To grow or survive: Plants modulate Brassinosteroid-regulated transcription factor BES1 during drought to balance growth and stress responses

Understanding how plants balance growth and stress responses is essential to optimize crop yield in an ever-changing environment. Brassinosteroids (BRs) regulate plant growth and stress responses, including that of drought. BRs signal to control the activities of the BES1/BZR1 family transcription factors (TFs), which in turn mediate the expression of more than 5,000 BR-responsive genes. The network through which BES1 regulates the large number of target genes and the factors that modulate BES1 during stress are only beginning to be understood. In this thesis, I investigated several mechanisms that converge on BES1 to balance BR-regulated growth and stress responses. First, BES1 is degraded by selective autophagy during stress. BES1 interacts with the ubiquitin receptor protein DSK2 and is targeted to the autophagy pathway during stress via the interaction of DSK2 with ATG8, a ubiquitin-like protein directing autophagosome formation and cargo recruitment. DSK2 is phosphorylated by the GSK3-like kinase BIN2, a negative regulator in the BR pathway. BIN2 phosphorylation of DSK2 flanking its ATG8 interacting motifs (AIMs) promotes the interaction of DSK2 with ATG8, thereby targeting BES1 for degradation under stress conditions. Accordingly, loss-of-function dsk2 plants accumulate BES1, have altered global gene expression profiles, and have compromised responses to drought and fixed-carbon starvation stresses. In addition, BES1 interacts with other TFs to coordinate growth and drought responses. RD26 is induced by drought and inhibits the activity of BES1 on target gene promoters during drought conditions. In contrast, under growth promoting conditions BES1 cooperates with a large network of TFs including WRKY46/54/70 to inhibit drought responses, thereby enabling BR-regulated growth. To more fully characterize the BR-regulatory network, we used genome-wide chromatin immunoprecipitation (ChIP), transcriptome and TF interactome datasets to identify 657 BR-related Transcription Factors (BR-TFs). We then took an integrated approach involving computational modeling, phenomics and functional genomics to study the networks through which BRs, BES1/BZR1 and BR-TFs function. Initially, 11,760 publicly available microarray datasets were used to build comprehensive gene regulatory networks (GRNs). BR-TFs are significantly enriched for BR and drought target genes in the GRNs, suggesting that these TFs function in growth and stress responses. BR-TFs were prioritized for functional studies using NEST (Network Essentiality Scoring Tool). Next, we developed BR response assays to conduct BR phenomics experiments for over 300 BR-TFs using more than 1000 knockout or overexpression lines. These studies identified numerous BR-TF mutants that displayed altered BR responses, allowing us to characterize the function of PLATZ and HMG as A/T-rich binding TFs that oppositely regulate BR-responsive gene expression. Finally, BR and drought phenomics experiments in soil-grown plants using time-lapse imaging and a robotic phenotyping system revealed that tcp mutants have increased BR-regulated growth and improved survival during drought compared to wild-type. These studies provide a paradigm for network-based discovery and characterization of hormone response pathways through the integration of genomics, network analysis and phenomics. Taken together, BES1 is emerging as a critical hub for BR-drought crosstalk, allowing plants to efficiently balance growth and stress responses

The Mechanism of Variegation in immutans Provides Insight into Chloroplast Biogenesis

The immutans (im) variegation mutant of Arabidopsis has green and white-sectored leaves due to the absence of fully functional plastid terminal oxidase (PTOX), a plastoquinol oxidase in thylakoid membranes. PTOX appears to be at the nexus of a growing number of biochemical pathways in the plastid, including carotenoid biosynthesis, PSI cyclic electron flow, and chlororespiration. During the early steps of chloroplast biogenesis, PTOX serves as an alternate electron sink and is a prime determinant of the redox poise of the developing photosynthetic apparatus. Whereas a lack of PTOX causes the formation of photooxidized plastids in the white sectors of im, compensating mechanisms allow the green sectors to escape the effects of the mutation. This manuscript provides an update on PTOX, the mechanism of im variegation, and findings about im compensatory mechanisms

AP2/ERF Transcription Factor Regulatory Networks in Hormone and Abiotic Stress Responses in Arabidopsis

Dynamic environmental changes such as extreme temperature, water scarcity and high salinity affect plant growth, survival, and reproduction. Plants have evolved sophisticated regulatory mechanisms to adapt to these unfavorable conditions, many of which interface with plant hormone signaling pathways. Abiotic stresses alter the production and distribution of phytohormones that in turn mediate stress responses at least in part through hormone- and stress-responsive transcription factors. Among these, the APETALA2/ETHYLENE RESPONSIVE FACTOR (AP2/ERF) family transcription factors (AP2/ERFs) have emerged as key regulators of various stress responses, in which they also respond to hormones with improved plant survival during stress conditions. Apart from participation in specific stresses, AP2/ERFs are involved in a wide range of stress tolerance, enabling them to form an interconnected stress regulatory network. Additionally, many AP2/ERFs respond to the plant hormones abscisic acid (ABA) and ethylene (ET) to help activate ABA and ET dependent and independent stress-responsive genes. While some AP2/ERFs are implicated in growth and developmental processes mediated by gibberellins (GAs), cytokinins (CTK), and brassinosteroids (BRs). The involvement of AP2/ERFs in hormone signaling adds the complexity of stress regulatory network. In this review, we summarize recent studies on AP2/ERF transcription factors in hormonal and abiotic stress responses with an emphasis on selected family members in Arabidopsis. In addition, we leverage publically available Arabidopsis gene networks and transcriptome data to investigate AP2/ERF regulatory networks, providing context and important clues about the roles of diverse AP2/ERFs in controlling hormone and stress responses

Examining Differences Between Asian Americans and Whites for Gambling and Drinking

Limited research have examined the ethnic differences in gambling and drinking among emerging adults, therefore; little is known about what is placing Asian American at-risk for involvement with these behaviors. The purpose of this study was to examine ethnic differences in risk factors between Asian Americans and White Americans for gambling and drinking at the two levels of involvement: abstinence and problems. The main objective 1) if there are ethnic differences among emerging adults in gambling and drinking involvement, 2) why these differences exist, and 3) which impulsivity and psychological distress variables are specific to each of these ethnic groups. The sample consisted of college students, A total of 823 participants, 414 (50.3%) females and 409 (49.7%) males, with an age range of 18-36 from the psychology subject pool of a large West Coast university were selected for this study. The sample was composed of 464 (56.4%) Asian Americans and 359 (43.6%) White Americans. White Americans were found to abstain less and develop more problems associated with drinking. No ethnic differences were found for gambling abstinence. Asian Americans were developing more problems gambling compared to White Americans. Ethnic differences were also found through risk-factors at each level of involvement with Asian Americans being highly correlated to psychological distress risk-factors and White Americans impulsivity factors. Of these risk factors, depression and belief in good luck (BIGL) mediated for problem gambling and social anxiety mediated for drinking abstinence. Overall, results indicated an increase need for more culturally sensitive and comprehensive based treatment and prevention programs

Identification of transcription factors that regulate ATG8 expression and autophagy in Arabidopsis

Autophagy is a conserved catabolic process in eukaryotes that contributes to cell survival in response to multiple stresses and is important for organism fitness. In Arabidopsis thaliana, the core machinery of autophagy is well defined, but its transcriptional regulation is largely unknown. The ATG8 (autophagy-related 8) protein plays central roles in decorating autophagosomes and binding to specific cargo receptors to recruit cargo to autophagosomes. We propose that the transcriptional control of ATG8 genes is important during the formation of autophagosomes and therefore contributes to survival during stress. Here, we describe a yeast one-hybrid (Y1H) screen for transcription factors (TFs) that regulate ATG8 gene expression in Arabidopsis, using the promoters of 4 ATG8 genes. We identified a total of 225 TFs from 35 families that bind these promoters. The TF-ATG8 promoter interactions revealed a wide array of diverse TF families for different promoters, as well as enrichment for families of TFs that bound to specific fragments. These TFs are not only involved in plant developmental processes but also in the response to environmental stresses. TGA9 (TGACG (TGA) motif-binding protein 9)/AT1G08320 was confirmed as a positive regulator of autophagy. TGA9 overexpression activated autophagy under both control and stress conditions and transcriptionally up-regulated expression of ATG8B, ATG8E and additional ATG genes via binding to their promoters. Our results provide a comprehensive resource of TFs that regulate ATG8 gene expression and lay a foundation for understanding the transcriptional regulation of plant autophagy

Histone Lysine Methyltransferase SDG8 Is Involved in Brassinosteroid-Regulated Gene Expression in Arabidopsis thaliana

Citation: Wang, X., Chen, J., Xie, Z., Liu, S., Nolan, T., Ye, H., et al. (2014). Histone lysine methyltransferase SDG8 is involved in brassinosteroid- regulated gene expression in arabidopsis thaliana.The plant steroid hormones, brassinosteroids (BRs), play important roles in plant growth, development and responses to environmental stresses. BRs signal through receptors localized to the plasma membrane and other signaling components to regulate the BES1/BZR1 family of transcription factors, which modulates the expression of 4,000-5,000 genes. How BES1/BZR1 and their interacting proteins function to regulate the large number of genes are not completely understood. Here we report that histone lysine methyltransferase SDG8, implicated in Histone 3 lysine 36 di- and tri-methylation (H3K36me2 and me3), is involved in BR-regulated gene expression. BES1 interacts with SDG8, directly or indirectly through IWS1, a transcription elongation factor involved in BR-regulated gene expression. The knockout mutant sdg8 displays a reduced growth phenotype with compromised BR responses. Global gene expression studies demonstrated that SDG8 plays a major role in BR-regulated gene expression as more than half of BR-regulated genes are differentially affected in sdg8 mutant. A Chromatin Immunoprecipitation (ChIP) experiment showed that H3K36me3 is reduced in BR-regulated genes in the sdg8 mutant. Based on these results, we propose that SDG8 plays an essential role in mediating BR-regulated gene expression. Our results thus reveal a major mechanism by which histone modifications dictate hormonal regulation of gene expression

Selective Autophagy of BES1 Mediated by DSK2 Balances Plant Growth and Survival

Plants encounter a variety of stresses and must fine-tune their growth and stress-response programs to best suit their environment. BES1 functions as a master regulator in the brassinosteroid (BR) pathway that promotes plant growth. Here, we show that BES1 interacts with the ubiquitin receptor protein DSK2 and is targeted to the autophagy pathway during stress via the interaction of DSK2 with ATG8, a ubiquitin-like protein directing autophagosome formation and cargo recruitment. Additionally, DSK2 is phosphorylated by the GSK3-like kinase BIN2, a negative regulator in the BR pathway. BIN2 phosphorylation of DSK2 flanking its ATG8 interacting motifs (AIMs) promotes DSK2-ATG8 interaction, thereby targeting BES1 for degradation. Accordingly, loss-of-function dsk2 mutants accumulate BES1, have altered global gene expression profiles, and have compromised stress responses. Our results thus reveal that plants coordinate growth and stress responses by integrating BR and autophagy pathways and identify the molecular basis of this crosstalk

The mass assembly of galaxy groups and the evolution of the magnitude gap

We investigate the assembly of groups and clusters of galaxies using the Millennium dark matter simulation and the associated gas simulations and semi-analytic catalogues of galaxies. In particular, in order to find an observable quantity that could be used to identify early-formed groups, we study the development of the difference in magnitude between their brightest galaxies to assess the use of magnitude gaps as possible indicators. We select galaxy groups and clusters at redshift z=1 with dark matter halo mass M(R200) > 1E13/h Msun, and trace their properties until the present time (z=0). We consider only the systems with X-ray luminosity L_X> 0.25E42/h^2 erg/s at z=0. While it is true that a large magnitude gap between the two brightest galaxies of a particular group often indicates that a large fraction of its mass was assembled at an early epoch, it is not a necessary condition. More than 90% of fossil groups defined on the basis of their magnitude gaps (at any epoch between 0<z<1) cease to be fossils within 4 Gyr, mostly because other massive galaxies are assembled within their cores, even though most of the mass in their haloes might have been assembled at early times. We show that, compared to the conventional definition of fossil galaxy groups based on the magnitude gap Delta m(12)> 2 (in the R-band, within 0.5R200 of the centre of the group), an alternative criterion Delta m(14)>2.5 (within the same radius) finds 50% more early-formed systems, and those that on average retain their fossil phase longer. However, the conventional criterion performs marginally better at finding early-formed groups at the high-mass end of groups. Nevertheless, both criteria fail to identify a majority of the early-formed systems.Comment: 16 pages, 11 figures, 2 tables. Accepted for publication in MNRA