Assessing Tolerance to Heavy-Metal Stress in Arabidopsis thaliana Seedlings

Alternative splicing as a driver of natural variation in abscisic acid response

Abscisic acid (ABA) is a crucial player in plant responses to the environment. It accumulates under stress, activating downstream signaling to implement molecular responses that restore homeostasis. Natural variance in ABA sensitivity remains barely understood, and the ABA pathway has been mainly studied at the transcriptional level, despite evidence that posttranscriptional regulation, namely, via alternative splicing, contributes to plant stress tolerance. Here, we identified the Arabidopsis accession Kn-0 as less sensitive to ABA than the reference Col-0, as shown by reduced effects of the hormone on seedling establishment, root branching, and stomatal closure, as well as by decreased induction of ABA marker genes. An in-depth comparative transcriptome analysis of the ABA response in the two variants revealed lower expression changes and fewer genes affected for the least ABA-sensitive ecotype. Notably, Kn-0 exhibited reduced levels of the ABA-signaling SnRK2 protein kinases and lower basal expression of ABA-reactivation genes, consistent with our finding that Kn-0 contains less endogenous ABA than Col-0. ABA also markedly affected alternative splicing, primarily intron retention, with Kn-0 being less responsive regarding both the number and magnitude of alternative splicing events, particularly exon skipping. We find that alternative splicing introduces a more ecotype-specific layer of ABA regulation and identify ABA-responsive splicing changes in key ABA pathway regulators that provide a functional and mechanistic link to the differential sensitivity of the two ecotypes. Our results offer new insight into the natural variation of ABA responses and corroborate a key role for alternative splicing in implementing ABA-mediated stress responses.

Arabidopsis CML13 and CML14 Have Essential and Overlapping Roles in Plant Development

Calmodulin (CaM)-like proteins (CMLs) are the largest family of calcium-binding proteins in plants, yet the functions of most CMLs are unknown. Arabidopsis CML13 and CML14 are closely related paralogs that interact with the isoleucine-glutamine (IQ) domains of myosins, IQ-domain proteins and CaM-binding transcription activators (CAMTAs). Here, we explored the physiological roles of CML13 and CML14 during development by using dexamethasone (Dex)-inducible RNA silencing to suppress either CML13 or CML14 transcript levels. In the absence of inducible suppression, CML13- and CML14-RNA-interference lines were indistinguishable from wild-type (WT) plants throughout development. In contrast, induction of silencing treatment led to rapid increases in RNA-hairpin production that correlated with a targeted reduction in CML13 or CML14 transcript levels and a range of developmental and morphological effects. RNA-suppression treatment did not impair the germination of CML13- or 14-RNA-interference lines, but these seedlings were chlorotic, displayed high mortality and failed to achieve seedling establishment. Under Dex treatment, seeds of CML13- and CML14-RNA-interference lines exhibited differential sensitivity to exogenous ABA compared to WT seeds. Induced RNA suppression of mature plants led to reduced silique length, shorter roots and rapid leaf senescence in CML13- and 14-RNA-interference plants, which correlated with increased gene expression of the senescence marker Senescence-Associated Gene13 (SAG13). Plants induced for RNA suppression at 2 weeks post-germination exhibited a much stronger phenotype than treatment of 3-, 4- or 5-week-old plants. Collectively, our data indicate that both CML13 and CML14 are essential for normal development and function across a broad range of tissues and developmental stages.

Quantitative Estimation of Synergistic Toxicity of Cu and Zn on Growth of Arabidopsis thaliana by Isobolographic Method

Heavy metal is one of the most frequent soil contaminants and contaminated soils generally include numerous metals. Although exposure to multiple metals may increase the toxicity to humans and ecosystems, only additive effects are considered in the risk assessment. In this study, the synergistic effect of heavy metals (Cu and Zn) on a model plant, Arabidopsis thaliana, was quantified by the isobolographic method. The plant was cultured via the growth assay method on a plant agar containing individual heavy metals or combinations of Cu + Zn in a growth chamber. The concentration of Cu varied by eight levels from 0 to 200 μM and the concentration of Zn also varied by eight levels from 0 to 400 μM. In the combination of metals, each of the three levels of Cu (25–75 μM) and Zn (20–100 μM) were applied. After 8 days, plants were harvested for root/shoot weight and measured for leaf chlorophyll and carotenoid content. The primary and secondary root elongation of A. thaliana was estimated using image analysis to calculate total root length. The EC50 values of Cu and Zn on A. thaliana, based on the total root length, were 40.0 and 76.4 μM, respectively. When two heavy metals were administered in combination, the EC values decreased less than those of the individual metals. The average value of the combination index was 0.6, proving the synergistic toxic effect on the root growth of A. Thaliana. As a result, the isobolograhic method is a useful tool for estimating the quantitative toxic effect of chemicals on plants.

Cadmium-tolerant endophytic Pseudomonas rhodesiae strains isolated from Typha latifolia modify the root architecture of Arabidopsis thaliana Col-0 in presence and absence of Cd

In this work, we isolated four Cd-tolerant endophytic bacteria from Typha latifolia roots that grow at a Cd-contaminated site. Bacterial isolates GRC065, GRC066, GRC093, and GRC140 were identified as Pseudomonas rhodesiae. These bacterial isolates tolerate cadmium and have abilities for phosphate solubilization, siderophore production, indole acetic acid (IAA) synthesis, and ACC deaminase activity, suggesting that they are plant growth-promoting rhizobacteria. Bacterial inoculation in Arabidopsis thaliana seedlings showed that P. rhodesiae strains increase total fresh weight and number of lateral roots concerning non-inoculated plants. These results indicated that P. rhodesiae strains promote A. thaliana seedlings growth by modifying the root system. On the other hand, in A. thaliana seedlings exposed to 2.5 mg/l of Cd, P. rhodesiae strains increased the number and density of lateral roots concerning non-inoculated plants, indicating that they modify the root architecture of A. thaliana seedlings exposed to cadmium. The results showed that P. rhodesiae strains promote the development of lateral roots in A. thaliana seedlings cultivated in both conditions, with and without cadmium. These results suggest that P. rhodesiae strains could exert a similar role inside the roots of T. latifolia that grow in the Cd-contaminated environment.

Protein lysine methylation contributes to modulating the response of sensitive and tolerant Arabidopsis species to cadmium stress

The mechanisms underlying the response and adaptation of plants to excess of trace elements are not fully described. Here, we analysed the importance of protein lysine methylation for plants to cope with cadmium. We analysed the effect of cadmium on lysine-methylated proteins and protein lysine methyltransferases (KMTs) in two cadmium-sensitive species, Arabidopsis thaliana and A. lyrata, and in three populations of A. halleri with contrasting cadmium accumulation and tolerance traits. We showed that some proteins are differentially methylated at lysine residues in response to Cd and that a few genes coding KMTs are regulated by cadmium. Also, we showed that 9 out of 23 A. thaliana mutants disrupted in KMT genes have a tolerance to cadmium that is significantly different from that of wild-type seedlings. We further characterized two of these mutants, one was knocked out in the calmodulin lysine methyltransferase gene and displayed increased tolerance to cadmium, and the other was interrupted in a KMT gene of unknown function and showed a decreased capacity to cope with cadmium. Together, our results showed that lysine methylation of non-histone proteins is impacted by cadmium and that several methylation events are important for modulating the response of Arabidopsis plants to cadmium stress.

Heterologous expression of the yeast Tpo1p or Pdr5p membrane transporters in Arabidopsis confers plant xenobiotic tolerance

Soil contamination is a major hindrance for plant growth and development. The lack of effective strategies to remove chemicals released into the environment has raised the need to increase plant resilience to soil pollutants. Here, we investigated the ability of two Saccharomyces cerevisiae plasma-membrane transporters, the Major Facilitator Superfamily (MFS) member Tpo1p and the ATP-Binding Cassette (ABC) protein Pdr5p, to confer Multiple Drug Resistance (MDR) in Arabidopsis thaliana. Transgenic plants expressing either of the yeast transporters were undistinguishable from the wild type under control conditions, but displayed tolerance when challenged with the herbicides 2,4-D and barban. Plants expressing ScTPO1 were also more resistant to the herbicides alachlor and metolachlor as well as to the fungicide mancozeb and the Co²⁺, Cu²⁺, Ni²⁺, Al³⁺ and Cd²⁺ cations, while ScPDR5-expressing plants exhibited tolerance to cycloheximide. Yeast mutants lacking Tpo1p or Pdr5p showed increased sensitivity to most of the agents tested in plants. Our results demonstrate that the S. cerevisiae Tpo1p and Pdr5p transporters are able to mediate resistance to a broad range of compounds of agricultural interest in yeast as well as in Arabidopsis, underscoring their potential in future biotechnological applications.