To date, almost no information is available in roots and shoots of the model plant Arabidopsis thaliana about the hierarchic relationship between metal accumulation, phytohormone levels, and glutathione/phytochelatin content, and how this relation affects root development. For this purpose, specific concentrations of cadmium, copper and zinc, alone or in triple combination, were supplied for 12 days to in vitro growing seedlings. The accumulation of these metals was measured in roots and shoots, and a significant competition in metal uptake was observed. Microscopic analyses revealed that root morphology was affected by metal exposure, and that the levels of trans-zeatin riboside, dihydrozeatin riboside, indole-3-acetic acid and the auxin/cytokinin ratio varied accordingly. By contrast, under metal treatments, minor modifications in gibberellic acid and abscisic acid levels occurred. Real-time polymerase chain reaction analysis of some genes involved in auxin and cytokinin synthesis (e.g. AtNIT in roots and AtIPT in shoots) showed on average a metal up-regulated transcription. The production of thiol-peptides was induced by all the metals, alone or in combination, and the expression of the genes involved in thiol-peptide synthesis (AtGSH1, AtGSH2, AtPCS1 and AtPCS2) was not stimulated by the metals, suggesting a full post-transcriptional control. Results show that the Cd/Cu/Zn-induced changes in root morphology are caused by a hormonal unbalance, mainly governed by the auxin/cytokinin ratio.
Arabidopsis thaliana L. is a model plant but little information is available about morphological root changes as part of a phytohormonal common response against both biotic and abiotic stressors. For this purpose, two-week-old Arabidopsis seedlings were treated with 10 μM CdSO4 or infected with CMV. After 12 days the entire aerial parts and the root system were analyzed, and the presence of CMV or the accumulation of Cd were detected. Microscopic analysis revealed that both CMV and Cd influenced root morphology by a marked development in the length of root hairs and an intense root branching if compared to controls. Among the three treatments, Cd-treated seedlings showed a shorter root axis length and doubled their lateral root diameter, while the lateral roots of CMV-infected seedlings were the longest. The root growth patterns were accompanied by significant changes in the levels of indole-3-acetic acid, trans-zeatin riboside, dihydrozeatin riboside, as a probable consequence of the regulation of some genes involved in their biosynthesis/degradation. The opposite role on root development played by the phythormones studied is discussed in detail. The results obtained could provide insights into novel strategies for plant defense against pathogens and plant protection against pollutants.
Trichoderma harzianum strain T-22 (T22) is one of the most effective strains of this fungus that is able to colonise the roots of most plant species across a wide range of soil types. This fungus is used as a biocontrol agent during crop production, and for the improvement of the rooting and acclimatisation phases in plant nurseries. In vitro-cultured shoots of GiSeLa6 ® (Prunus cerasus ϫ P. canescens) and of GF677 (P. amygdalus ϫ P. persica), two important Prunus varieties used as commercial rootstocks, were inoculated with T22. The results showed that early inoculation of the fungus (at the stage of shoot transfer to root-inducing medium) seriously damaged both GiSeLa6 ® and GF677 plants; whereas, following later inoculation (7 d after shoot transfer to root-inducing medium), the plants survived and showed significant increases in shoot growth and root development. In particular, root lengths in GiSeLa6 ® and GF677 plants increased by 180% and 136%, respectively, compared to non-inoculated controls. Microscopic analysis revealed T22 hyphae spreading on the root surface in GiSeLa6 ® (fungus colonisation frequency = 20%), but not in GF677 roots. Our results demonstrate that the application of T22 during the rooting phase resulted in greater shoot lengths, as well as increased numbers of leaves, roots, and stem diameters. These morphological characteristics could increase the quality and viability of nursery planting material and provide advantages during the plant acclimatisation phase.
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