RESEARCH NOTE

Metal- and Organ-Specific Response to Heavy Metal-Induced Stress Mediated by Antioxidant Enzymes' Activities, Polyamines, and Plant Hormones Levels in Populus deltoides

Besides anthropogenic factors, climate change causes altered precipitation patterns that indirectly affect the increase of heavy metals in soils due to hydrological effects and enhanced leaching (i.e., Cd and Ni), especially in the vicinity of mines and smelters. Phytoextraction is a well-known, powerful "green" technique for environmental clean-up that uses plants to extract, sequester, and/or detoxify heavy metals, and it makes significant contributions to the removal of persistent inorganic pollutants from soils. Poplar species, due to their growth features, high transpiration rate, large biomass, and feasible reproduction represent great candidates for phytoextraction technology. However, the consequences of concomitant oxidative stress upon plant metabolism and the mechanism of the poplar's tolerance to heavy metal-induced stress are still not completely understood. In this study, cuttings of poplar species (Populus deltoides W. Bartram ex Marshall) were separately exposed to two heavy metals (Cd²⁺ and Ni²⁺) that were triple the maximum allowed amount (MAA) (according to national legislation). The aim of the study was to estimate the effects of heavy metals on: (I) the accumulation of free and conjugated polyamines, (II) plant hormones (including abscisic acid-ABA and indole-3-acetic acid-IAA), and (III) the activities of different antioxidant enzymes at root and leaf levels. By using the selected ion monitoring (SIM) mode of gas chromatography with mass spectrometry (GC/MS) coupled with the isotopically labeled technique, amounts of ABA and IAA were quantified, while polyamine amounts were determined by using high-performance liquid chromatography (HPLC) with fluorometric detection after derivatization. The results showed that P. deltoides responded to elevated concentrations of heavy metals in soils by exhibiting metal- and organ-specific tolerance. Knowledge about tolerance mechanisms is of great importance for the development of phytoremediation technology and afforestation programs for polluted soils.

AM fungi root colonization increases the production of essential isoprenoids vs. nonessential isoprenoids especially under drought stress conditions or after jasmonic acid application

Previous studies have shown that root colonization by arbuscular mycorrhiza (AM) fungi enhances plant resistance to abiotic and biotic stressors and finally plant growth. However, little is known about the effect of AM on isoprenoid foliar and root content. In this study we tested whether the AM symbiosis affects carbon resource allocation to different classes of isoprenoids such as the volatile nonessential isoprenoids (monoterpenes and sesquiterpenes) and the non-volatile essential isoprenoids (abscisic acid, chlorophylls and carotenoids). By subjecting the plants to stressors such as drought and to exogenous application of JA, we wanted to test their interaction with AM symbiosis in conditions where isoprenoids usually play a role in resistance to stress and in plant defence. Root colonization by AM fungi favoured the leaf production of essential isoprenoids rather than nonessential ones, especially under drought stress conditions or after JA application. The increased carbon demand brought on by AM fungi might thus influence not only the amount of carbon allocated to isoprenoids, but also the carbon partitioning between the different classes of isoprenoids, thus explaining the not previously shown decrease of root volatile isoprenoids in AM plants. We propose that since AM fungi are a nutrient source for the plant, other carbon sinks normally necessary to increase nutrient uptake can be avoided and therefore the plant can devote more resources to synthesize essential isoprenoids for plant growth.

Genomic and physiological studies of early cryptochrome 1 action demonstrate roles for auxin and gibberellin in the control of hypocotyl growth by blue light

Blue light inhibits elongation of etiolated Arabidopsis thaliana hypocotyls during the first 30 min of irradiation by a mechanism that depends on the phototropin 1 (phot1) photoreceptor. The cryptochrome 1 (cry1) photoreceptor begins to exert control after 30 min. To identify genes responsible for the cry1 phase of growth inhibition, mRNA expression profiles of cry1 and wild-type seedlings were compared using DNA microarrays. Of the roughly 420 genes found to be differentially expressed at the point of cry1 response incipience, approximately half were expressed higher and half lower in cry1 relative to the wild type. Many of the cry1-dependent genes encoded kinases, transcription factors, cell cycle regulators, cell wall metabolism enzymes, gibberellic acid (GA) biosynthesis enzymes, and auxin response factors. High-resolution growth studies supported the hypothesis that genes in the last two categories were indeed relevant to cry1-mediated growth control. Inhibiting GA4 biosynthesis with a 3beta-hydroxylase inhibitor (Ca-prohexadione) restored wild-type response kinetics in cry1 and completely suppressed its long-hypocotyl phenotype in blue light. Co-treatment of cry1 seedlings with Ca-prohexadione plus GA4 completely reversed the effects of the inhibitor, restoring the long-hypocotyl phenotype typical of the mutant. Treatment of wild-type seedlings with GA4 was not sufficient to phenocopy cry1 seedlings, but co-treatment with IAA plus GA4 produced cry1-like growth kinetics for a period of approximately 5 h. The genomic and physiological data together indicate that blue light acting through cry1 quickly affects the expression of many genes, a subset of which suppresses stem growth by repressing GA and auxin levels and/or sensitivity.