Zonal responses of sensitive vs. tolerant wheat roots during Al exposure and recovery

Aluminum-Induced Alterations to the Cell Wall and Antioxidant Enzymes Involved in the Regulation of the Aluminum Tolerance of Chinese Fir (Cunninghamia lanceolata)

Chinese fir (Cunninghamia lanceolata), which is an important coniferous tree species in China, is mainly planted in acidic soils with toxic aluminum (Al) levels. However, the consequences of Al toxicity and its resistance mechanism in Chinese fir remain largely uncharacterized. In this study, the Al-induced modification and possible role of cell wall in regulating Al tolerance in Chinese fir were investigated by using seedlings with contrasting Al tolerance, namely, Al-sensitive (YX02) and Al-resistant (YX01) genotypes. The results in present work showed that Al treatment resulted in a dose- and time-dependent inhibition of root growth and oxidative damage in both genotypes, but more in YX02 than in YX01. The severe oxidative damage observed in YX02 under Al stress was found to correlate with lower antioxidant enzyme activities as compared with YX01. The greater root growth inhibition observed in YX02 compared with YX01 was associated with a higher accumulation of Al in pectin and hemicllulose 1 (HC1) fraction because of the higher pectin and HC1 contents and the lower degree of pectin demethylation due to enhanced pectin methylesterase activity in YX02, which ultimately enhanced cell wall binding capacity for Al in YX02. Taken together, our results suggested that enhancement of antioxidant enzyme activities and cell wall modification-induced Al exclusion are the two mechanisms responsible for the Al tolerance of Chinese fir.

Titanium and Zinc Based Nanomaterials in Agriculture: A Promising Approach to Deal with (A)biotic Stresses?

Abiotic stresses, such as those induced by climatic factors or contaminants, and biotic stresses prompted by phytopathogens and pests inflict tremendous losses in agriculture and are major threats to worldwide food security. In addition, climate changes will exacerbate these factors as well as their negative impact on crops. Drought, salinity, heavy metals, pesticides, and drugs are major environmental problems that need deep attention, and effective and sustainable strategies to mitigate their effects on the environment need to be developed. Besides, sustainable solutions for agrocontrol must be developed as alternatives to conventional agrochemicals. In this sense, nanotechnology offers promising solutions to mitigate environmental stress effects on plants, increasing plant tolerance to the stressor, for the remediation of environmental contaminants, and to protect plants against pathogens. In this review, nano-sized TiO₂ (nTiO₂) and ZnO (nZnO) are scrutinized, and their potential to ameliorate drought, salinity, and xenobiotics effects in plants are emphasized, in addition to their antimicrobial potential for plant disease management. Understanding the level of stress alleviation in plants by these nanomaterials (NM) and relating them with the application conditions/methods is imperative to define the most sustainable and effective approaches to be adopted. Although broad-spectrum reviews exist, this article provides focused information on nTiO₂ and nZnO for improving our understanding of the ameliorative potential that these NM show, addressing the gaps in the literature.

Spatial responses of antioxidative system to aluminum stress in roots of wheat (Triticum aestivum L.) plants

Aluminum (Al) toxicity associated with acid soils represents one of the biggest limitations to crop production worldwide. The root apex of plants is the major perception site of Al toxicity. In Al stressed wheat primary roots, Al accumulation and loss of plasma membrane integrity were highest in the root apex (0-5mm), and decreased along the root axis (5-25mm). To further understand these responses in wheat, spatial profiles of antioxidant responses to Al along the 0-25mm root tip of two wheat genotypes differing in Al tolerance were analyzed. Under Al stress, the lowest root elongation was in the 0-5mm root tip, and more severe inhibition was observed in Al-sensitive genotype than Al-tolerant genotype. The highest increase of Al and hydrogen peroxide (H₂O₂) was in the 0-5mm zone, with the most pronounced increase of malondialdehyde content and Evans blue uptake after Al exposure, especially in Al-sensitive genotype. The activities of superoxides dismutase (SOD), ascrobate peroxidase (APX), catalase (CAT) and peroxidase (POD) and levels of antioxidants (ascorbic acid, reduced glutathione, dehydroascorbate, glutathione disulfide) were significantly increased along the root tip under Al stress, with the 0-5mm region again being the most active zone. In the same zone, the activities of CAT, APX and contents of antioxidants were higher in Al-tolerant genotype while SOD and POD activities were lower. Our results indicate that Al-induced changes in H₂O₂ production and antioxidative system in root tip are regulated in a spatially-specific manner, suggesting that this response may play an important role in wheat adaptation to Al toxicity.

Different mechanisms of the metalliferous Zygophyllum fabago shoots and roots to cope with Pb toxicity

Lead (Pb) remains classified as a priority pollutant. Zygophyllum fabago is considered an early colonizer of heavy metal-polluted soils under semiarid conditions, but physiological mechanisms underlying this colonizing capacity remain unclear. In order to characterize Z. fabago plants' performance on Pb-contaminated soils, we evaluated how Pb influenced root and shoot growth, carbon metabolism, and oxidative status. For that, 30-day-old seedlings from one population colonizing a mine tailing ("Mercader") at Murcia (southeast Spain) were exposed to 500-μM Pb(NO₃)₂ for 1 week. Results showed that this high dose of Pb induced no plant mortality nor senescence, though promoting plant nanism. Besides the efficiency of roots to accumulate Pb, shoots also demonstrate a high efficiency to translocate and accumulate this metal. Pb exposure decreased Zn uptake to the aerial part and reduced net photosynthetic rate (A), RuBisCO activity, chlorophyll, and soluble sugar contents in shoots. Moreover, in shoots, Pb exposure increased the levels of O₂^- and decreased antioxidant capacity, culminating with a loss of cell membrane integrity (electrolyte leakage) and increased protein oxidation. Compared to controls, exposed roots had less Mn and Zn levels, and despite the rise in H₂O₂ levels, they were able to modulate non-protein thiols presenting a robust defense capacity. This capacity may support the roots' ability to maintain cell membrane integrity (electrolyte leakage) with regard to control. Principal component analysis (PCA) contributed to elucidate how this species adjusts physiological mechanisms to cope with Pb toxicity, showing that roots and shoots evolved different antioxidant defenses, which demonstrates the importance of organ specificity in the response of Z. fabago to heavy metals.

Endopolyploidy levels in barley vary in different root types and significantly decrease under phosphorus deficiency

Increased endopolyploidy is important for plant growth and development as well as for adaptation to environmental stresses. However, little is known about the role of reduced endopolyploidy, especially in root systems. In this report, endopolyploidy variations were examined in different types of barley (Hordeum vulgare L.) roots, and the effects of phosphorus (P) deficiency and salinity (NaCl) stress on root endopolyploidy were also studied. The results showed that the endopolyploidy levels were lower in lateral roots than in either primary or nodal roots. The lower endopolyploidy in lateral roots was attributed to cortical cells. P deficiency reduced the endopolyploidy levels in lateral roots and mature zone of primary roots. By contrast, salinity had no effects on the endopolyploidy levels in either lateral or primary roots, but had a minor effect on nodal roots. Transcript analysis of cell cycle-related genes showed that multiple cell cycle-related genes were more highly expressed in lateral roots than in primary roots, suggesting their roles in lowering endopolyploidy. P deficiency reduced HvCCS52A1 transcripts in the mature zone of primary roots, but had little effect on the transcripts of 12 cell cycle-related genes in lateral roots, suggesting that endopolyploidy regulation differs between lateral roots and primary roots. Our results revealed that endopolyploidy reduction in root systems could be an integrated part of endopolyploidy plasticity in barley growth and development as well as in adaptation to a low P environment.

Pb low doses induced genotoxicity in Lactuca sativa plants

Soil and water contamination by lead (Pb) remains a topic of great concern, particularly regarding crop production. The admissible Pb values in irrigation water in several countries range from ≈0.1 to ≈5 mg L-1. In order to evaluate putative effects of Pb within legal doses on crops growth, we exposed Lactuca sativa seeds and seedlings to increasing doses of Pb(NO3)2 up to 20 mg L-1. The OECD parameter seed germination and seedling/plant growth were not affected by any of the Pb-concentrations used. However, for doses higher than 5 mg L-1 significant DNA damage was detected: Comet assay detected DNA fragmentation at ≥ 5 mg L-1 and presence of micronuclei (MN) were detected for 20 mg L-1. Also, cell cycle impairment was observed for doses as low as 0.05 mg L-1 and 0.5 mg L-1 (mostly G2 arrest). Our data show that for the low doses of Pb used, the OECD endpoints were not able to detect toxicity, while more sensitive endpoints (related with DNA damage and mitotic/interphase disorders) identified genotoxic and cytostatic effects. Furthermore, the nature of the genotoxic effect was dependent on the concentration. Finally, we recommend that MN test and the comet assay should be included as sensitive endpoints in (eco)toxicological assays.

Pure anatase and rutile + anatase nanoparticles differently affect wheat seedlings

TiO2-nanoparticles (TiO2-NPs) are increasingly released to the environment. The present work investigates the cytotoxicity, genotoxicity and uptake of TiO2-NPs in Triticum aestivum. Wheat seeds were exposed to 5-150 mg L(-1) of anatase (ana) or rutile + anatase (rut + ana) TiO2-NPs for 5 d. After exposure, germination and growth rates were determined. Cytotoxic effects were evaluated by changes in the cell cycle dynamics and in the membrane integrity. Genotoxicity was assessed by ploidy mutations and DNA-damage, and by mitotic abnormalities. NP uptake was analyzed by Energy Dispersive X-ray Spectroscopy (EDS). Ana-TiO2 revealed higher toxicity regarding the rate of germination, but no negative effects were detected concerning growth. Although roots and shoots showed no EDS-detectable levels of Ti, despite cyto- and genotoxicity was observed in ana and rut + ana-NPs exposed roots. Cell cycle profile was formulation dependent with rut + ana presenting a higher capability to induce a cell cycle arrest at G0/G1. Both formulations induced genotoxic effects by increasing micronucleated cells: for rut + ana a dose-dependent response is evident and seems to be more genotoxic than ana at lower concentrations. Rut + ana also increased membrane permeability. The observed higher cytotoxicity of rut + ana may be explained by the higher photoactivity of this mixture. Overall, these data indicate that during germination, TiO2-NPs induce severe cyto/genotoxic effects, which are dependent on the TiO2-NP formulation.

Pb2+ exposure induced microsatellite instability in Pisum sativum in a locus related with glutamine metabolism

Lead (Pb) is a toxic element, but its putative mutagenic effects in plant cells, using molecular markers, remain to unveil. To evaluate if Pb induces mutagenicity, Pisum sativum L. seedlings were exposed to Pb(2+) (up to 2000 mg L(-1)) for 28 days and the instability of microsatellites (or Simple Sequence Repeats, SSR) was analyzed in leaves and roots. The analysis of eight selected microsatellites (SSR1-SSR8) demonstrated that only at the highest dosage microsatellite instability (MSI) occurred, at a frequency of 4.2%. Changes were detected in one microsatellite (SSR6) that is inserted in the locus for glutamine synthetase. SSR6 products of roots exposed to the highest concentration of Pb were 3 bp larger than those of the control. Our data demonstrate that: (a) SSR technique is sensitive to detect Pb-induced mutagenicity in plants. MSI instability is Pb dose dependent and organ dependent (roots are more sensitive); (b) the Pb-sensitive SSR6 is inserted in the glutamine synthetase locus, with still unknown relation with functional changes of this enzyme; (c) Pb levels inducing MSI are much above the maximum admitted levels in some European Union countries for agricultural purpose waters. In conclusion, we propose here the potential use of SSR to evaluate Pb(2+)-induced mutagenicity, in combination with other genetic markers.