Genome-wide identification of grain filling genes regulated by the OsSMF1 transcription factor in rice

OsMYB14, an R2R3-MYB transcription factor, regulates plant height through the control of hormone metabolism in rice

Plant growth must be regulated throughout the plant life cycle. The myeloblastosis (MYB) transcription factor (TF) family is one of the largest TF families and is involved in metabolism, lignin biosynthesis, and developmental processes. Here, we showed that OsMYB14, a rice R2R3-MYB TF, was expressed in leaves and roots, especially in rice culm and panicles, and that it localized to the nucleus. Overexpression of OsMYB14 (OsMYB14-ox) in rice resulted in a 30% reduction in plant height compared to that of the wild type (WT), while the height of the osmyb14-knockout (osmyb14-ko) mutant generated using the CRISPR/Cas9 system was not significantly different. Microscopic observations of the first internode revealed that the cell size did not differ significantly among the lines. RNA sequencing analysis revealed that genes associated with plant development, regulation, lipid metabolism, carbohydrate metabolism, and gibberellin (GA) and auxin metabolic processes were downregulated in the OsMYB14-ox line. Hormone quantitation revealed that inactive GA19 accumulated in OsMYB14-ox but not in the WT or knockout plants, suggesting that GA20 generation was repressed. Indole-3-acetic acid (IAA) and IAA-aspartate accumulated in OsMYB14-ox and osmyb14-ko, respectively. Indeed, real-time PCR analysis revealed that the expression of OsGA20ox1, encoding GA20 oxidase 1, and OsGH3-2, encoding IAA-amido synthetase, was downregulated in OsMYB14-ox and upregulated in osmyb14-ko. A protein-binding microarray revealed the presence of a consensus DNA-binding sequence, the ACCTACC-like motif, in the promoters of the OsGA20ox1 and GA20ox2 genes. These results suggest that OsMYB14 may act as a negative regulator of biological processes affecting plant height in rice by regulating GA biosynthesis and auxin metabolism.

A Novel Allele in the Promoter of Wx Decreases Gene Expression and Confers Lower Apparent Amylose Contents in Japonica Rice (Oryza sativa L.)

Wx is the key gene that controls amylose content (AC), and various alleles have been found in rice populations. Wxb is the major allele in japonica and produces moderate AC (15~18%). It was recently found that editing the promoter of Wx could produce a series of alleles that have different Wx activities. Although some studies have edited the promoter, few studies have focused on the natural variations in Wx. Here, we used the Rice3K database to investigate variations in the Wx promoter and found that the allele Wx1764178 (A/G) has a higher LD (linkage disequilibrium) with the two key SNPs (1765751, T/G; 1768006, A/C), which could produce different Wx alleles and influence AC, as reported previously. Further study showed that the Wx1764178 allele (A/G) is functional and influences the expression of Wx positively. Editing the A allele using CRISPR‒Cas9 produced 36 and 3 bp deletions and caused a decrease in the expression of Wx. The apparent amylose content (AAC) in the edited lines was decreased by 7.09% and 11.50% compared with that of the wild type, which was the japonica variety Nipponbare with Wxb and the A allele at 1764178, while a complementary line with the G allele showed a lower AAC than the A allele with no effect on other agronomic traits. The AAC of the edited lines showed a higher increase than that of the wild type (Nipponbare, Wxb) in low-nitrogen conditions relative to high-nitrogen conditions. We also developed a dCAPS marker to identify the allele and found that the G allele has widely been used (82.95%) in japonica-bred varieties from Jiangsu Province, China. Overall, we found a functional allele (Wx1764178, A/G) in the Wx promoter that could affect AAC in japonica cultivars and be developed as markers for quality improvement in rice breeding programs.

Non-Tandem CCCH-Type Zinc-Finger Protein CpZF_CCCH1 Improves Fatty Acid Desaturation and Stress Tolerance in Chlamydomonas reinhardtii

The properties and nutritional value of microalgal bioproducts depend significantly on fatty acid desaturation, which is generally modulated by manipulating the culture conditions or associated gene expressions. Here, we investigated the role of CpZF_CCCH1, a non-tandem CCCH-type zinc-finger (non-TZF) protein, in elevating polyunsaturated fatty acid (PUFA) content (11.00-16.36%) in Chlamydomonas reinhardtii. Through lipidomic and flow cytometry analyses, we observed reduced triacylglycerol accumulation (7.01-21.15%) and elevated levels of membrane lipids containing PUFAs (7.81-46.18%) in C. reinhardtii overexpressing CpZF_CCCH1. Additionally, overexpression of nucleus-located CpZF_CCCH1 downregulated genes associated with triacylglycerol assembly and lipid turnover from 2.00- to 2.90-fold, likely by binding to GCN4 motif and promoter of 3-phosphate-glycerol acyltransferase. Furthermore, overexpression of CpZF_CCCH1 alleviated reactive oxygen species levels by 59.28-73.26% and enhanced stress tolerance under adverse conditions. These findings expanded the roles of non-TZF proteins in lipid metabolism, opening new avenues for metabolic engineering to enhance the nutritional value and stress tolerance of microalgae and agricultural crops.

Comparing genomes of Fructus Amomi-producing species reveals genetic basis of volatile terpenoid divergence

Wurfbainia longiligularis and Wurfbainia villosa are both rich in volatile terpenoids and are 2 primary plant sources of Fructus Amomi used for curing gastrointestinal diseases. Metabolomic profiling has demonstrated that bornyl diphosphate (BPP)-related terpenoids are more abundant in the W. villosa seeds and have a wider tissue distribution in W. longiligularis. To explore the genetic mechanisms underlying the volatile terpenoid divergence, a high-quality chromosome-level genome of W. longiligularis (2.29 Gb, contig N50 of 80.39 Mb) was assembled. Functional characterization of 17 terpene synthases (WlTPSs) revealed that WlBPPS, along with WlTPS 24/26/28 with bornyl diphosphate synthase (BPPS) activity, contributes to the wider tissue distribution of BPP-related terpenoids in W. longiligularis compared to W. villosa. Furthermore, transgenic Nicotiana tabacum showed that the GCN4-motif element positively regulates seed expression of WvBPPS and thus promotes the enrichment of BPP-related terpenoids in W. villosa seeds. Systematic identification and analysis of candidate TPS in 29 monocot plants from 16 families indicated that substantial expansion of TPS-a and TPS-b subfamily genes in Zingiberaceae may have driven increased diversity and production of volatile terpenoids. Evolutionary analysis and functional identification of BPPS genes showed that BPP-related terpenoids may be distributed only in the Zingiberaceae of monocot plants. This research provides valuable genomic resources for breeding and improving Fructus Amomi with medicinal and edible value and sheds light on the evolution of terpenoid biosynthesis in Zingiberaceae.

Exploring the Binding Affinity of the ARR2 GARP DNA Binding Domain via Comparative Methods

Plants have evolved signaling mechanisms such as the multi-step phosphorelay (MSP) to respond to different internal and external stimuli. MSP responses often result in gene transcription regulation that is modulated through transcription factors such as B-type Arabidopsis response regulator (ARR) proteins. Among these proteins, ARR2 is a key component that is expressed ubiquitously and is involved in many aspects of plant development. Although it has been noted that B-type ARRs bind to their cognate genes through a DNA-binding domain termed the GARP domain, little is known about the structure and function of this type of DNA-binding domain; thus, how ARRs bind to DNA at a structural level is still poorly understood. In order to understand how the MSP functions in planta, it is crucial to unravel both the kinetics as well as the structural identity of the components involved in such interactions. For this reason, this work focusses on resolving how the GARP domain of ARR2 (GARP2) binds to the promoter region of ARR5, one of its native target genes in cytokinin signaling. We have established that GARP2 specifically binds to the ARR5 promoter with three different bi-molecular interaction systems-qDPI-ELISA, FCS, and MST-and we also determined the KD of this interaction. In addition, structural modeling of the GARP2 domain confirms that GARP2 entails a HTH motif, and that protein-DNA interaction most likely occurs via the α3-helix and the N-terminal arm of this domain since mutations in this region hinder ARR2's ability to activate transcription.

Genetic dissection of protein and starch during wheat grain development using QTL mapping and GWAS

Protein, starch, and their components are important for wheat grain yield and end-products, which are affected by wheat grain development. Therefore, QTL mapping and a genome-wide association study (GWAS) of grain protein content (GPC), glutenin macropolymer content (GMP), amylopectin content (GApC), and amylose content (GAsC) were performed on wheat grain development at 7, 14, 21, and 28 days after anthesis (DAA) in two environments using a recombinant inbred line (RIL) population of 256 stable lines and a panel of 205 wheat accessions. A total of 29 unconditional QTLs, 13 conditional QTLs, 99 unconditional marker-trait associations (MTAs), and 14 conditional MTAs significantly associated (p < 10^-4) with four quality traits were found to be distributed on 15 chromosomes, with the phenotypic variation explained (PVE) ranging from 5.35% to 39.86%. Among these genomic variations, three major QTLs [QGPC3B, QGPC2A, and QGPC(S3|S2)3B] and SNP clusters on the 3A and 6B chromosomes were detected for GPC, and the SNP TA005876-0602 was stably expressed during the three periods in the natural population. The QGMP3B locus was detected five times in three developmental stages in two environments with 5.89%-33.62% PVE, and SNP clusters for GMP content were found on the 3A and 3B chromosomes. For GApC, the QGApC3B.1 locus had the highest PVE of 25.69%, and SNP clusters were found on chromosomes 4A, 4B, 5B, 6B, and 7B. Four major QTLs of GAsC were detected at 21 and 28 DAA. Most interestingly, both QTL mapping and GWAS analysis indicated that four chromosomes (3B, 4A, 6B, and 7A) were mainly involved in the development of protein, GMP, amylopectin, and amylose synthesis. Of these, the wPt-5870-wPt-3620 marker interval on chromosome 3B seemed to be most important because it played an important role in the synthesis of GMP and amylopectin before 7 DAA, in the synthesis of protein and GMP from 14 to 21 DAA, and in the development of GApC and GAsC from 21 to 28 DAA. Using the annotation information of IWGSC Chinese Spring RefSeq v1.1 genome assembly, we predicted 28 and 69 candidate genes for major loci from QTL mapping and GWAS, respectively. Most of them have multiple effects on protein and starch synthesis during grain development. These results provide new insights and information for the potential regulatory network between grain protein and starch synthesis.

Double DAP-seq uncovered synergistic DNA binding of interacting bZIP transcription factors

Many eukaryotic transcription factors (TF) form homodimer or heterodimer complexes to regulate gene expression. Dimerization of BASIC LEUCINE ZIPPER (bZIP) TFs are critical for their functions, but the molecular mechanism underlying the DNA binding and functional specificity of homo- versus heterodimers remains elusive. To address this gap, we present the double DNA Affinity Purification-sequencing (dDAP-seq) technique that maps heterodimer binding sites on endogenous genomic DNA. Using dDAP-seq we profile twenty pairs of C/S1 bZIP heterodimers and S1 homodimers in Arabidopsis and show that heterodimerization significantly expands the DNA binding preferences of these TFs. Analysis of dDAP-seq binding sites reveals the function of bZIP9 in abscisic acid response and the role of bZIP53 heterodimer-specific binding in seed maturation. The C/S1 heterodimers show distinct preferences for the ACGT elements recognized by plant bZIPs and motifs resembling the yeast GCN4 cis-elements. This study demonstrates the potential of dDAP-seq in deciphering the DNA binding specificities of interacting TFs that are key for combinatorial gene regulation.

The hot science in rice research: How rice plants cope with heat stress

Global climate change has great impacts on plant growth and development, reducing crop productivity worldwide. Rice (Oryza sativa L.), one of the world's most important food crops, is susceptible to high-temperature stress from seedling stage to reproductive stage. In this review, we summarize recent advances in understanding the molecular mechanisms underlying heat stress responses in rice, including heat sensing and signalling, transcriptional regulation, transcript processing, protein translation, and post-translational regulation. We also highlight the irreversible effects of high temperature on reproduction and grain quality in rice. Finally, we discuss challenges and opportunities for future research on heat stress responses in rice.

Environmental Stimuli: A Major Challenge during Grain Filling in Cereals

Light, temperature, water, and fertilizer are arguably the most important environmental factors regulating crop growth and productivity. Environmental stimuli, including low light, extreme temperatures, and water stresses caused by climate change, affect crop growth and production and pose a growing threat to sustainable agriculture. Furthermore, soil salinity is another major environmental constraint affecting crop growth and threatening global food security. The grain filling stage is the final stage of growth and is also the most important stage in cereals, directly determining the grain weight and final yield. However, the grain filling process is extremely vulnerable to different environmental stimuli, especially for inferior spikelets. Given the importance of grain filling in cereals and the deterioration of environmental problems, understanding environmental stimuli and their effects on grain filling constitutes a major focus of crop research. In recent years, significant advances made in this field have led to a good description of the intricate mechanisms by which different environmental stimuli regulate grain filling, as well as approaches to adapt cereals to changing climate conditions and to give them better grain filling. In this review, the current environmental stimuli, their dose-response effect on grain filling, and the physiological and molecular mechanisms involved are discussed. Furthermore, what we can do to help cereal crops adapt to environmental stimuli is elaborated. Overall, we call for future research to delve deeper into the gene function-related research and the commercialization of gene-edited crops. Meanwhile, smart agriculture is the development trend of the future agriculture under environmental stimuli.

Carotenoid Cleavage Dioxygenase 1 Catalyzes Lutein Degradation To Influence Carotenoid Accumulation and Color Development in Foxtail Millet Grains

Foxtail millet is a minor but economically important crop in certain regions of the world. Millet color is often used to judge grain quality, yet the molecular determinants of millet coloration remain unclear. Here, we explored the relationship between SiCCD1 and millet coloration in yellow and white millet varieties. Carotenoid levels declined with grain maturation and were negatively correlated with SiCCD1 expression, which was significantly higher in white millet as compared to yellow millet during the color development stage. Cloning of the SiCCD1 promoter and CDS sequences from these different millet varieties revealed the presence of two additional cis-regulatory elements within the SiCCD1 promoter in white millet varieties, including an enhancer-like GC motif element associated with anoxic specific inducibility and a GCN4-motif element associated with endosperm expression. Dual-luciferase reporter assays confirmed that SiCCD1 promoter fragments containing these additional cis-acting elements derived from white millet varieties were significantly more active than those from yellow millet varieties, consistent with the observed SiCCD1 expression patterns. Further in vitro enzyme detection assays confirmed that SiCCD1 primarily targets and degrades lutein. Together, these data suggest that SiCCD1 promoter variation was a key factor associated with the observed differences in SiCCD1 expression, which in turn led to the difference in millet coloration.

Regulators of Starch Biosynthesis in Cereal Crops

Starch is the main food source for human beings and livestock all over the world, and it is also the raw material for production of industrial alcohol and biofuel. A considerable part of the world’s annual starch production comes from crops and their seeds. With the increasing demand for starch from food and non-food industries and the growing loss of arable land due to urbanization, understanding starch biosynthesis and its regulators is essential to produce the desirable traits as well as more and better polymers via biotechnological approaches in cereal crops. Because of the complexity and flexibility of carbon allocation in the formation of endosperm starch, cereal crops require a broad range of enzymes and one matching network of regulators to control the providential functioning of these starch biosynthetic enzymes. Here, we comprehensively summarize the current knowledge about regulatory factors of starch biosynthesis in cereal crops, with an emphasis on the transcription factors that directly regulate starch biosynthesis. This review will provide new insights for the manipulation of bioengineering and starch biosynthesis to improve starch yields or qualities in our diets and in industry.

A Meloidogyne graminicola Pectate Lyase Is Involved in Virulence and Activation of Host Defense Responses

Plant-parasitic nematodes secrete an array of cell-wall-degrading enzymes to overcome the physical barrier formed by the plant cell wall. Here, we describe a novel pectate lyase gene Mg-PEL1 from M. graminicola. Quantitative real-time PCR assay showed that the highest transcriptional expression level of Mg-PEL1 occurred in pre-parasitic second-stage juveniles, and it was still detected during the early parasitic stage. Using in situ hybridization, we showed that Mg-PEL1 was expressed exclusively within the subventral esophageal gland cells of M. graminicola. The yeast signal sequence trap system revealed that it possessed an N-terminal signal peptide with secretion function. Recombinant Mg-PEL1 exhibited hydrolytic activity toward polygalacturonic acid. Rice plants expressing RNA interference vectors targeting Mg-PEL1 showed an increased resistance to M. graminicola. In addition, using an Agrobacterium-mediated transient expression system and plant immune response assays, we demonstrated that the cell wall localization of Mg-PEL1 was required for the activation of plant defense responses, including programmed plant cell death, reactive oxygen species (ROS) accumulation and expression of defense-related genes. Taken together, our results indicated that Mg-PEL1 could enhance the pathogenicity of M. graminicola and induce plant immune responses during nematode invasion into plants or migration in plants. This provides a new insight into the function of pectate lyases in plants-nematodes interaction.

Rice protein-binding microarrays: a tool to detect cis-acting elements near promoter regions in rice

The present study showed that a rice (Oryza sativa)-specific protein-binding microarray (RPBM) can be applied to analyze DNA-binding motifs with a TF where binding is evaluated in extended natural promoter regions. The analysis may facilitate identifying TFs and their downstream genes and constructing gene networks through cis-elements. Transcription factors (TFs) regulate gene expression at the transcriptional level by binding a specific DNA sequence. Thus, predicting the DNA-binding motifs of TFs is one of the most important areas in the functional analysis of TFs in the postgenomic era. Although many methods have been developed to address this challenge, many TFs still have unknown DNA-binding motifs. In this study, we designed RPBM with 40-bp probes and 20-bp of overlap, yielding 49 probes spanning the 1-kb upstream region before the translation start site of each gene in the entire genome. To confirm the efficiency of RPBM technology, we selected two previously studied TFs, OsWOX13 and OsSMF1, and an uncharacterized TF, OsWRKY34. We identified the ATTGATTG and CCACGTCA DNA-binding sequences of OsWOX13 and OsSMF1, respectively. In total, 635 and 932 putative feature genes were identified for OsWOX13 and OsSMF1, respectively. We discovered the CGTTGACTTT DNA-binding sequence and 195 putative feature genes of OsWRKY34. RPBM could be applicable in the analysis of DNA-binding motifs for TFs where binding is evaluated in the promoter and 5' upstream CDS regions. The analysis may facilitate identifying TFs and their downstream genes and constructing gene networks through cis-elements.

The NAC Transcription Factors OsNAC20 and OsNAC26 Regulate Starch and Storage Protein Synthesis

Starch and storage proteins determine the weight and quality of cereal grains. Synthesis of these two grain components has been comprehensively investigated, but the transcription factors responsible for their regulation remain largely unknown. In this study, we investigated the roles of NAM, ATAF, and CUC (NAC) transcription factors, OsNAC20, and OsNAC26 in starch and storage protein synthesis in rice (Oryza sativa) endosperm. OsNAC20 and OsNAC26 showed high levels of amino acid sequence similarity. Both were localized in the aleurone layer, starchy endosperm, and embryo. Mutation of OsNAC20 or OsNAC26 alone had no effect on the grain, while the osnac20/26 double mutant had significantly decreased starch and storage protein content. OsNAC20 and OsNAC26 alone could directly transactivate the expression of starch synthaseI (SSI), pullulanase (Pul), glutelin A1 (GluA1), glutelin B4/5 (GluB4/5), α-globulin, and 16 kD prolamin and indirectly influenced plastidial disproportionating enzyme1 (DPE1) expression to regulate starch and storage protein synthesis. Although they could also bind to the promoters of ADP-Glc pyrophosphorylase small subunit 2b (AGPS2b), ADP-Glc pyrophosphorylase large subunit 2 (AGPL2), and starch branching enzymeI (SBEI), and the expression of the three genes was largely decreased in the osnac20/26 mutant, ADP-Glc pyrophosphorylase and starch branching enzyme activities were unchanged in this double mutant. In addition, OsNAC20 and OsNAC26 are main regulators of Pul, GluB4, α-globulin, and 16 kD prolamin In conclusion, OsNAC20 and OsNAC26 play an essential and redundant role in the regulation of starch and storage protein synthesis.

Diverse Roles of MAX1 Homologues in Rice

Cytochrome P450 enzymes encoded by MORE AXILLARY GROWTH1 (MAX1)-like genes produce most of the structural diversity of strigolactones during the final steps of strigolactone biosynthesis. The diverse copies of MAX1 in Oryza sativa provide a resource to investigate why plants produce such a wide range of strigolactones. Here we performed in silico analyses of transcription factors and microRNAs that may regulate each rice MAX1, and compared the results with available data about MAX1 expression profiles and genes co-expressed with MAX1 genes. Data suggest that distinct mechanisms regulate the expression of each MAX1. Moreover, there may be novel functions for MAX1 homologues, such as the regulation of flower development or responses to heavy metals. In addition, individual MAX1s could be involved in specific functions, such as the regulation of seed development or wax synthesis in rice. Our analysis reveals potential new avenues of strigolactone research that may otherwise not be obvious.

Proteomic profiling reveals differentially expressed proteins associated with amylose accumulation during rice grain filling

BACKGROUND

Amylose accumulation in rice grains is controlled by genetic and environmental factors. Amylose content is a determinant factor of rice quality in terms of cooking and eating. Great variations in amylose content in indica rice cultivars have been observed. The current study was to identify differentially expressed proteins in starch and sucrose metabolism and glycolysis/gluconeogenesis pathways and their relationships to amylose synthesis using two rice cultivars possess contrasting phenotypes in grain amylose content.

RESULTS

Synthesis and accumulation of amylose in rice grains significantly affected the variations between rice cultivars in amylose contents. The high amylose content cultivar has three down-regulated differentially expressed proteins, i.e., LOC_Os01g62420.1, LOC_Os02g36600.1, and LOC_Os08g37380.2 in the glycolysis/gluconeogenesis pathway, which limit the glycolytic process and decrease the glucose-1-phosphate consumption. In the starch and sucrose metabolic pathway, an up-regulated protein, i.e., LOC_Os06g04200.1 and two down-regulated proteins, i.e., LOC_Os05g32710.1 and LOC_Os04g43360.1 were identified (Figure 4). Glucose-1-phosphate is one of the first substrates in starch synthesis and glycolysis that are catalyzed to form adenosine diphosphate glucose (ADPG), then the ADPG is catalyzed by granule-bound starch synthase I (GBSS I) to elongate amylose.

CONCLUSIONS

The results indicate that decreasing the consumption of glucose-1-phosphate in the glycolytic process is essential for the formation of ADPG and UDPG, which are substrates for amylose synthesis. In theory, amylose content in rice can be regulated by controlling the fate of glucose-1-phosphate.

Genotypic variation in the grain photosynthetic contribution to grain filling in rice

Grain filling in rice, a staple cereal crop worldwide, is a critical determinant of grain yield and quality. However, there is little available information on the relationship between grain filling and grain photosynthetic capacity in rice. This study evaluated the genetic diversity among six rice cultivars for their grain filling rate (GR₀) and the relationships with the grain chlorophyll contents and grain net photosynthetic rate (P_N). Significant variations in GR₀, P_N, and the chlorophyll contents (a, b, and total) in the grains of the cultivars were observed. Approximately 90 % of the variation in GR₀ was explained by the grain P_N. General linear model regression revealed significant positive correlations between P_N/GR₀ and the chlorophyll contents (a, b, and total) in the grains. There was also a significant positive correlation between P_N and GR₀. These positive correlations suggest a direct positive relationship between the grain filling rate and grain chlorophyll contents, which is indicative of the high photosynthetic capacity of the grains during the early grain filling period. These results suggest that the grain chlorophyll contents could be used as a molecular marker in marker-assisted breeding programs for rice cultivars with high grain net photosynthetic capacity during the early period of grain filling to improve grain yield.

Over-Expressing TaSPA-B Reduces Prolamin and Starch Accumulation in Wheat (Triticum aestivum L.) Grains

Starch and prolamin composition and content are important indexes for determining the processing and nutritional quality of wheat (Triticum aestivum L.) grains. Several transcription factors (TFs) regulate gene expression during starch and protein biosynthesis in wheat. Storage protein activator (TaSPA), a member of the basic leucine zipper (bZIP) family, has been reported to activate glutenin genes and is correlated to starch synthesis related genes. In this study, we generated TaSPA-B overexpressing (OE) transgenic wheat lines. Compared with wild-type (WT) plants, the starch content was slightly reduced and starch granules exhibited a more polarized distribution in the TaSPA-B OE lines. Moreover, glutenin and ω- gliadin contents were significantly reduced, with lower expression levels of related genes (e.g., By15, Dx2, and ω-1,2 gliadin gene). RNA-seq analysis identified 2023 differentially expressed genes (DEGs). The low expression of some DEGs (e.g., SUSase, ADPase, Pho1, Waxy, SBE, SSI, and SS II a) might explain the reduction of starch contents. Some TFs involved in glutenin and starch synthesis might be regulated by TaSPA-B, for example, TaPBF was reduced in TaSPA-B OE-3 lines. In addition, dual-luciferase reporter assay indicated that both TaSPA-B and TaPBF could transactivate the promoter of ω-1,2 gliadin gene. These results suggest that TaSPA-B regulates a complex gene network and plays an important role in starch and protein biosynthesis in wheat.

Overexpression of a wheat (Triticum aestivum L.) bZIP transcription factor gene, TabZIP6, decreased the freezing tolerance of transgenic Arabidopsis seedlings by down-regulating the expression of CBFs

The basic leucine zipper (bZIP) proteins play important roles against abiotic stress in plants, including cold stress. However, most bZIPs involved in plant freezing tolerance are positive regulators. Only a few bZIPs function negatively in cold stress response. In this study, TabZIP6, a Group C bZIP transcription factor gene from common wheat (Triticum aestivum L.), was cloned and characterized. The transcript of TabZIP6 was strongly induced by cold treatment (4 °C). TabZIP6 is a nuclear-localized protein with transcriptional activation activity. Arabidopsis plants overexpressing TabZIP6 showed decreased tolerance to freezing stress. Microarray as well as quantitative real-time PCR (qRT-PCR) analysis showed that CBFs and some key COR genes, including COR47 and COR15B, were down-regulated by cold treatment in TabZIP6-overexpressing Arabidopsis lines. TabZIP6 was capable of binding to the G-box motif and the CBF1 and CBF3 promoters in yeast cells. A yeast two-hybrid assay revealed that TabZIP6, as well as the other two Group S bZIP proteins involved in cold stress tolerance in wheat, Wlip19 and TaOBF1, can form homodimers by themselves and heterodimers with each other. These results suggest that TabZIP6 may function negatively in the cold stress response by binding to the promoters of CBFs, and thereby decreasing the expression of downstream COR genes in TabZIP6-overexpressing Arabidopsis seedlings.