Poor adventitious root formation is a major obstacle in micropropagation and in conventional propagation. This paper reviews recent progress in the understanding of adventitious root formation as a developmental process focusing on the role of plant hormones and on the effect of rooting conditions on plant performance. Since the discovery of the rhizogenic effect of auxin ca. 70 yr ago, no new broadly applicable rooting treatments have been developed. Recent research, though, may lead to new rooting procedures. Application of wounding-related compounds may be effective in difficult-to-root crops. Furthermore, by adapting conditions during the propagation phase, microcuttings with an enhanced capability to root may be produced. These conditions include elongation of stems (by etiolation or double-layer culture) and repeated subculture (rejuvenation; i.e. transition from adult to juvenile). Data are presented that show that during tissue culture maturation (transition from juvenile to adult) also occurs. The conditions during the in vitro rooting treatment may have a tremendous effect on performance after transfer ex vitro. In particular, accumulation of ethylene during in vitro rooting may have a devastating effect. Addition of stress-protecting compounds during propagation or rooting in vitro may enhance the performance ex vitro
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