ReviewReviewing current knowledge on olive (Olea europaea L.) adventitious root formation

- Olive may become a model species for studies on adventitious rooting.
- Fundamental research on adventitious root formation is scarce.
- Analytical methods are difficult to establish in woody perennials.
- Studies must be redirected to specific tissues and cells.
- Biochemical analyses are inefficient without concomitant genetic evaluation.

Olive (Olea europaea) is one of the most important fruit species in the Mediterranean basin, where 95% of the world's olive orchards are planted, and it has become an economically valuable crop worldwide, due to an increasing interest in olive oil for human consumption. New olive orchards are being planted outside the Mediterranean, calling for an effort to identify the genotypes best adapted to the new conditions. However, some olive cultivars remain difficult to propagate, which significantly reduces the capacity to use the full genetic diversity of the species. Improving rooting ability in cuttings from recalcitrant olive cultivars has become a critical topic, which implies fundamental research on the anatomy, physiology, biochemistry and genetics of the adventitious root formation process. Besides, the existence of different rooting behaviors among olive cultivars also makes the species a candidate model plant for these studies. Olive propagation techniques evolved through time from field- or nursery-planted hardwood cuttings, to semi-hardwood cuttings in greenhouses under mist, and, more recently, to in vitro culture techniques. Nevertheless, research about adventitious root formation carried on each propagation method was mostly based on trial and error approaches. Researchers have mainly investigated different factors involved in the process of adventitious rooting by testing their effect in the rooting capacity of different cultivars, leading to a high dispersion and fragmentation of the available information. The goal of this review is to present the most relevant results achieved on adventitious root formation in olive cuttings, aiming to provide an integrated perspective of the current knowledge.