Dehydrating stresses trigger the accumulation of abscisic acid (ABA), a key plant stresssignaling hormone that activates Snf1-Related Kinases (SnRK2s) to mount adaptive responses.However, the regulatory circuits that terminate the SnRK2s signal relay after acclimation or poststress conditions remain to be defined. Here, we show that the desensitization of the ABA-signal is achieved by the regulation of OST1 (SnRK2.6) protein stability via the E3-ubiquitin-ligase HOS15. Upon ABA signal, HOS15-induced degradation of OST1 is inhibited and stabilized OST1 promotes the stress-response. When the ABA signal terminates, protein phosphatases ABI1/2 recruit HOS15 to OST1 to promote the rapid degradation of OST1. Notably, we found that even in the presence of ABA, OST1 levels were also depleted within hours of ABA signal onset. The unexpected dynamics of OST1 abundance was resolved by a systematic mathematical modeling demonstrating a desensitizing feedback loop by which OST1-induced up-regulation of ABI1/2 leads to the degradation of OST1. This model illustrates the complex rheostat dynamics underlying the ABA-induced stress response and desensitization. signaling components. Several members of the PYR/PYL/RCAR family of ABA receptors are specifically recognized by different E3 ubiquitin ligases and targeted for degradation through proteasome action (Irigoyen et al., 2014). ABI1, a PP2C phosphatase that inhibits ABA-related SnRK2 kinases such as OST1, is ubiquitinated by PUB12/PUB13 (U-box E3 ligases) and also degraded by the proteasome in the presence of ABA signal (Kong et al., 2015), which then facilitates the activation of SnRK2 kinases and of their downstream transcription factors (TFs).Eventually, the TFs that accumulate in response to ABA need to be degraded when the signal ceases. When ABA signaling stops, ABI FIVE BINDING PROTEIN1 (AFP1) and KEG (KEEP ON GOING) facilitate UPS-mediated proteolysis of ABI5 and ABF1/ABF3 (Lopez-Molina et al., 2003;Stone et al., 2006;Chen et al., 2013;Liu et al., 2013). In addition, DWA1/DWA2 (DWD HYPERSENSITIVE TO ABA1/2), and ABD1 (ABA-HYPERSENSITIVE DCAF1), substrate receptors for the DDB1 CULLIN4-based E3 ligases, command the degradation of ABI5 (Seo et al., 2014;Lee et al., 2010). The positive signaling effectors SnRK2.2, SnRK2.3 and SnRK2.6/OST1 are known to be degraded by an ubiquitination-and proteasome-dependent mechanism, but the mechanism involved has not been identified with the exception of SnKR2.3 that was shown to be degraded by AtPP2-B11 (Kim et al., 2013;Cheng et al., 2017). In summary, the degradation of positive signaling effectors leads to deactivation of the ABA signal pathway.The ubiquitin-26S proteasome system (UPS) proceeds via sequential reactions performed by three distinct sets of enzymes: ubiquitin activating enzymes (E1), ubiquitin conjugating enzymes (E2) and ubiquitin protein ligases (E3). Because target specificity is conferred by the E3 ligases, plant genomes encode hundreds of E3 ligases that recruit specific target proteins in multiple biological pro...

To survive salt stress, plants must maintain a balance between sodium and potassium ions. High-affinity potassium transporters (HKTs) play a key role in reducing Na+ toxicity through K+ uptake. Eutrema parvula (formerly known as Thellungiella parvula), a halophyte closely related to Arabidopsis, has two HKT1 genes that encode EpHKT1;1 and EpHKT1;2. In response to high salinity, the EpHKT1;2 transcript level increased rapidly; by contrast, the EpHKT1;1 transcript increased more slowly in response to salt treatment. Yeast cells expressing EpHKT1;2 were able to tolerate high concentrations of NaCl, whereas EpHKT1;1-expressing yeast cells remained sensitive to NaCl. Amino acid sequence alignment with other plant HKTs showed that EpHKT1;1 contains an asparagine residue (Asn-213) in the second pore-loop domain, but EpHKT1;2 contains an aspartic acid residue (Asp-205) at the same position. Yeast cells expressing EpHKT1;1, in which Asn-213 was substituted with Asp, were able to tolerate high concentrations of NaCl. In contrast, substitution of Asp-205 by Asn in EpHKT1;2 did not enhance salt tolerance and rather resulted in a similar function to that of AtHKT1 (Na+ influx but no K+ influx), indicating that the presence of Asn or Asp determines the mode of cation selectivity of the HKT1-type transporters. Moreover, Arabidopsis plants (Col-gl) overexpressing EpHKT1;2 showed significantly higher tolerance to salt stress and accumulated less Na+ and more K+ compared to those overexpressing EpHKT1;1 or AtHKT1. Taken together, these results suggest that EpHKT1;2 mediates tolerance to Na+ ion toxicity in E. parvula and is a major contributor to its halophytic nature.

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In this study, near-infrared (NIR) and attenuated-total-reflectance infrared (ATR-IR) spectroscopy techniques in hyphenation with partial least squares (PLS) regression were utilized to determine the antioxidant capacity of Primulae flos cum calycibus samples. Folin-Ciocalteu (FC), ferric ion reducing antioxidant power (FRAP), 2,2-diphenyl-picrylhydrazyl (DPPH), 2,2 0 -azino-bis-(3-ethylbenzothiazoline-6sulfonic acid) diammonium salt (ABTS) and cupric reducing antioxidant capacity (CUPRAC) assays were performed as reference methods. Different spectral pretreatments such as standard normal variate (SNV), 1st or 2nd derivative, were applied to remove scattering effects. For all assays, cross and test-set validations were performed. The ability of the two spectroscopic techniques to replace the five assays was evaluated and compared. The standard error of prediction (SEP) and the ratio performance deviation (RPD) were determined and corrected for the imprecision of the reference data to obtain the respective SEP corr and RPD corr values. In general, NIR demonstrated advantages over ATR-IR spectroscopy and resulted best for the ABTS assay (R 2 : 0.94, RPD corr : 4.66; test-set validation). Also with ATR-IR spectroscopy, the best prediction power was obtained for the ABTS assay (R 2 : 0.94, RPD corr : 4.10; testset validation). The feasibility of vibrational spectroscopy as a fast and simple tool to replace wet chemistry assays for the measurement of the antioxidant capacity of Primulae flos cum calycibus samples was demonstrated.

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Seed germination is a complex biological process controlled by various regulators, including phytohormones. Among these, abscisic acid and gibberellic acid inhibit and promote seed germination, respectively. Many studies have addressed the biological roles of auxin in plant growth and development, but very few have considered its role in seed germination. Here, we identified a novel function of the auxin signaling repressor Aux/IAA8 during seed germination. The IAA8 loss-of-function mutant iaa8-1 exhibited delayed seed germination. The phenotype of iaa8-1 was restored by ectopic expression of IAA8. Interestingly, IAA8 accumulated to high levels during seed germination, which was achieved not only by increased protein synthesis but also by the stabilization of IAA8 protein. We also showed that IAA8 down-regulates the transcription of ABSCISIC ACID INSENSITIVE3 (ABI3), a negative regulator of seed germination. Our study, thus strongly suggest that the auxin signaling repressor IAA8 acts as a positive regulator of seed germination in Arabidopsis thaliana.

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