambial zone of <italic>A. retusus</italic> and <italic>F. pilosus</italic>, we did not detect <italic>KNAT7</italic> ortholog expression in the cambial zone of <italic>P. lychnidiflora</italic>. Differences in the transcriptional repressor activity of secondary cell wall biosynthesis by the <italic>KNAT7</italic> orthologs could therefore explain the differences in wood development in the cactus species.</p>