The gapped xylem mutant identifies a common regulatory step in secondary cell wall deposition

The phenotype of the novel gapped xylem (gpx) mutant is described. gpx plants exhibit gaps in the xylem in positions where xylem elements would normally be located. These gaps are not part of the transpiration stream and result in gpx plants having fewer functional xylem elements. The gaps are due to the absence of a secondary cell wall in developing xylem elements, resulting in complete degradation of these elements during cell death, and illustrate the importance of the secondary cell wall in retaining a functional xylem element following programmed cell death. Consequently the gpx phenotype suggests that the processes of secondary cell wall formation and cell death are independently regulated in developing xylem. gpx plants also exhibit a highly irregular pattern of secondary cell wall thickening in interfascicular cells, with some cells apparently undergoing little or no secondary cell wall deposition. Secondary cell wall deposition in plants involves the co-ordinate regulation of several complex metabolic pathways. The gpx mutant identifies a key step involved in regulating the deposition of secondary cell wall material in both xylem and interfascicular cells, and suggests that a common regulatory step controls secondary cell wall formation in these diverse cell types. The gpx mutant offers a unique opportunity to elucidate the mechanism by which the complex processes involved in secondary cell wall formation are co-ordinately regulated.