Salicylic acid (SA) is a very simple phenolic compound (a C7H6O3 compound composed of an aromatic ring, one carboxylic and a hydroxyl group) and this simplicity contrasts with its high versatility and the involvement of SA in several plant processes either in optimal conditions or in plants facing environmental cues, including heavy metal (HM) stress. Nowadays, a huge body of evidence has unveiled that SA plays a pivotal role as plant growth regulator and influences intra- and inter-plant communication attributable to its methyl ester form, methyl salicylate, which is highly volatile. Under stress, including HM stress, SA interacts with other plant hormones (e.g., auxins, abscisic acid, gibberellin) and promotes the stimulation of antioxidant compounds and enzymes thereby alerting HM-treated plants and helping in counteracting HM stress. The present literature survey reviews recent literature concerning the roles of SA in plants suffering from HM stress with the aim of providing a comprehensive picture about SA and HM, in order to orientate the direction of future research on this topic.

https://www.mdpi.com/1420-3049/25/3/540/pdf

The Role of Salicylic Acid in Plants Exposed to Heavy Metals

Cadmium accumulation in crops and the possibility of Cd entering the food chain are serious concerns for public health. This review discusses the deleterious effects of Cd in plants and the toleran...

Cadmium stress in plants: A critical review of the effects, mechanisms, and tolerance strategies

The heavy metal contamination in plant-soil environment has increased manifold recently. In order to reduce the harmful effects of metal stress in plants, the application of beneficial soil microbes is gaining much attention. In the present research, the role of Serratia marcescens BM1 in enhancing cadmium (Cd) stress tolerance and phytoremediation potential of soybean plants, was investigated. Exposure of soybean plants to two Cd doses (150 and 300 µM) significantly reduced plant growth, biomass, gas exchange attributes, nutrients uptake, antioxidant capacity, and the contents of chlorophyll, total phenolics, flavonoids, soluble sugars, and proteins. Additionally, Cd induced the stress levels of Cd, proline, glycine betaine, hydrogen peroxide, malondialdehyde, antioxidant enzymes (i.e., catalase, CAT; ascorbate peroxidase, APX; superoxide dismutase, SOD; peroxidise, POD), and the expression of stress-related genes (i.e., APX, CAT, Fe-SOD, POD, CHI, CHS, PHD2, VSO, NR, and P5CS) in soybean leaves. On the other hand, inoculation of Cd-stressed soybean plants with Serratia marcescens BM1 significantly enhanced the plant growth, biomass, gas exchange attributes, nutrients uptake, antioxidant capacity, and the contents of chlorophyll, total phenolics, flavonoids, soluble sugars, and proteins. Moreover, Serratia marcescens BM1 inoculation reduced the levels of cadmium and oxidative stress markers, but significantly induced the activities of antioxidant enzymes and the levels of osmolytes and stress-related genes expression in Cd-stressed plants. The application of 300 µM CdCl2 and Serratia marcescens triggered the highest expression levels of stress-related genes. Overall, this study suggests that inoculation of soybean plants with Serratia marcescens BM1 promotes phytoremediation potential and Cd stress tolerance by modulating the photosynthetic attributes, osmolytes biosynthesis, antioxidants machinery, and the expression of stress-related genes.

Serratia marcescens BM1 Enhances Cadmium Stress Tolerance and Phytoremediation Potential of Soybean Through Modulation of Osmolytes, Leaf Gas Exchange, Antioxidant Machinery, and Stress-Responsive Genes Expression

Nanoparticle-based amelioration of drought stress and cadmium toxicity in rice via triggering the stress responsive genetic mechanisms and nutrient acquisition.

Hormesis, which describes the stimulatory effect of low doses of toxic substances on growth, is a well-known phenomenon in the plant and animal kingdoms. However, the mechanisms that are involved in this phenomenon are still poorly understood. We performed preliminary studies on corn coleoptile sections, which showed a positive correlation between the stimulation of growth by Cd or Pb and an increase in the auxin and H2O2 content in the coleoptile sections. Subsequently, we grew corn seedlings in hydroponic culture and tested a wide range of Cd or Pb concentrations in order to determine hormetic growth stimulation. In these seedlings the gas exchange and the chlorophyll a fluorescence, as well as the content of chlorophyll, flavonol, auxin and hydrogen peroxide, were measured. We found that during the hormetic stimulation of growth, the response of the photosynthetic apparatus to Cd and Pb differed significantly. While the application of Cd mostly caused a decrease in various photosynthetic parameters, the application of Pb stimulated some of them. Nevertheless, we discovered that the common features of the hormetic stimulation of shoot growth by heavy metals are an increase in the auxin and flavonol content and the maintenance of hydrogen peroxide at the same level as the control plants.

https://www.mdpi.com/1422-0067/21/6/2099/pdf

Hormesis in Plants: The Role of Oxidative Stress, Auxins and Photosynthesis in Corn Treated with Cd or Pb.

Prospects and applications of plant growth promoting rhizobacteria to mitigate soil metal contamination: A review.

Effects of salicylic acid, Epi-brassinolide and calcium on stress alleviation and Cd accumulation in tomato plants.

Accumulation and fixation of Cd by tomato cell wall pectin under Cd stress

Effects of salicylic acid, Fe(II) and plant growth-promoting bacteria on Cd accumulation and toxicity alleviation of Cd tolerant and sensitive tomato genotypes.

Cadmium (Cd) contamination of agricultural soil is a serious global problem. Plant growth-promoting bacteria could modulate growth and Cd uptake of plants. In this study, the effects of rhizobacterium Burkholderia sp. D54 on growth and Cd uptake of tomato, ryegrass and soybean plants were investigated. The bacterial inoculation increased at least one pigment content of all tested plants. In soybean plants, chlorophyll a, chlorophyll b and carotenoid concentrations were increased by 63.19%, 73.15% and 57.58%, respectively. The dry mass of aerial parts was increased by 67.38%, 19.13% and 67.41% in tomato, ryegrass and soybean plants, respectively. The total Cd uptake in tomato and ryegrass plants was increased upon bacterial inoculation, while the Cd concentration varied among plant species. Burkholderia sp. D54 inoculation decreased Cd concentration in aerial parts of tomato and Cd translocation from roots to aerial parts (p < 0.05). The Cd concentration in roots and aerial parts of ryegrass was increased, and Cd translocation from roots to aerial parts was enhanced. By contrast, Cd concentration in aerial parts and roots of the soybean plants was decreased. Burkholderia sp. D54 also modulated the nutrient uptake of the tested plants. The K, Ca and Zn concentrations were decreased in tomato and soybean but enhanced in ryegrass. These results indicated that Burkholderia sp. D54 could modulate growth, nutrient element concentration and Cd uptake of the tested plants. This study might provide a promising way of using PGPR control Cd uptake in food crops or assisting phytoremediation by hyperaccumulators.

Jia Honglei

Papers

Prospects and applications of plant growth promoting rhizobacteria to mitigate soil metal contamination: A review.

Effects of salicylic acid, Epi-brassinolide and calcium on stress alleviation and Cd accumulation in tomato plants.

Accumulation and fixation of Cd by tomato cell wall pectin under Cd stress

Effects of salicylic acid, Fe(II) and plant growth-promoting bacteria on Cd accumulation and toxicity alleviation of Cd tolerant and sensitive tomato genotypes.

Effects of Burkholderia sp. D54 on growth and cadmium uptake of tomato, ryegrass and soybean plants