Ectopic deposition of lignin in the pith of stems of two Arabidopsis mutants

The biosynthesis of lignin in vascular plants is regulated both development ally and environmentally. In the inflorescence stems of Arabidopsis, lignin is mainly deposited in the walls of xylem cells and interfascicular fiber cells during normal plant growth and development. The mechanisms controllin g the spatial deposition of lignin remain unknown. By screening ethyl metha nesulfonate-mutagenized populations of Arabidopsis, we have isolated two al lelic elp1 (ectopic deposition of lignin in pith) mutants with altered lign in deposition patterns. In elp1 stems, lignin was ectopically deposited in the walls of pith parenchyma cells in addition to its normal deposition in the walls of xylem and fiber cells. Lignin appeared to be deposited in patc hes of parenchyma cells in the pith of both young and mature elp1 stems. Th e ectopic deposition of lignin in the pith of elp1 stems was accompanied by an increase in the activities of enzymes in the lignin biosynthetic pathwa y and with the ectopic expression of caffeoyl coenzyme A O-methyltransferas e in pith cells. These results indicate that the ELP1 locus is involved in the repression of the lignin biosynthetic pathway in the pith. Isolation of the elp1 mutants provides a novel means with which to study the molecular mechanisms underlying the spatial control of lignification.