The rhizosphere microbiota of the zinc and cadmium hyperaccumulators Arabidopsis halleri and Noccaea caerulescens is highly convergent in Prayon (Belgium)

The Prayon site is known as a zinc-polluted area where two zinc and cadmium hyperaccumulator plant species currently coexist, although Arabidopsis halleri was introduced more recently than Noccaea caerulescens . While soil microorganisms may influence metal uptake, the microbial community present in the rhizosphere of hyperaccumulators remains poorly known. Plants of both species were sampled with their bulk and rhizosphere soil from different plots of the Prayon site. Soil components (ionome, pH, water composition, temperature) were analyzed, as well as shoot ionome and expression levels of metal transporter genes ( HMA3 , HMA4 , ZIP4 / ZNT1 , ZIP6 , MTP1 ). The taxonomic diversity of the microorganisms in soil samples was then determined by 16S rRNA metabarcoding and compared at the Operational Taxonomy Unit (OTU) level and across different taxonomic levels. Our elemental analyses confirmed that the site is still highly contaminated with zinc and cadmium and that both plant species indeed hyperaccumulate these elements in situ . Although the pollution is overall high, it is heterogenous at the site scale and correlates with the expression of some metal transporter genes. Metabarcoding analyses revealed a decreasing gradient of microbial diversity, with more OTUs discovered in the rhizosphere than in the soil bulk, especially at the bottom of the cores. However, the variability gradient increases with the distance from roots. Using an ad hoc pseudo-taxonomy to bypass the biases caused by a high proportion of unclassified and unknown OTUs, we identified Chloroflexi, Armatimonadetes, Pirellulaceae, Gemmatimonadetes and Chitinophagaceae as the drivers of the differences in the gradient along the cores. In contrast, no significant difference was identified between the rhizosphere composition of A. halleri and N. caerulescens . This suggests that, despite their distinct colonization history in Prayon, the two plant species have now recruited highly convergent microbial communities in the rhizosphere.

Between the devil and the deep blue sea: herbivory induces foraging for and uptake of cadmium in a metal hyperaccumulating plant

Plants have been shown to change their foraging behaviour in response to resource heterogeneity. However, an unexplored hypothesis is that foraging could be induced by environmental stressors, such as herbivory, which might increase the demand for particular resources, such as those required for herbivore defence. This study examined the way simulated herbivory affects both root foraging for and uptake of cadmium (Cd), in the metal-hyperaccumulating plant Arabidopsis halleri , which uses this heavy metal as herbivore defence. Simulated herbivory elicited enhanced relative allocation of roots to Cd-rich patches as well as enhanced Cd uptake, and these responses were exhibited particularly by plants from non-metalliferous origin, which have lower metal tolerance. By contrast, plants from a metalliferous origin, which are more tolerant to Cd, did not show any preference in root allocation, yet enhanced Cd sharing between ramets when exposed to herbivory. These results suggest that foraging for heavy metals, as well as their uptake and clonal-sharing, could be stimulated in A. halleri by herbivory impact. Our study provides first support for the idea that herbivory can induce not only defence responses in plants but also affect their foraging, resource uptake and clonal sharing responses.

Artificial mimicry of seasonal transcriptome dynamics in Arabidopsis thaliana reveals short- and long-term responses to environmental conditions

Plants must respond to various environmental factors that change seasonally. In a previous study, seasonally oscillating genes were identified by a massive time-series transcriptome analysis in a wild population of Arabidopsis halleri ssp. gemmifera, a sister species of Arabidopsis thaliana. To analyze the function of these seasonally oscillating genes, we established an experimental system to mimic seasonal expression trends using A. thaliana. Arabidopsis thaliana plants were cultured under conditions that mimicked average monthly temperatures and daylengths in a "smart growth chamber mini," a hand-made low-cost small chamber. Under different short-term incubations, the seasonal trends of 1627 seasonally oscillating genes were mimicked. These seasonally oscillating genes had varying temporal responsiveness (constant, transient, and incremental). Our findings suggest that plants perceive and integrate information about environmental stimuli in the field by combining seasonally oscillating genes with temporal responsiveness.

Homology-Based Interactions between Small RNAs and Their Targets Control Dominance Hierarchy of Male Determinant Alleles of Self-Incompatibility in Arabidopsis lyrata

Self-incompatibility (SI) is conserved among members of the Brassicaceae plant family. This trait is controlled epigenetically by the dominance hierarchy of the male determinant alleles. We previously demonstrated that a single small RNA (sRNA) gene is sufficient to control the linear dominance hierarchy in Brassica rapa and proposed a model in which a homology-based interaction between sRNAs and target sites controls the complicated dominance hierarchy of male SI determinants. In Arabidopsis halleri, male dominance hierarchy is reported to have arisen from multiple networks of sRNA target gains and losses. Despite these findings, it remains unknown whether the molecular mechanism underlying the dominance hierarchy is conserved among Brassicaceae. Here, we identified sRNAs and their target sites that can explain the linear dominance hierarchy of Arabidopsis lyrata, a species closely related to A. halleri. We tested the model that we established in Brassica to explain the linear dominance hierarchy in A. lyrata. Our results suggest that the dominance hierarchy of A. lyrata is also controlled by a homology-based interaction between sRNAs and their targets.

Effects of Growth Stage and Cd Chemical Form on Cd and Zn Accumulation in Arabidopsis halleri ssp. gemmifera

Cadmium is a hazardous heavy metal and causes contamination globally. Phytoremediation can potentially become a low-cost and eco-friendly technique for mitigating Cd contamination. Arabidopsis halleri ssp. gemmifera hyper-accumulates Cd and Zn, and may be used to remediate Cd-contaminated sites. However, few studies have focused on Cd accumulation by A. halleri ssp. gemmifera. Herein, we demonstrate the accumulation of Cd by A. halleri ssp. gemmifera. The biomass, Cd, and Zn concentration of the plant increased in the 103 days of experimentation. Cd concentration of soil significantly decreased compared to its initial concentration (≈10%). The material balance of Cd uptake by plant and Cd decrement from soil ranged from 63.3% to 83.7% in each growth stage. Analysis indicated that the water-eluted and exchangeable forms of Cd were stable during the experiment. However, Cd concentration extracted with 0.1 M HCl decreased (25% of initial), and this fraction was not bioavailable. The study exhibits the mass balance of Cd between plant uptake and decrement from the soil and the changes in the chemical form of Cd during stages of A. halleri ssp. gemmifera cultivation.