Molecular characterisation of a novel plant homeobox gene expressed in thematuring xylem zone of Populus tremula X tremuloides

In trees, wood is produced from the vascular cambium. However, little is kn own about how differentiation of the cambial derivatives is controlled at t he molecular level. Hybrid aspen, Populus tremula x tremuloides, is a fast- growing, wood-forming plant from which an efficient transformation and rege neration protocol has been developed. We use this species as a model to stu dy xylem cell differentiation. In a cross-section of the stem, cells in the cambial region at different stages of development can be seen in a radial distribution. We dissected out the cambial region and constructed a cDNA-li brary from this tissue. The library was screened with a degenerated primer mix complementary to the third helix region of the homeobox. A number of cD NAs hybridising to the probe were isolated, purified, cloned and sequenced. Characterisation of the expression pattern of two such homeobox cDNAs reve aled that one of the cDNAs, denoted PttHB1, was specifically expressed in t he xylem maturation zone, indicating a role in the regulation of secondary wall formation, while the other, denoted PttHB2 was expressed in differenti ating xylem and phloem cells. Structurally, the two hybrid aspen homeobox g enes are homologous to each other, but no close relationship could be ident ified between these genes and any other homeobox sequence described thus fa r, in plants or other eukaryotes. We therefore conclude that the PttHB1 and PttHB2 genes belong to a novel hitherto undescribed sequence class of home obox genes. We designate this as PALE.