Differential Gene Expression During Maturation-Caused Decline in Adventitious Rooting Ability in Loblolly Pine (Pinus taeda L.)

Abstract

Loss of adventitious rooting ability occurs very quickly in loblolly pine seedlings, where hypocotyl cuttings from 50-day-old seedlings root readily in response to auxin, while epicotyl cuttings from the same seedling do not, despite their anatomical similarity (Diaz-Sala et al., 1996a). In hypocotyl cuttings, auxin causes root meristems to organize in the cambial region centrifugal to the former primary xylem poles over a 12-day period. In epicotyl cuttings, the cambium dedifferentiates in response to auxin, but roots rarely form, and then only after 50 or more days. A brief exposure (a 5 min pulse) to auxin is sufficient to promote rooting. While both types of cutting transport auxin in a polar manner, a pulse of N-(l-napthl)phthalamic acid (NPA), which inhibits auxin efflux, also delays rooting in hypocotyls only if applied within the first 3 days after the auxin pulse; thereafter it has no effect. These observations show that key events that determine root meristem formation occur in the first 72 hours, and that auxin binding to the efflux carrier that mediates polar auxin transport, may be necessary for rooting (Diaz-Sala et al., 1996a).