Expression of theKNOTTED HOMEOBOXGenes in the Cactaceae Cambial Zone Suggests Their Involvement in Wood Development.
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. TheKNOTTED HOMEOBOX (KNOX)gene family encodes transcription factors that regulate plant development. The role of class IKNOXgenes 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 IIKNOXgenes are less well understood, although theArabidopsis thalianaclass II KNOX protein KNAT7 is known to regulate secondary cell wall biosynthesis. To explore the involvement of theKNOXgenes in the enormous variability of wood in Cactaceae, we identified orthologous genes expressed in species with fibrous (Pereskia lychnidifloraandPilosocereus alensis), non-fibrous (Ariocarpus retusus), and dimorphic (Ferocactus pilosus) wood. Both class I and class IIKNOXgenes were expressed in the cactus cambial zone, including one or two class I paralogs ofKNAT1, as well as one or two class II paralogs ofKNAT3-KNAT4-KNAT5. While theKNOXgeneSHOOTMERISTEMLESS(STM)and its orthologARK1are expressed during secondary growth in theArabidopsisandPopulusstem, respectively, we did not findSTMorthologs in the Cactaceae cambial zone, which suggests possible differences in the vascular cambium genetic regulatory network in these species. Importantly, while two class IIKNOXparalogs from theKNAT7clade were expressed in the cambial zone ofA. retususandF. pilosus, we did not detectKNAT7ortholog expression in the cambial zone ofP. lychnidiflora. Differences in the transcriptional repressor activity of secondary cell wall biosynthesis by theKNAT7orthologs could therefore explain the differences in wood development in the cactus species.
Departamento de Botánica, Instituto de Biología, Universidad Nacional Autónoma de México Mexico City, Mexico.
Jorge
J
Reyes-Rivera
Departamento de Biología Molecular de Plantas, Instituto de Biotecnología, Universidad Nacional Autónoma de México Cuernavaca, Mexico.
Gustavo
G
Rodríguez-Alonso
Departamento de Botánica, Instituto de Biología, Universidad Nacional Autónoma de México Mexico City, Mexico.
Emilio
E
Petrone
Departamento de Botánica, Instituto de Biología, Universidad Nacional Autónoma de México Mexico City, Mexico.
Alejandra
A
Vasco
Jardín Botánico, Instituto de Biología, Universidad Nacional Autónoma de México Mexico City, Mexico.
Francisco
F
Vergara-Silva
Departamento de Biología Molecular de Plantas, Instituto de Biotecnología, Universidad Nacional Autónoma de México Cuernavaca, Mexico.
Svetlana
S
Shishkova
Departamento de Botánica, Instituto de Biología, Universidad Nacional Autónoma de México Mexico City, Mexico.
Teresa
T
Terrazas
Cactaceae; KNAT7 ortholog; KNOX transcription factors; dimorphic wood; fibrous wood; non-fibrous wood; vascular cambium; wood lignification