The wheat TabZIP2 transcription factor is activated by the nutrient starvation-responsive SnRK3/CIPK protein kinase.

KEY MESSAGE: The understanding of roles of bZIP factors in biological processes during plant development and under abiotic stresses requires the detailed mechanistic knowledge of behaviour of TFs. Basic leucine zipper (bZIP) transcription factors (TFs) play key roles in the regulation of grain development and plant responses to abiotic stresses. We investigated the role and molecular mechanisms of function of the TabZIP2 gene isolated from drought-stressed wheat plants. Molecular characterisation of TabZIP2 and derived protein included analyses of gene expression and its target promoter, and the influence of interacting partners on the target promoter activation. Two interacting partners of TabZIP2, the 14-3-3 protein, TaWIN1 and the bZIP transcription factor TaABI5L, were identified in a Y2H screen. We established that under elevated ABA levels the activity of TabZIP2 was negatively regulated by the TaWIN1 protein and positively regulated by the SnRK3/CIPK protein kinase WPK4, reported previously to be responsive to nutrient starvation. The physical interaction between the TaWIN1 and the WPK4 was detected. We also compared the influence of homo- and hetero-dimerisation of TabZIP2 and TaABI5L on DNA binding. TabZIP2 gene functional analyses were performed using drought-inducible overexpression of TabZIP2 in transgenic wheat. Transgenic plants grown under moderate drought during flowering, were smaller than control plants, and had fewer spikes and seeds per plant. However, a single seed weight was increased compared to single seed weights of control plants in three of four evaluated transgenic lines. The observed phenotypes of transgenic plants and the regulation of TabZIP2 activity by nutrient starvation-responsive WPK4, suggest that the TabZIP2 could be the part of a signalling pathway, which controls the rearrangement of carbohydrate and nutrient flows in plant organs in response to drought.

Basic leucine zipper (bZIP) transcription factors involved in abiotic stresses: A molecular model of a wheat bZIP factor and implications of its structure in function.

BACKGROUND: Basic leucine zipper (bZIP) genes encode transcription factors (TFs) that control important biochemical and physiological processes in plants and all other eukaryotic organisms. SCOPE OF REVIEW: Here we present (i) the homo-dimeric structural model of bZIP consisting of basic leucine zipper and DNA binding regions, in complex with the synthetic Abscisic Acid-Responsive Element (ABREsyn); (ii) discuss homo- and hetero-dimerisation patterns of bZIP TFs; (iii) summarise the current progress in understanding the molecular mechanisms of function of bZIP TFs, including features determining the specificity of their binding to DNA cis-elements, and (iv) review information on interaction partners of bZIPs during plant development and stress response, as well as on types and roles of post-translational modifications, and regulatory aspects of protein-degradation mediated turn-over. Finally, we (v) recapitulate on the recent advances regarding functional roles of bZIP factors in major agricultural crops, and discuss the potential significance of bZIP-based genetic engineering in improving crop yield and tolerance to abiotic stresses. MAJOR CONCLUSIONS: An accurate analysis and understanding of roles of plant bZIP TFs in different biological processes requires the knowledge of interacting partners, time and location of expression in plant organs, and the information on mechanisms of homo- and hetero-dimerisation of bZIP TFs. GENERAL SIGNIFICANCE: Studies on molecular mechanisms of plant bZIP TFs at the atomic levels will provide novel insights into the regulatory processes during plant development, and responses to abiotic and biotic stresses.

Molecular interactions of the γ-clade homeodomain-leucine zipper class I transcription factors during the wheat response to water deficit.

The γ-clade of class I homeodomain-leucine zipper (HD-Zip I) transcription factors (TFs) constitute members which play a role in adapting plant growth to conditions of water deficit. Given the importance of wheat (Triticum aestivum L.) as a global food crop and the impact of water deficit upon grain yield, we focused on functional aspects of wheat drought responsive HD-Zip I TFs. While the wheat γ-clade HD-Zip I TFs share significant sequence similarities with homologous genes from other plants, the clade-specific features in transcriptional response to abiotic stress were detected. We demonstrate that wheat TaHDZipI-3, TaHDZipI-4, and TaHDZipI-5 genes respond differentially to a variety of abiotic stresses, and that proteins encoded by these genes exhibit pronounced differences in oligomerisation, strength of DNA binding, and trans-activation of an artificial promoter. Three-dimensional molecular modelling of the protein-DNA interface was conducted to address the ambiguity at the central nucleotide in the pseudo-palindromic cis-element CAATNATTG that is recognised by all three HD-Zip I proteins. The co-expression of these genes in the same plant tissues together with the ability of HD-Zip I TFs of the γ-clade to hetero-dimerise suggests a role in the regulatory mechanisms of HD-Zip I dependent transcription. Our findings highlight the complexity of TF networks involved in plant responses to water deficit. A better understanding of the molecular complexity at the protein level during crop responses to drought will enable adoption of efficient strategies for production of cereal plants with enhanced drought tolerance.

Change of function of the wheat stress-responsive transcriptional repressor TaRAP2.1L by repressor motif modification.

Plants respond to abiotic stresses by changes in gene regulation, including stress-inducible expression of transcriptional activators and repressors. One of the best characterized families of drought-related transcription factors are dehydration-responsive element binding (DREB) proteins, known as C-repeat binding factors (CBF). The wheat DREB/CBF gene TaRAP2.1L was isolated from drought-affected tissues using a dehydration-responsive element (DRE) as bait in a yeast one-hybrid screen. TaRAP2.1L is induced by elevated abscisic acid, drought and cold. A C-terminal ethylene responsive factor-associated amphiphilic repression (EAR) motif, known to be responsible for active repression of target genes, was identified in the TaRAP2.1L protein. It was found that TaRAP2.1L has a unique selectivity of DNA-binding, which differs from that of DREB activators. This binding selectivity remains unchanged in a TaRAP2.1L variant with an inactivated EAR motif (TaRAP2.1Lmut). To study the role of the TaRAP2.1L repressor activity associated with the EAR motif in planta, transgenic wheat overexpressing native or mutated TaRAP2.1L was generated. Overexpression of TaRAP2.1L under constitutive and stress-inducible promoters in transgenic wheat and barley led to dwarfism and decreased frost tolerance. By contrast, constitutive overexpression of the TaRAP2.1Lmut gene had little or no negative influence on wheat development or grain yield. Transgenic lines with the TaRAP2.1Lmut transgene had an enhanced ability to survive frost and drought. The improved stress tolerance is attributed to up-regulation of several stress-related genes known to be downstream genes of DREB/CBF activators.

The homeodomain transcription factor TaHDZipI-2 from wheat regulates frost tolerance, flowering time and spike development in transgenic barley.

Homeodomain leucine zipper class I (HD-Zip I) transcription factors (TFs) play key roles in the regulation of plant growth and development under stresses. Functions of the TaHDZipI-2 gene isolated from the endosperm of developing wheat grain were revealed. Molecular characterization of TaHDZipI-2 protein included studies of its dimerisation, protein-DNA interactions and gene activation properties using pull-down assays, in-yeast methods and transient expression assays in wheat cells. The analysis of TaHDZipI-2 gene functions was performed using transgenic barley plants. It included comparison of developmental phenotypes, yield components, grain quality, frost tolerance and the levels of expression of potential target genes in transgenic and control plants. Transgenic TaHDZipI-2 lines showed characteristic phenotypic features that included reduced growth rates, reduced biomass, early flowering, light-coloured leaves and narrowly elongated spikes. Transgenic lines produced 25-40% more seeds per spike than control plants, but with 50-60% smaller grain size. In vivo lipid imaging exposed changes in the distribution of lipids between the embryo and endosperm in transgenic seeds. Transgenic lines were significantly more tolerant to frost than control plants. Our data suggest the role of TaHDZipI-2 in controlling several key processes underlying frost tolerance, transition to flowering and spike development.

An autoactive mutant of the M flax rust resistance protein has a preference for binding ATP, whereas wild-type M protein binds ADP.

Resistance (R) proteins are key regulators of the plant innate immune system and are capable of pathogen detection and activation of the hypersensitive cell death immune response. To understand the molecular mechanism of R protein activation, we undertook a phenotypic and biochemical study of the flax nucleotide binding (NB)-ARC leucine-rich repeat protein, M. Using Agrobacterium-mediated transient expression in flax cotyledons, site-directed mutations of key residues within the P-loop, kinase 2, and MHD motifs within the NB-ARC domain of M were shown to affect R protein function. When purified using a yeast expression system and assayed for ATP and ADP, these mutated proteins exhibited marked differences in the quantity and identity of the bound nucleotide. ADP was bound to recombinant wild-type M protein, while the nonfunctional P-loop mutant did not have any nucleotides bound. In contrast, ATP was bound to an autoactive M protein mutated in the highly conserved MHD motif. These data provide direct evidence supporting a model of R protein function in which the "off" R protein binds ADP and activation of R protein defense signaling involves the exchange of ADP for ATP.

Purification of the M flax-rust resistance protein expressed in Pichia pastoris.

The M flax-rust resistance (R) gene is predicted to encode a 150-kDa protein of the Toll-interleukin-like receptor-nucleotide binding site-leucine rich repeat (TIR-NBS-LRR) class of plant disease resistance proteins and provides resistance against the Melampsora lini (flax rust) fungus carrying the AvrM avirulence gene. The extremely low level of this class of R proteins found in plant tissue has precluded their biochemical and structural analysis, and the study of these proteins has been largely restricted to genetic analyses and in vivo investigations. Here we report the production and purification of the M protein in the methalotrophic yeast, Pichia pastoris. Expression trials with five different constructs reveals optimum levels of soluble native M protein can be obtained as an N-terminally 9x His-tagged protein, in which the first 21 amino acids of the predicted wild-type protein are deleted. Expression was achieved using a high cell density fed-batch bioreactor culture at low temperature. M protein was purified to near homogeneity from whole-cell lysates using cation exchange, immobilised metal ion affinity chromatography and gel filtration with a final yield of approximately 3 mg of protein/1000 g wet weight of yeast cells lysed. The successful expression and purification of soluble M protein opens the way for biochemical and structural analysis of this class of important plant proteins.