The GATA-type transcription factors GNC and GNL/CGA1 repress gibberellin signaling downstream from DELLA proteins and PHYTOCHROME-INTERACTING FACTORS.

The phytohormone gibberellin (GA) regulates various developmental processes in plants such as germination, greening, elongation growth, and flowering time. DELLA proteins, which are degraded in response to GA, repress GA signaling by inhibitory interactions with PHYTOCHROME-INTERACTING FACTOR (PIF) family transcription factors. How GA signaling is controlled downstream from the DELLA and PIF regulators is, at present, unclear. Here, we characterize GNC (GATA, NITRATE-INDUCIBLE, CARBON-METABOLISM INVOLVED) and GNL/CGA1 (GNC-LIKE/CYTOKININ-RESPONSIVE GATA FACTOR1), two homologous GATA-type transcription factors from Arabidopsis thaliana that we initially identified as GA-regulated genes. Our genetic analyses of loss-of-function mutants and overexpression lines establish that GNC and GNL are functionally redundant regulators of germination, greening, elongation growth and flowering time. We further show by chromatin immunoprecipitation that both genes are potentially direct transcription targets of PIF transcription factors, and that their expression is up-regulated in pif mutant backgrounds. In line with a key role of GNC or GNL downstream from DELLA and PIF signaling, we find that their overexpression leads to gene expression changes that largely resemble those observed in a ga1 biosynthesis mutant or a pif quadruple mutant. These findings, together with the fact that gnc and gnl loss-of-function mutations suppress ga1 phenotypes, support the hypothesis that GNC and GNL are important repressors of GA signaling downstream from the DELLA and PIF regulators.

Convergence of auxin and gibberellin signaling on the regulation of the GATA transcription factors GNC and GNL in Arabidopsis thaliana.

Plant growth is regulated by a complex network of signaling events. Points of convergence for the signaling cross-talk between the phytohormones auxin and gibberellin (GA), which partly control overlapping processes during plant development, are largely unknown. At the cellular level, auxin responses are controlled by members of the AUXIN RESPONSE FACTOR (ARF) family of transcription factors as well as AUXIN/INDOLE-3-ACETIC ACID INDUCIBLE (AUX/IAA) proteins that repress the activity of at least a subset of ARFs. Here, we show that the two paralogous GATA transcription factors GATA, NITRATE-INDUCIBLE, CARBON-METABOLISM INVOLVED (GNC) and GNC-LIKE (GNL)/CYTOKININ-RESPONSIVE GATA FACTOR1 (CGA1) are direct and critical transcription targets downstream from ARF2 in the control of greening, flowering time, and senescence. Mutants deficient in the synthesis or signaling of the phytohormone GA are also impaired in greening, flowering, and senescence, and interestingly, GNC and GNL were previously identified as important transcription targets of the GA signaling pathway. In line with a critical regulatory role for GNC and GNL downstream from both auxin and GA signaling, we show here that the constitutive activation of GA signaling is sufficient to suppress arf2 mutant phenotypes through repression of GNC and GNL. In addition, we show that GA promotes ARF2 protein abundance through a translation-dependent mechanism that could serve to override the autoinhibitory negative feedback regulation of ARF2 on its own transcription and thereby further promote GA signaling.

Functional diversification within the family of B-GATA transcription factors through the leucine-leucine-methionine domain.

The transcription of the Arabidopsis (Arabidopsis thaliana) GATA transcription factors GATA, NITRATE-INDUCIBLE, CARBON METABOLISM-INVOLVED (GNC) and GNC-LIKE (GNL)/CYTOKININ-RESPONSIVE GATA FACTOR1 is controlled by several growth regulatory signals including light and the phytohormones auxin, cytokinin, and gibberellin. To date, GNC and GNL have been attributed functions in the control of germination, greening, flowering time, floral development, senescence, and floral organ abscission. GNC and GNL belong to the 11-member family of B-class GATA transcription factors that are characterized to date solely by their high sequence conservation within the GATA DNA-binding domain. The degree of functional conservation among the various B-class GATA family members is not understood. Here, we identify and examine B-class GATAs from Arabidopsis, tomato (Solanum lycopersicon), Brachypodium (Brachypodium distachyon), and barley (Hordeum vulgare). We find that B-class GATAs from these four species can be subdivided based on their short or long N termini and the presence of the 13-amino acid C-terminal leucine-leucine-methionine (LLM) domain with the conserved motif LLM. Through overexpression analyses and by complementation of a gnc gnl double mutant, we provide evidence that the length of the N terminus may not allow distinguishing between the different B-class GATAs at the functional level. In turn, we find that the presence and absence of the LLM domain in the overexpressors has differential effects on hypocotyl elongation, leaf shape, and petiole length, as well as on gene expression. Thus, our analyses identify the LLM domain as an evolutionarily conserved domain that determines B-class GATA factor identity and provides a further subclassification criterion for this transcription factor family.

ML3 is a NEDD8- and ubiquitin-modified protein.

NEDD8 (NEURAL PRECURSOR CELL-EXPRESSED, DEVELOPMENTALLY DOWN-REGULATED PROTEIN8) is an evolutionarily conserved 8-kD protein that is closely related to ubiquitin and that can be conjugated like ubiquitin to specific lysine residues of target proteins in eukaryotes. In contrast to ubiquitin, for which a broad range of substrate proteins are known, only a very limited number of NEDD8 target proteins have been identified to date. Best understood, and also evolutionarily conserved, is the NEDD8 modification (neddylation) of cullins, core subunits of the cullin-RING-type E3 ubiquitin ligases that promote the polyubiquitylation of degradation targets in eukaryotes. Here, we show that Myeloid differentiation factor-2-related lipid-recognition domain protein ML3 is an NEDD8- as well as ubiquitin-modified protein in Arabidopsis (Arabidopsis thaliana) and examine the functional role of ML3 in the plant cell. Our analysis indicates that ML3 resides in the vacuole as well as in endoplasmic reticulum (ER) bodies. ER bodies are Brassicales-specific ER-derived organelles and, similar to other ER body proteins, ML3 orthologs can only be identified in this order of flowering plants. ML3 gene expression is promoted by wounding as well as by the phytohormone jasmonic acid and repressed by ethylene, signals that are known to induce and repress ER body formation, respectively. Furthermore, ML3 protein abundance is dependent on NAI1, a master regulator of ER body formation in Arabidopsis. The regulation of ML3 expression and the localization of ML3 in ER bodies and the vacuole is in agreement with a demonstrated importance of ML3 in the defense to herbivore attack. Here, we extend the spectrum of ML3 biological functions by demonstrating a role in the response to microbial pathogens.

Safeners recruit multiple signalling pathways for the orchestrated induction of the cellular xenobiotic detoxification machinery in Arabidopsis.

Safeners enhance herbicide tolerance in crop plants but not in target weeds, thus improving herbicide selectivity. The safeners isoxadifen-ethyl and mefenpyr-diethyl protect cereal crops from sulfonyl urea herbicides in postemergence application. The two safeners were shown here to induce the cellular xenobiotic detoxification machinery in Arabidopsis thaliana when applied to leaves in a way mimicking field application. Gene expression profiling revealed the induction of 446 genes potentially involved in the detoxification process. Transgenic Arabidopsis plants expressing a reporter gene under control of a safener-responsive maize promoter were used as a model system to study the safener signalling pathway. Reporter gene analysis in the tga2/3/5/6, sid2-2 and npr1 mutants as compared with the wild-type background showed that safener inducibility required TGA transcription factors and salicylic acid (SA) in a NON-EXPRESSOR of PR-1 (NPR1)-independent pathway converging on two as-1 promoter elements. For the majority of the safener-responsive Arabidopsis genes, a similar dependence on TGA transcription factors and/or SA was shown by gene expression profiling in wild-type plants as compared with the tga2/3/5/6 and sid2-2 mutants. Thirty-eight percent of the genes, however, were induced by safeners in a TGA/SA-independent manner. These genes are likely to be controlled by WRKY transcription factors and cognate W-boxes in their promoters.

B-GATA transcription factors - insights into their structure, regulation, and role in plant development.

GATA transcription factors are evolutionarily conserved transcriptional regulators that recognize promoter elements with a G-A-T-A core sequence. In comparison to animal genomes, the GATA transcription factor family in plants is comparatively large with approximately 30 members. Here, we review the current knowledge on B-GATAs, one of four GATA factor subfamilies from Arabidopsis thaliana. We show that B-GATAs can be subdivided based on structural features and their biological function into family members with a C-terminal LLM- (leucine-leucine-methionine) domain or an N-terminal HAN- (HANABA TARANU) domain. The paralogous GNC (GATA, NITRATE-INDUCIBLE, CARBON-METABOLISM INVOLVED) and CGA1/GNL (CYTOKININ-INDUCED GATA1/GNC-LIKE) are introduced as LLM-domain containing B-GATAs from Arabidopsis that control germination, greening, senescence, and flowering time downstream from several growth regulatory signals. Arabidopsis HAN and its monocot-specific paralogs from rice (NECK LEAF1), maize (TASSEL SHEATH1), and barley (THIRD OUTER GLUME) are HAN-domain-containing B-GATAs with a predominant role in embryo development and floral development. We also review GATA23, a regulator of lateral root initiation from Arabidopsis that is closely related to GNC and GNL but has a degenerate LLM-domain that is seemingly specific for the Brassicaceae family. The Brassicaceae-specific GATA23 and the monocot-specific HAN-domain GATAs provide evidence that neofunctionalization of B-GATAs was used during plant evolution to expand the functional repertoire of these transcription factors.