Abstract
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 KNOTTED HOMEOBOX (KNOX) gene family encodes transcription factors that regulate plant development. The role of class I KNOX 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 KNOX genes are less well understood, although the Arabidopsis thaliana class II KNOX protein KNAT7 is known to regulate secondary cell wall biosynthesis. To explore the involvement of the KNOX genes in the enormous variability of wood in Cactaceae, we identified orthologous genes expressed in species with fibrous (Pereskia lychnidiflora and Pilosocereus alensis), non-fibrous (Ariocarpus retusus), and dimorphic (Ferocactus pilosus) wood. Both class I and class II KNOX genes were expressed in the cactus cambial zone, including one or two class I paralogs of KNAT1, as well as one or two class II paralogs of KNAT3-KNAT4-KNAT5. While the KNOX gene SHOOTMERISTEMLESS (STM) and its ortholog ARK1 are expressed during secondary growth in the Arabidopsis and Populus stem, respectively, we did not find STM 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 KNOX paralogs from the KNAT7 clade were expressed in the cambial zone of A. retusus and F. pilosus, we did not detect KNAT7 ortholog expression in the cambial zone of P. lychnidiflora. Differences in the transcriptional repressor activity of secondary cell wall biosynthesis by the KNAT7 orthologs could therefore explain the differences in wood development in the cactus species.
Highlights
As reservoirs of pluripotent cells, meristems have played a leading role in the diversification of angiosperm growth forms
The young xylem of A. retusus with non-fibrous wood and F. pilosus with dimorphic wood forms wide-band tracheids and vessel elements with annular and helical secondary wall patterns formed during early development
It is well established that some KNOTTED HOMEOBOX (KNOX) transcription factors are important for the maintenance of the shoot apical meristem, their role in vascular cambium activity and secondary growth is less well understood
Summary
As reservoirs of pluripotent cells, meristems have played a leading role in the diversification of angiosperm growth forms. The wood found in smaller species is generally scarce and non-fibrous, with abundant wideband tracheids and vessel elements similar to those typical of proto- and metaxylem In these species, the level of wood lignification is insignificant and the lignin has a heterogeneous chemical composition (Reyes-Rivera et al, 2015). The mechanisms shaping the development of the vascular cambium and its derivatives in the Cactaceae are mostly unknown; it was suggested that the wide variation in wood anatomy of different cacti species might be attributed to a variation of gene expression patterns and gene expression level (Mauseth and Plemons, 1995; Mauseth, 2006; Landrum, 2008; Vázquez-Sánchez and Terrazas, 2011)
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