Vascular expression of Populus LRR-RLK genes and the effects of their overexpression on wood formation

Abstract

Leucine-rich repeat receptor-like kinases (LRR-RLKs) constitute the largest subfamily of receptor-like kinases (RLKs) in land plants. Although some LRR-RLKs have been characterized as playing key roles in cell-to-cell communications, the physiological roles of most LRR-RLKs remain to be determined. In this study, we first screened the Populus trichocarpa expressed sequence tag and microarray database to look for PtLRR-RLKs differentially and preferentially expressed across the vascular cambial zone. The cell-specific expression of seven selected PtLRR-RLK genes was then verified by the GUS (β-glucuronidase) reporter gene assay. Nearly all of the seven types of plants transformed with PtLRR-RLKpro:GUS fusions showed a high level of transcription in the developing primary xylem and interfascicular cambium during the early stages of xylem differentiation. In stems undergoing secondary growth, the GUS activities driven by the seven PtLRR-RLK promoters were mainly associated with the cambial zone. These seven PtLRR-RLK genes were then individually overexpressed in hybrid poplars, and most transgenic poplars did not exhibit significant phenotypic changes. However, overexpression of PtLRR-RLK1, the putative ortholog of TDR/PXY in Arabidopsis, led to an ectopic accumulation of lignin in the pith along with an enlarged secondary xylem and increased lignin content without perceptible effect on secondary cell wall thickness, which made this transgene a likely candidate as a transducer of signals during plant secondary growth and wood formation.