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Tracking biochemical changes during adventitious root formation in olive (Olea europaea L.)

Abstract:

The activity of oxidative enzymes and the levels of free auxins were determined during adventitious root formation in olive explants. Rooting trials were performed both with in vitro-cultured microshoots of the cultivar 'Galega Vulgar', treated with indole-3-butyric acid (IBA) and with salicylhydroxamic acid (SHAM) + IBA, as well as with semi-hardwood cuttings of the cultivars 'Galega Vulgar' (difficult-to-root) and 'Cobrançosa' (easy-to-root), treated with IBA. The auxin (IBA) was used in all experiments as a rooting promoter, while SHAM was used in micropropagation trials as rooting inhibitor, providing a negative control. Free indole-3-acetic acid (IAA) and IBA concentrations were determined in microshoots, as well as in semi-hardwood cuttings, throughout the rooting period at pre-established time-points. At the same time-points, the enzymatic activity of polyphenol oxidases (PPO), peroxidases (POX), and IAA oxidase (IAAox) was evaluated in the microshoots. Microshoots treated with SHAM + IBA revealed higher POX and IAAox activity, as well as lower PPO activity, than those treated only with IBA. IAA levels were higher in IBA-treated microshoots during induction phase, but lower during early initiation phase. In contrast, free IBA levels were higher in microshoots treated with SHAM + IBA during induction, but lower during initiation. A similar pattern of free auxin levels was observed in semi-hardwood cuttings of the two contrasting cultivars under evaluation. The similarities found on the auxin patterns of microshoots treated with SHAM and those of semi-hardwood cuttings of the difficult-to-root olive cultivar allow considering SHAM a reliable control for when simulation of a difficult-to-root behavior is necessary. The inhibitory effect of SHAM in root formation could be related with 1) the inhibition of alternative oxidase (AOX), leading to a downregulation of phenylpropanoid biosynthetic pathways, which would decrease the concentration of phenolic substrates for PPO; 2) an increase in IAAox activity resulting in lower free IAA levels or 3) a defective conversion of IBA into IAA.