Modulation of external and internal aluminum resistance by ALS3-dependent STAR1-mediated promotion of STOP1 degradation.

The ALMT1 transporter aids malate secretion, chelating Al3+ ions to form nontoxic Al-malate complexes, believed to exclude Al from the roots. However, the extent to which malate secreted by ALMT1 is solely used for the exclusion of Al3+ or can be reutilized by plant roots for internal Al tolerance remains uncertain. In our investigation, we explored the impact of malate secretion on both external and internal Al resistance in Arabidopsis thaliana. Additionally, we delved into the mechanism by which the tonoplast-localized bacterial-type ATP-binding cassette (ABC) transporter complex STAR1/ALS3 promotes the degradation of the Al resistance transcription factor STOP1 to regulate ALMT1 expression. Our study demonstrates that the level of secreted malate influences whether the Al-malate complex is excluded from the roots or transported into root cells. The nodulin 26-like intrinsic protein (NIP) subfamily members NIP1;1 and NIP1;2, located in the plasma membrane, coordinate with STAR1/ALS3 to facilitate Al-malate transport from root apoplasm to the symplasm and eventually to the vacuoles for the internal Al detoxification. ALS3-dependent STAR1 interacts with and promotes the degradation of STOP1, regulating malate exudation. Our findings demonstrate the dual roles of malate exudation in external Al exclusion and Al absorption for internal Al detoxification.

An exclusion mechanism is epistatic to an internal detoxification mechanism in aluminum resistance in Arabidopsis.

BACKGROUND: In Arabidopsis, the aluminum (Al) exclusion mechanism is mainly facilitated by ALMT1-mediated malate exudation and MATE-mediated citrate releases from the root. Recently, we have demonstrated that coordinated functioning between an ALMT1-mediated Al exclusion mechanism, via exudation of malate from the root tip, and a NIP1;2-facilitated internal detoxification mechanism, via removal of Al from the root cell wall and subsequent root-to-shoot Al translocation, plays critical roles in achieving overall Al resistance. However, the genetic relationship between ALMT1 and NIP1;2 in these processes remained unclear. RESULTS: Through genetic and physiological analyses, we demonstrate that unlike ALMT1 and MATE, which function independently and additively, ALMT1 and NIP1;2 show an epistatic relationship in Al resistance. These results indicate that ALMT1 and NIP1;2 function in the same biochemical pathway, whereas ALMT1 and MATE in different ones. CONCLUSION: The establishment of the epistatic relationship and the coordinated functioning between the ALMT1 and NIP1;2-mediated exclusion and internal detoxification mechanisms are pivotal for achieving overall Al resistance in the non-accumulating Arabidopsis plant. We discuss and emphasize the indispensable roles of the root cell wall for the implementation of the Al exclusion mechanism and for the establishment of an epistatic relationship between the ALMT1-mediated exclusion mechanism and the NIP1;2-facilitated internal detoxification mechanism.

Dittrichia viscosa Selection Strategy Based on Stress Produces Stable Clonal Lines for Phytoremediation Applications.

Dittrichia viscosa uptake and translocation of the metalloid As is not fully understood and some data are contradictory, but its adaptability to this pollutant is known and is dependent on its genetic variability. D. viscosa is not a hyperaccumulator plant, but it can grow in high-drought conditions while still producing large biomass, even tolerating significant concentrations of As3+ and As5+. In spite of these remarkable characteristics, adaptive modification of performances is not predictable in wild populations. In previous work, we established experimental clonal populations to perform a functional study on the aquaporin NIP1.1. Here, we propose a strategy to select a clonal population of D. viscosa with a defined phenotype related to As tolerance and to reduced NIP1.1 expression levels for phytoremediation applications. From the previous work, we selected four independent clones, two of them belonging to the weak population (W8 and W9) and the other two belonging to the strong population (S1 and S3). The weak and strong populations differ for a different expression ratio root/shoot of DvNip1;1 that brings a different tolerance to As presence. The stress response of the populations, revealed by the CAT enzymatic test, was statistically correlated to the clones, but not to As uptake. Performance of the selected plants on a second unrelated metallic pollutant, Cd, was evaluated, showing that Cd uptake is also independent from the tolerant phenotype. In vitro culture methods using solid media and temporary immersion bioreactors were compared to propose an optimized combined protocol. The procedure yielded propagation of genetically stable tolerant clonal lines with good uptake of As and Cd. The plants, mass-produced with the developed in vitro protocol, were able to maintain their acquired abilities and are potentially able be later applied in phytoremediation or contaminated areas' re-naturalization.

Evaluation of Dittrichia viscosa Aquaporin Nip1.1 Gene as Marker for Arsenic-Tolerant Plant Selection.

Dittrichia viscosa (L.) Greuter is gaining attention for its high genetic plasticity and ability to adapt to adverse environmental conditions, including heavy metal and metalloid pollution. Uptake and translocation of cadmium, copper, iron, nickel, lead, and zinc to the shoots have been characterized, but its performance with arsenic is less known and sometimes contradictory. Tolerance to As is not related to a reduced uptake, but the null mutation of the aquaporin Nip1.1 gene in Arabidopsis makes the plant completely resistant to the metalloid. This aquaporin, localized in the endoplasmic reticulum, is responsible for arsenite and antimony (Sb) membrane permeation, but the uptake of arsenite occurs also in the null mutant, suggesting a more sophisticated action mechanism than direct uptake. In this study, the DvNip1 gene homologue is cloned and its expression profile in roots and shoots is characterized in different arsenic stress conditions. The use of clonal lines allowed to evidence that DvNip1.1 expression level is influenced by arsenic stress. The proportion of gene expression in roots and shoots can be used to generate an index that appears to be a promising putative selection marker to predict arsenic-resistant lines of Dittrichia viscosa plants.

Variation in Membrane Trafficking Linked to SNARE AtSYP51 Interaction With Aquaporin NIP1;1.

SYP51 and 52 are the two members of the SYP5 Qc-SNARE gene family in Arabidopsis thaliana. These two proteins, besides their high level of sequence identity (85%), have shown to have differential functional specificity and possess a different interactome. Here we describe a unique and specific interaction of SYP51 with an ER aquaporin, AtNIP1;1 (also known as NLM1) indicated to be able to transport arsenite [As(III)] and previously localized on PM. In the present work we investigate in detail such localization in vivo and characterize the interaction with SYP51. We suggest that this interaction may reveal a new mechanism regulating tonoplast invagination and recycling. We propose this interaction to be part of a regulatory mechanism associated with direct membrane transport from ER to tonoplast and Golgi mediated vesicle trafficking. We also demonstrate that NIP1;1 is important for plant tolerance to arsenite but does not alter its uptake or translocation. To explain such phenomenon the hypothesis that SYP51/NIP1;1 interaction modifies ER and vacuole ability to accumulate arsenite is discussed.

Aluminum-activated root malate and citrate exudation is independent of NIP1;2-facilitated root-cell-wall aluminum removal in Arabidopsis.

In Arabidopsis, aluminum (Al) exclusion from the root is mainly facilitated by Al-activated root malate and citrate exudation through the ALMT1 malate transporter and the MATE citrate transporter, respectively. However, the nature of an internal Al tolerance mechanism remains largely unknown. In a recent study, we showed that NIP1;2 facilitates Al-malate transport from the root cell wall into the root symplasm and subsequent root-to-shoot translocation and thus NIP1;2 plays key roles in Al detoxification and internal tolerance in Arabidopsis. We discovered that the NIP1;2-mediated Al removal from the root cell wall requires a functional ALMT1-mediated malate exudation system, which allows the formation of an Al-malate complex in the root cell wall. Thus, a coordinated function between the exclusion and the internal resistance mechanisms, linked by the ALMT1-mediated root malate exudation and the NIP1;2-mediated Al uptake system, is critical for Al resistance in Arabidopsis.