What we have learned about the physiology of in vitro adventitious rooting of woody plants and how it relates to improvements in the practice

Natural auxins and synthetic analogs are the most powerful exogenous stimulators of adventitious rooting of ligneous as well as for herbaceous cuttings. For several years we have investigated the physiology of rooting, including the metabolism of endogenous auxins and their molecular mechanisms of action. Our work has resulted in discoveries leading to practical strategies allowing faster and improved rooting of cuttings from shoots raised in vitro, and subsequently resulting in roots better adapted for acclimatization and to overcome so-called (apparent) recalcitrance-to-rooting of some species. In this chapter we describe: (i) how variation in auxin treatments during rooting can be used to regulate the endogenous levels of auxins; (ii) the importance of not maintaining auxin treatment throughout the whole rooting process, thereby the endogenous auxin level itself not always being high; (iii) the division of the organogenetic process into at least three main rooting interdependent phases, i.e. induction, initiation, and expression, with different durations depending on the species and with different requirements for each phase, and (iv) how polyamines, even if less effective from outside, and their metabolism are as important as auxins in the rooting process. We will also describe how cross-talk between hormones, manipulations with related genes and mutants, and some similarities between the induction of rooting and evocation of flowering, offer new tools for further progress in the field.