Overexpression of Arabidopsis YUCCA6 in Potato Results in High-Auxin Developmental Phenotypes and Enhanced Resistance to Water Deficit

Indole-3-acetic acid （IAA）, a major plant auxin, is produced in both tryptophan-dependent and tryptophan-independent pathways. A major pathway in Arabidopsis thaliana generates IAA in two reactions from tryptophan. Step one converts tryptophan to indole-3-pyruvic acid （IPA） by tryptophan aminotransferases followed by a rate-limiting step converting IPA to IAA catalyzed by YUCCA proteins. We identified eight putative StYUC （Solanum tuberosum YUCCA） genes whose deduced amino acid sequences share 50%-70% identity with those of Arabidopsis YUCCA proteins. All include canonical, conserved YUCCA sequences： FATGY motif, FMO signature sequence, and FAD-binding and NADP-binding sequences. In addition, five genes were found with -50% amino acid sequence identity to Arabidopsis trypto-phan aminotransferases. Transgenic potato （Solanum tuberosum cv. Jowon） constitutively overexpressing Arabidopsis AtYUC6 displayed high-auxin phenotypes such as narrow downward-curled leaves, increased height, erect stature, and longevity. Transgenic potato plants overexpressing AtYUC6 showed enhanced drought tolerance based on reduced water loss. The phenotype was correlated with reduced levels of reactive oxygen species in leaves. The results suggest a func-tional YUCCA pathway of auxin biosynthesis in potato that may be exploited to alter plant responses to the environment.

Arabidopsis thaliana RECEPTOR DEAD KINASE1 Functions as a Positive Regulator in Plant Responses to ABA

Abscisic acid （ABA） is a major phytohormone involved in important stress-related and developmental plant processes. Membrane-delimited ABA signal transduction plays an important role in early ABA signaling, but the molecular mechanisms connecting core signaling components to the plasma membrane remain un- clear. Plants have evolved a large number of receptor-like kinases （RLKs） to modulate diverse biological processes by perceiving extracellular stimuli and activating downstream signaling responses. In this study, a putative leucine-rich repeat-RLK gene named RECEPTOR DEAD KINASE1 （AtRDK1） was identified and characterized in Arabidopsis thaliana. RDK1 promoter-GUS analysis revealed that RDK1 is expressed ubiq- uitously in the various tissues in Arabidopsis, and its expression is mainly induced by ABA. In the presence of ABA, RDKl-deficient rdkl-1 and rdkl-2 lines showed significant resistance in cotyledon greening and root growth, whereas RDKl-overexpressing lines showed enhanced sensitivity. Consistently, the expres- sion of ABA-responsive genes was significantly downregulated in rdkl mutant seedlings, which were also hypersensitive to drought stress with increased water loss. Interestingly, RDK1 was found to be an atypical kinase localized to the plasma membrane and did not require its kinase activity during ABA-mediated inhi- bition of seedling development. Accordingly, RDK1 interacted in the plasma membrane with type 2C protein phosphatase ABSClSIC ACID INSENSITIVE1 （ABI1）; this interaction was further enhanced by exogenous application of ABA, suggesting that RDKl-mediated recruitment of ABI1 onto the plasma membrane is important for ABA signaling. Taken together, these results reveal an important role for RDK1 in plant responses to abiotic stress conditions in an ABA-dependent manner.

Rheostatic Control of ABA Signaling through HOS15-Mediated OST1 Degradation

Dehydrating stresses trigger the accumulation of abscisic acid(ABA),a key plant stress-signaling hormone that activates Snf1-Related Kinases(SnRK2s)to mount adaptive responses.However,the regulatory circuits that terminate the SnRK2s signal relay after acclimation or post-stress 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 promote rapid degradation of OST1 via HOS15.Notably,we found that even in the presence of ABA,OST1 levels are also depleted within hours of ABA signal onset.The unexpected dynamics of OST1 abundance are then resolved by systematic mathematical modeling,demonstrating a desensitizing feedback loop by which OST1-induced upregulation of ABI1/2 leads to the degradation of OST1.This model illustrates the complex rheostat dynamics underlying the ABA-induced stress response and desensitization.