Dealing with plant bioregulators in Italy: From the introduction to now

Plant bioregulators (PBRs) are natural or synthetic substances able to interfere with the plant's hormone system controlling important physiological processes in trees. The discovery of plant hormones (auxins, gibberellins, cytokinins, ethylene and abscisic acid) and the research and development of PBRs has been always an evolving process. PBRs research started in 1930s, when the natural auxin indoleacetic acid (IAA) was discovered, and the first Italian experience began in the late 1940s, when auxin-like compounds as indolebutyric acid (IBA), α-naphthalenacetic acid (1-NAA) and its amide (NAAm) found practical uses. Gibberellic acid (GA3) was isolated in the 1950s and identified a decade later. In Italy, Berelex (GA3) was tested as fruit-setting agent, while Regulex (GA4-7) was tested in the 1980s for apple fruit skin russeting control. The study on GA metabolism allowed to understand the action of "growth retardants", whose research began in the mid-1960s with chlormequat (CCC) and daminozide (SADH), in the 1980s with paclobutrazol, and, finally, in 2003 with prohexadione-Ca. Zeatin, the first cytokinin, was discovered at the beginning of the 1960s. Nowadays, 6-benzyladenine (6-BA) is widely used as thinning agent on pomefruit and the mixture with GA4+7 allows to control fruit shape elongation in apple. In the 1960s, ethylene was recognized as gaseous hormone for fruit ripening control. Ethylene releasing compounds were used for enhancing fruit coloration in table grape, as a mechanical harvesting aid, and as a thinning agent for stonefruit. The study on ethylene biosynthesis recently led to find inhibitors of its action (aminoetoxyvinylglycine, ReTain) and perception (1-methylcyclopropene, SmartFresh). Abscisic acid (ABA) induces fruit abscission, stomatal closure and fruit ripening and colour enhancement. A commercial formulation (ProTone) has been recently used in table grapes. Other growth substances have been also shown to regulate physiological processes in trees: brassinosteroids, polyamines, jasmonates, and strigolactones, the latter being a new class of hormones regulating plantmycorrhizal interactions and playing a role in controlling apical dominance. As a conclusive remark, PBRs represent an important tool, although they must be considered as part of a larger portfolio of options for the grower to control vigour and improving cropping.