Heavy metal ATPase 3 (HMA3) confers cadmium hypertolerance on the cadmium/zinc hyperaccumulator Sedum plumbizincicola

Vacuolar Transporters for Cadmium and Arsenic in Plants and their Applications in Phytoremediation and Crop Development

Soil contamination by heavy metals and metalloids such as cadmium (Cd) and arsenic (As) poses a major threat to the environment and to human health. Vacuolar sequestration is one of the main mechanisms by which plants control toxic materials including Cd and As. Understanding the mechanisms of heavy metal tolerance and accumulation can be useful for both phytoremediation and safe crop development. In this review, we summarize recent advances in deciphering the molecular mechanisms underlying vacuolar sequestration of Cd and As, and discuss potential biotechnological applications of this knowledge and efforts towards attaining these goals.

Genome-Wide Association Study of Cadmium Accumulation at the Seedling Stage in Rapeseed (Brassica napus L.)

Cadmium is a potentially toxic heavy metal to human health. Rapeseed (Brassica napus L.), a vegetable and oilseed crop, might also be a Cd hyperaccumulator, but there is little information on this trait in rapeseed. We evaluated Cd accumulation in different oilseed accessions and employed a genome-wide association study to identify quantitative trait loci (QTLs) related to Cd accumulation. A total of 419 B. napus accessions and inbred lines were genotyped with a 60K Illumina Infinium SNP array of Brassica. Wide genotypic variations in Cd concentration and translocation were found. Twenty-five QTLs integrated with 98 single-nucleotide polymorphisms (SNPs) located at 15 chromosomes were associated with Cd accumulation traits. These QTLs explained 3.49-7.57% of the phenotypic variation observed. Thirty-two candidate genes were identified in these genomic regions, and they were 0.33-497.97 kb away from the SNPs. We found orthologs of Arabidopsis thaliana located near the significant SNPs on the B. napus genome, including NRAMP6 (natural resistance-associated macrophage protein 6), IRT1 (iron-regulated transporter 1), CAD1 (cadmium-sensitive 1), and PCS2 (phytochelatin synthase 2). Of them, four candidate genes were verified by qRT-PCR, the expression levels of which were significantly higher after exposure to Cd than in the controls. Our results might facilitate the study of the genetic basis of Cd accumulation and the cloning of candidate Cd accumulation genes, which could be used to help reduce Cd levels in edible plant parts and/or create more efficient hyperaccumulators.

Conceptualizing plant systems evolution

Organisms inhabiting extreme environments are emerging models in systems evolution, enabling us to identify the molecular alterations effecting major phenotypic divergence through comparative approaches. Here I discuss possible physiological mechanisms underlying evolutionary adaptations to extreme environments both theoretically and in relation to experimental observations. Reasoning leads me to the 'conserved steady-state' hypothesis of evolutionary adaptation: Between closely related plants adapted to differently composed soils, the homeostatically controlled steady-state set point cytosolic (buffered) concentrations of mineral ions are conserved. Subsequently, I compare molecular alterations expected to contribute to physiological adaptations with our present knowledge. Key roles of enhanced gene product dosage in plant evolutionary adaptations question the widespread stimulus response-centric paradigm. As a broader implication, co-regulation networks can lack decisive functional network elements. With this article, I hope to stimulate a discussion across research fields and provide an initial conceptual framework for future experimental testing and for quantitative modelling.

The Effects of the Endophytic Bacterium Pseudomonas fluorescens Sasm05 and IAA on the Plant Growth and Cadmium Uptake of Sedum alfredii Hance

Endophytic bacteria have received attention for their ability to promote plant growth and enhance phytoremediation, which may be attributed to their ability to produce indole-3-acetic acid (IAA). As a signal molecular, IAA plays a key role on the interaction of plant and its endomicrobes. However, the different effects that endophytic bacteria and IAA may have on plant growth and heavy metal uptake is not clear. In this study, the endophytic bacterium Pseudomonas fluorescens Sasm05 was isolated from the stem of the zinc (Zn)/cadmium (Cd) hyperaccumulator Sedum alfredii Hance. The effects of Sasm05 and exogenous IAA on plant growth, leaf chlorophyll concentration, leaf Mg²⁺-ATPase and Ca²⁺-ATPase activity, cadmium (Cd) uptake and accumulation as well as the expression of metal transporter genes were compared in a hydroponic experiment with 10 μM Cd. The results showed that after treatment with 1 μM IAA, the shoot biomass and chlorophyll concentration increased significantly, but the Cd uptake and accumulation by the plant was not obviously affected. Sasm05 inoculation dramatically increased plant biomass, Cd concentration, shoot chlorophyll concentration and enzyme activities, largely improved the relative expression of the three metal transporter families ZRT/IRT-like protein (ZIP), natural resistance associated macrophage protein (NRAMP) and heavy metal ATPase (HMA). Sasm05 stimulated the expression of the SaHMAs (SaHMA2, SaHMA3, and SaHMA4), which enhanced Cd root to shoot translocation, and upregulated SaZIP, especially SaIRT1, expression to increase Cd uptake. These results showed that although both exogenous IAA and Sasm05 inoculation can improve plant growth and photosynthesis, Sasm05 inoculation has a greater effect on Cd uptake and translocation, indicating that this endophytic bacterium might not only produce IAA to promote plant growth under Cd stress but also directly regulate the expression of putative key Cd uptake and transport genes to enhance Cd accumulation of plant.