Biochemical and morphological changes in micropropagated shoots of GiSeLa6® (Prunus spp.) rootstock inoculated with Trichoderma harzianum strain T-22

Trichoderma harzianum strain T-22 (T22) has the ability of enhancing root growth and plant development. The aim of this research is to explain the biochemical basis of the direct plant-growth-promoting activity of T22. Seven days after the transfer to root-inducing medium, in vitro-cultured shoots of GiSeLa6® (Prunus cerasus x P. canescens), an important Prunus rootstock for sweet and sour cherry varieties, were inoculated with T22. Root and shoot growth were significantly affected by T22 (+76 and +61%, respectively). Indole-3-acetic acid (IAA), trans-zeatin riboside (t-ZR) and dihydrozeatin riboside (DHZR) were analyzed by HPLC/MS. Acidification of the medium by plant, T22, and plant + T22 were assessed, whereas root morphological changes were observed by light and epifluorescence microscopic analysis. Nine days after inoculation, the levels of indole-3-acetic acid (IAA), trans-zeatin riboside (t-ZR), dihydrozeatin riboside (DHZR), gibberellic acid (GA3) and abscisic acid (ABA) were analyzed by high performance liquid chromatography coupled with mass spectrometry. The results showed that after T22-inoculation, IAA and GA3 significantly increased in both leaves (+49 and +71%, respectively) and roots (+40 and +143%, respectively) whereas t-ZR decreased (–51% in leaves and –37% in roots). Changes in DHZR were observed in T22-inoculated roots (–32%) but not in leaves, whereas the levels of ABA did not differ between the two treatments. Root activity determined a decline of medium acidity, and this effect was more marked in T22-inoculated plants (up to pH 4). Microscopic analysis revealed changes in root cell wall suberification in the exoderm and endoderm, with an increase in suberized cellular layers from 1 to 2-3, and an enhancement of cell wall epifluorescence. All these T22-induced changes promote rooting and shoot growth, and they could increase plant survival during the acclimatisation phase of nursery processes.