ABI4 Mediates Antagonistic Effects of Abscisic Acid and Gibberellins at Transcript and Protein Levels

Abscisic acid (ABA) and gibberellins (GA) are plant hormones which antagonistically mediate numerous physiological processes, and their optimal balance is essential for normal plant development. However, the molecular mechanism underlying ABA and GA antagonism still needs to be determined. Here, we report that ABA- INSENSITIVE 4 (ABI4) is a central factor for GA/ABA homeostasis and antagonism in post-germination stages. ABI4 over-expression in Arabidopsis (OE-ABI4) leads to developmental defects including a decrease in plant height and poor seed production. The transcription of a key ABA biosynthetic gene, NCED6, and of a key GA catabolic gene, GA2ox7, is significantly enhanced by ABI4 over-expression. ABI4 activates NCED6 and GA2ox7 transcription by directly binding to the promoters, and genetic analysis revealed that mutation in these two genes partially rescues the dwarf phenotype of ABI4 overexpressing plants. Consistently, ABI4 overexpressing seedlings have a lower GA/ABA ratio compared to the wild type. We further show that ABA induces GA2ox7 transcription while GA represses NCED6 expression in an ABI4-dependent manner; and that ABA stabilizes the ABI4 protein, whereas GA promotes its degradation. Taken together, these results propose that ABA and GA antagonize each other by oppositely acting on ABI4 transcript and protein levels

Almond Shell-Derived, Biochar-Supported, Nano-Zero-Valent Iron Composite for Aqueous Hexavalent Chromium Removal: Performance and Mechanisms

Nano-zero-valent iron biochar derived from almond shell (nZVI-ASBC) was used for hexavalent chromium (CR) removal. Experiments showed that pH was the main factor (p < 0.01) that affected the experimental results. At a dosage of 10 mg.L-1 and pH of 2-6, in the first 60 min, nZVI-ASBC exhibited a removal efficiency of 99.8%, which was approximately 20% higher than the removal yield at pH 7-11. Fourier transform infrared spectroscopy results indicated N-H was the main functional group that influenced the chemisorption process. The pseudo second-order dynamics and Langmuir isotherm models proved to be the most suitable. Thermodynamic studies showed that the reaction was exothermic and spontaneous at low temperatures (T < 317 K). Various interaction mechanisms, including adsorption and reduction, were adopted for the removal of Cr(VI) using the nZVI-ASBC composite. The findings showed that the BC-modified nZVI prepared with almond shell exerts a good effect and could be used for the removal of Cr(VI)

ABI4 Regulates Primary Seed Dormancy by Regulating the Biogenesis of Abscisic Acid and Gibberellins in Arabidopsis

<div><p>Seed dormancy is an important economic trait for agricultural production. Abscisic acid (ABA) and Gibberellins (GA) are the primary factors that regulate the transition from dormancy to germination, and they regulate this process antagonistically. The detailed regulatory mechanism involving crosstalk between ABA and GA, which underlies seed dormancy, requires further elucidation. Here, we report that ABI4 positively regulates primary seed dormancy, while negatively regulating cotyledon greening, by mediating the biogenesis of ABA and GA. Seeds of the Arabidopsis <i>abi4</i> mutant that were subjected to short-term storage (one or two weeks) germinated significantly more quickly than Wild-Type (WT), and <i>abi4</i> cotyledons greened markedly more quickly than WT, while the rates of germination and greening were comparable when the seeds were subjected to longer-term storage (six months). The ABA content of dry <i>abi4</i> seeds was remarkably lower than that of WT, but the amounts were comparable after stratification. Consistently, the GA level of <i>abi4</i> seeds was increased compared to WT. Further analysis showed that <i>abi4</i> was resistant to treatment with paclobutrazol (PAC), a GA biosynthesis inhibitor, during germination, while <i>OE-ABI4</i> was sensitive to PAC, and exogenous GA rescued the delayed germination phenotype of <i>OE-ABI4</i>. Analysis by qRT-PCR showed that the expression of genes involved in ABA and GA metabolism in dry and germinating seeds corresponded to hormonal measurements. Moreover, chromatin immunoprecipitation qPCR (ChIP-qPCR) and transient expression analysis showed that ABI4 repressed <i>CYP707A1</i> and <i>CYP707A2</i> expression by directly binding to those promoters, and the ABI4 binding elements are essential for this repression. Accordingly, further genetic analysis showed that <i>abi4</i> recovered the delayed germination phenotype of <i>cyp707a1</i> and <i>cyp707a2</i> and further, rescued the non-germinating phenotype of <i>ga1-t</i>. Taken together, this study suggests that ABI4 is a key factor that regulates primary seed dormancy by mediating the balance between ABA and GA biogenesis.</p></div

Gene expression analysis in dry and imbibed seeds.

<p>Gene expression was investigated by qRT-PCR during the course of the imbibition process. Two-week stored seeds were used for mRNA extraction, and three replications were performed. Primers used in the qRT-PCR assay are listed in <a href="http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.1003577#pgen.1003577.s009" target="_blank">Table S1</a>. (A) GA biosynthesis genes. (B) GA catabolism genes. (C) <i>RGL3</i>, a negative regulator of GA signaling. (D) ABA biosynthesis genes. (E) ABA catabolism genes.</p

GA biogenesis is impaired in seeds of <i>abi4</i> mutant.

<p>(A) Two independent homozygous <i>OE-ABI4</i> lines were identified through qRT-PCR. (B) Western blot confirmed the two <i>OE-ABI4</i> transgenic lines. (C) <i>ABI5</i> expression analysis in <i>abi4</i>, WT and two <i>OE-ABI4</i> transgenic lines. (D)–(F) Germination analysis of WT, <i>abi4</i>, OE-1 and OE-2 seeds on 1/2 MS medium (D) 1/2 MS medium supplemented with 15 µM PAC. (E) 1/2 MS medium supplemented with 0.5 µM GA (F). Quantitative analysis of germination rates are shown in the right panels (n≥45). One representative image (time points indicated in figures) per genotype is shown (left panels). Bar = 0.25 mm. Percentages are average of three repeats ± standard error. (G) Endogenous GA₄ levels in <i>abi4</i> and WT seeds were determined. Two-week stored seeds were used for analysis. Percentages are average of three repeats ± standard error.</p

ABA quantification in <i>abi4</i> and WT dry and imbibed seeds.

<p>ABA contents were determined in dry and imbibed WT and <i>abi4</i> mutant seeds. Two-week stored seeds were used for analysis. The * stands for significant level of 0.05.</p

Decreased primary seed dormancy and vivipary phenotype of <i>abi4</i>.

<p>(A)–(F) Germination of WT and <i>abi4</i> seeds on 1/2 MS medium with or without stratification treatment. Seeds were stored for 1, 2 weeks or 6 months after harvest and subjected to analysis. Quantitative analysis of germination rates are shown in the right panels (n≥45). One representative image per genotype (1.5 days after sowing) is shown (left panels). Bar = 0.25 mm. Percentages are the average of three repeats ± standard error. (G) Representative images of vivipary phenotype of <i>abi4</i> on 1/2 MS medium (a, c) or on soil (b, d) are shown. Immature long-green siliques were collected from plants with various genotypes plants grown under the identical growth conditions.</p