A DE1 BINDING FACTOR 1-GLABRA2 module regulates rhamnogalacturonan I biosynthesis in Arabidopsis seed coat mucilage

Brassica napus BnaA09.MYB52 enhances seed coat mucilage accumulation and tolerance to osmotic stress during seed germination in Arabidopsis thaliana

Seed coat mucilage plays an important role in promoting seed germination under adversity. Previous studies have shown that Arabidopsis thaliana MYB52 (AtMYB52) can positively regulate seed coat mucilage accumulation. However, the role of Brassica napus MYB52 (BnaMYB52) in accumulation of seed coat mucilage and tolerance to osmotic stress during seed germination remains largely unknown. We cloned the BnaA09.MYB52 coding domain sequence from B. napus cv ZS11, identified its conserved protein domains and elucidated its relationship with homologues from a range of plant species. Transgenic plants overexpressing BnaA09.MYB52 in the A. thaliana myb52-1 mutant were generated through Agrobacterium-mediated transformation and used to assess the possible roles of BnaA09.MYB52 in accumulation of seed coat mucilage and tolerance to osmotic stress during seed germination. Subcellular localization and transcriptional activity assays demonstrated that BnaA09.MYB52 functions as a transcription factor. RT-qPCR results indicate that BnaA09.MYB52 is predominantly expressed in roots and developing seeds of B. napus cv ZS11. Introduction of BnaA09.MYB52 into myb52-1 restored thinner seed coat mucilage in this mutant to levels in the wild type. Consistently, expression levels of three key genes participating in mucilage formation in developing seeds of myb52-1 were also restored to wild type levels by overexpressing BnaA09.MYB52. Furthermore, BnaA09.MYB52 was induced by osmotic stress during seed germination in B. napus, and ectopic expression of BnaA09.MYB52 successfully corrected sensitivity of the myb52-1 mutant to osmotic stress during seed germination. These findings enhance our understanding of the functions of BnaA09.MYB52 and provide a novel strategy for future B. napus breeding.

Antagonistic MADS-box transcription factors SEEDSTICK and SEPALLATA3 form a transcriptional regulatory network that regulates seed oil accumulation

Transcriptional regulation is essential for balancing multiple metabolic pathways that influence oil accumulation in seeds. Thus far, the transcriptional regulatory mechanisms that govern seed oil accumulation remain largely unknown. Here, we identified the transcriptional regulatory network composed of MADS-box transcription factors SEEDSTICK (STK) and SEPALLATA3 (SEP3), which bridges several key genes to regulate oil accumulation in seeds. We found that STK, highly expressed in the developing embryo, positively regulates seed oil accumulation in Arabidopsis (Arabidopsis thaliana). Furthermore, we discovered that SEP3 physically interacts with STK in vivo and in vitro. Seed oil content is increased by the SEP3 mutation, while it is decreased by SEP3 overexpression. The chromatin immunoprecipitation, electrophoretic mobility shift assay, and transient dual-luciferase reporter assays showed that STK positively regulates seed oil accumulation by directly repressing the expression of MYB5, SEP3, and SEED FATTY ACID REDUCER 4 (SFAR4). Moreover, genetic and molecular analyses demonstrated that STK and SEP3 antagonistically regulate seed oil production and that SEP3 weakens the binding ability of STK to MYB5, SEP3, and SFAR4. Additionally, we demonstrated that TRANSPARENT TESTA 8 (TT8) and ACYL-ACYL CARRIER PROTEIN DESATURASE 3 (AAD3) are direct targets of MYB5 during seed oil accumulation in Arabidopsis. Together, our findings provide the transcriptional regulatory network antagonistically orchestrated by STK and SEP3, which fine tunes oil accumulation in seeds.

Morphological and anatomical changes during dormancy break of the seeds of Fritillaria taipaiensis

Fritillaria taipaiensis P. Y. Li is the most suitable species planted at low altitudes among other species used as Tendrilleaf Fritillary Bulb, whose seeds embracing the morphological and physiological dormancy need to experience a long-dormant time from sowing to germination. In this study, the developmental changes of F. taipaiensis seeds during dormancy period were observed by morphological and anatomical observation, and the cause of long-term dormancy of seeds was discussed from the perspective of embryonic development. The process of embryonic organogenesis was revealed during the dormancy stage by the paraffin section. The effects of testa, endosperm and temperature on dormant seeds were discussed. Furthermore, we found that the mainly dormant reason was caused by the morphological dormancy, which accounted for 86% of seed development time. The differentiation time of the globular or pear-shaped embryo into a short-rod embryo was longer, which was one of the chief reasons for the morphological dormancy and played an important role in embryonic formation. Testa and endosperm with mechanical constraint and inhibitors involved in the dormancy of F. taipaiensis seeds. The seeds of F. taipaiensis, the average ambient temperature of 6-12°C for morphological dormancy and 11-22°C for physiological dormancy, were unsuitable for seed growth. Therefore, we suggested that the dormancy time of F. taipaiensis seeds could be shortened by shortening the development time of the proembryo stage and stratification for the different stages of dormancy.

Auxin as an architect of the pectin matrix

Auxin is a versatile plant growth regulator that triggers multiple signalling pathways at different spatial and temporal resolutions. A plant cell is surrounded by the cell wall, a complex and dynamic network of polysaccharides. The cell wall needs to be rigid to provide mechanical support and protection and highly flexible to allow cell growth and shape acquisition. The modification of the pectin components, among other processes, is a mechanism by which auxin activity alters the mechanical properties of the cell wall. Auxin signalling precisely controls the transcriptional output of several genes encoding pectin remodelling enzymes, their local activity, pectin deposition, and modulation in different developmental contexts. This review examines the mechanism of auxin activity in regulating pectin chemistry at organ, cellular, and subcellular levels across diverse plant species. Moreover, we ask questions that remain to be addressed to fully understand the interplay between auxin and pectin in plant growth and development.

Recent advances in polysaccharide biomodification by microbial fermentation: production, properties, bioactivities, and mechanisms

Polysaccharides are natural chemical compounds that are extensively employed in the food and pharmaceutical industries. They exhibit a wide range of physical and biological properties. These properties are commonly improved by using chemical and physical methods. However, with the advancement of biotechnology and increased demand for green, clean, and safe products, polysaccharide modification via microbial fermentation has gained importance in improving their physicochemical and biological activities. The physicochemical and structural characteristics, biological activity, and modification mechanisms of microbially fermented polysaccharides were reviewed and summarized in this study. Polysaccharide modifications were categorized and discussed in terms of strains and fermentation techniques. The effects of microbial fermentation on the physicochemical characteristics of polysaccharides were highlighted. The impact of modification of polysaccharides on their antioxidant, immune, hypoglycemic, and other activities, as well as probiotic digestive enhancement, were also discussed. Finally, we investigated a potential enzyme-based process for polysaccharide modification via microbial fermentation. Modification of polysaccharides via microbial fermentation has significant value and application potential.

MYB-bHLH-TTG1 in a Multi-tiered Pathway Regulates Arabidopsis Seed Coat Mucilage Biosynthesis Genes Including PECTIN METHYLESTERASE INHIBITOR14 Required for Homogalacturonan Demethylesterification

MYB-bHLH-TTG1 (MBW) transcription factor (TF) complexes regulate Arabidopsis seed coat biosynthesis pathways via a multi-tiered regulatory mechanism. The MYB genes include MYB5, MYB23 and TRANSPARENT TESTA2 (TT2), which regulate GLABRA2 (GL2), HOMEODOMAIN GLABROUS2 (HDG2) and TRANSPARENT TESTA GLABRA2 (TTG2). Here, we examine the role of PECTIN METHYLESTERASE INHIBITOR14 (PMEI14) in seed coat mucilage pectin methylesterification and provide evidence in support of multi-tiered regulation of seed coat mucilage biosynthesis genes including PMEI14. The PMEI14 promoter was active in the seed coat and developing embryo. A pmei14 mutant exhibited stronger attachment of the outer layer of seed coat mucilage, increased mucilage homogalacturonan demethylesterification and reduced seed coat radial cell wall thickness, results consistent with decreased PMEI activity giving rise to increased PME activity. Reduced mucilage release from the seeds of myb5, myb23, tt2 and gl2, hdg2, ttg2 triple mutants indicated that HDG2 and MYB23 play minor roles in seed coat mucilage deposition. Chromatin immunoprecipitation analysis found that MYB5, TT8 and seven mucilage pathway structural genes are directly regulated by MYB5. Expression levels of GL2, HDG2, TTG2 and nine mucilage biosynthesis genes including PMEI14 in the combinatorial mutant seeds indicated that these genes are positively regulated by at least two of those six TFs and that TTG1 and TTG2 are major regulators of PMEI14 expression. Our results show that MYB-bHLH-TTG1 complexes regulate mucilage biosynthesis genes, including PMEI14, both directly and indirectly via a three-tiered mechanism involving GL2, HDG2 and TTG2.

The maize ZmVPS23-like protein relocates the nucleotide-binding leucine-rich repeat protein Rp1-D21 to endosomes and suppresses the defense response

Plants often utilize nucleotide-binding leucine-rich repeat (NLR) proteins to perceive pathogen infections and trigger a hypersensitive response (HR). The endosomal sorting complex required for transport (ESCRT) machinery is a conserved multisubunit complex that is essential for the biogenesis of multivesicular bodies and cargo protein sorting. VPS23 is a key component of ESCRT-I and plays important roles in plant development and abiotic stresses. ZmVPS23L, a homolog of VPS23-like in maize (Zea mays), was previously identified as a candidate gene in modulating HR mediated by the autoactive NLR protein Rp1-D21 in different maize populations. Here, we demonstrate that ZmVPS23L suppresses Rp1-D21-mediated HR in maize and Nicotiana benthamiana. Variation in the suppressive effect of HR by different ZmVPS23L alleles was correlated with variation in their expression levels. ZmVPS23 also suppressed Rp1-D21-mediated HR. ZmVPS23L and ZmVPS23 predominantly localized to endosomes, and they physically interacted with the coiled-coil domain of Rp1-D21 and mediated the relocation of Rp1-D21 from the nucleo-cytoplasm to endosomes. In summary, we demonstrate that ZmVPS23L and ZmVPS23 are negative regulators of Rp1-D21-mediated HR, likely by sequestrating Rp1-D21 in endosomes via physical interaction. Our findings reveal the role of ESCRT components in controlling plant NLR-mediated defense responses.

Regulation of seed coat mucilage production and modification in Arabidopsis

The Arabidopsis seed coat mucilage is a polysaccharide-rich matrix synthesized by the seed coat epidermal cells. It is a specialized cell wall mainly composed of three types of polysaccharides (i. e. pectin, hemicellulose, and cellulose), and represents as an ideal model system for plant cell wall research. A large number of genes responsible for the synthesis and modification of cell wall polysaccharides have been identified using this model system. Moreover, a subset of regulators controlling mucilage production and modification have been characterized, and the underlying transcriptional regulatory mechanisms have been elucidated. This substantially contributes to the understanding of the molecular mechanisms underlying mucilage synthesis and modification. In this review, we concisely summarize the various genes and regulators involved in seed coat cell differentiation, mucilage biosynthesis and modification, and secondary cell wall formation. In particular, we put emphasis on the latest knowledge gained regarding the transcriptional regulation of mucilage production, which is composed of a hierarchal cascade with three-layer transcriptional regulators. Collectively, we propose an updated schematic framework of the genetic regulatory network controlling mucilage production and modification in the Arabidopsis mucilage secretory cells.

Coregulation of glutamine synthetase1;2 (GLN1;2) and NADH-dependent glutamate synthase (GLT1) gene expression in Arabidopsis roots in response to ammonium supply

Ammonium absorbed by roots is assimilated into amino acids. The glutamine synthetase/glutamate synthase (glutamine 2-oxoglutarate aminotransferase) (GS/GOGAT) cycle is essential to this biological process. In Arabidopsis thaliana, GLN1;2 and GLT1 are the GS and GOGAT isoenzymes induced in response to ammonium supply and playing key roles in ammonium utilization. Although recent studies suggest gene regulatory networks involved in transcriptional regulation of ammonium-responsive genes, direct regulatory mechanisms for ammonium-induced expression of GS/GOGAT remain unclear. In this study, we revealed that the expression of GLN1;2 and GLT1 in Arabidopsis is not directly induced by ammonium but is regulated by glutamine or post-glutamine metabolites produced by ammonium assimilation. Previously, we identified a promoter region required for ammonium-responsive expression of GLN1;2. In this study, we further dissected the ammonium-responsive region of the GLN1;2 promoter and also performed a deletion analysis of the GLT1 promoter, which led to the identification of a conserved ammonium-responsive region. Yeast one-hybrid screening using the ammonium-responsive region of the GLN1;2 promoter as a decoy sequence revealed a trihelix family transcription factor DF1 that binds to this region. A putative DF1 binding site was also found in the ammonium-responsive region of the GLT1 promoter.

Single-cell RNA-seq reveals fate determination control of an individual fibre cell initiation in cotton (Gossypium hirsutum)

Cotton fibre is a unicellular seed trichome, and lint fibre initials per seed as a factor determines fibre yield. However, the mechanisms controlling fibre initiation from ovule epidermis are not understood well enough. Here, with single-cell RNA sequencing (scRNA-seq), a total of 14 535 cells were identified from cotton ovule outer integument of Xu142_LF line at four developmental stages (1.5, 1, 0.5 days before anthesis and the day of anthesis). Three major cell types, fibre, non-fibre epidermis and outer pigment layer were identified and then verified by RNA in situ hybridization. A comparative analysis on scRNA-seq data between Xu142 and its fibreless mutant Xu142 fl further confirmed fibre cluster definition. The developmental trajectory of fibre cell was reconstructed, and fibre cell was identified differentiated at 1 day before anthesis. Gene regulatory networks at four stages revealed the spatiotemporal pattern of core transcription factors, and MYB25-like and HOX3 were demonstrated played key roles as commanders in fibre differentiation and tip-biased diffuse growth respectively. A model for early development of a single fibre cell was proposed here, which sheds light on further deciphering mechanism of plant trichome and the improvement of cotton fibre yield.

Genome-Wide Characterization and Expression Profiling of the GRAS Gene Family in Salt and Alkali Stresses in Miscanthus sinensis

The GRAS family genes encode plant-specific transcription factors that play important roles in a diverse range of developmental processes and abiotic stress responses. However, the information of GRAS gene family in the bioenergy crop Miscanthus has not been available. Here, we report the genome-wide identification of GRAS gene family in Micanthus sinensis. A total of 123 MsGRAS genes were identified, which were divided into ten subfamilies based on the phylogenetic analysis. The co-linearity analysis revealed that 59 MsGRAS genes experienced segmental duplication, forming 35 paralogous pairs. The expression of six MsGRAS genes in responding to salt, alkali, and mixed salt-alkali stresses was analyzed by transcriptome and real-time quantitative PCR (RT-qPCR) assays. Furthermore, the role of MsGRAS60 in salt and alkali stress response was characterized in transgenic Arabidopsis. The MsGRAS60 overexpression lines exhibited hyposensitivity to abscisic acid (ABA) treatment and resulted in compromised tolerance to salt and alkali stresses, suggesting that MsGRAS60 is a negative regulator of salt and alkali tolerance via an ABA-dependent signaling pathway. The salt and alkali stress-inducible MsGRAS genes identified serve as candidates for the improvement of abiotic stress tolerance in Miscanthus.

Trihelix transcription factors GTL1 and DF1 prevent aberrant root hair formation in an excess nutrient condition

Root hair growth is tuned in response to the environment surrounding plants. While most previous studies focused on the enhancement of root hair growth during nutrient starvation, few studies investigated the root hair response in the presence of excess nutrients. We report that the post-embryonic growth of wild-type Arabidopsis plants is strongly suppressed with increasing nutrient availability, particularly in the case of root hair growth. We further used gene expression profiling to analyze how excess nutrient availability affects root hair growth, and found that RHD6 subfamily genes, which are positive regulators of root hair growth, are downregulated in this condition. However, defects in GTL1 and DF1, which are negative regulators of root hair growth, cause frail and swollen root hairs to form when excess nutrients are supplied. Additionally, we observed that the RHD6 subfamily genes are mis-expressed in gtl1-1 df1-1. Furthermore, overexpression of RSL4, an RHD6 subfamily gene, induces swollen root hairs in the face of a nutrient overload, while mutation of RSL4 in gtl1-1 df1-1 restore root hair swelling phenotype. In conclusion, our data suggest that GTL1 and DF1 prevent unnecessary root hair formation by repressing RSL4 under excess nutrient conditions.

Pectin methyltransferase QUASIMODO2 functions in the formation of seed coat mucilage in Arabidopsis

Pectin, cellulose, and hemicelluloses are major components of primary cell walls in plants. In addition to cell adhesion and expansion, pectin plays a central role in seed mucilage. Seed mucilage contains abundant pectic rhamnogalacturonan-I (RG-I) and lower amounts of homogalacturonan (HG), cellulose, and hemicelluloses. Previously, accumulated evidence has addressed the role of pectin RG-I in mucilage production and adherence. However, less is known about the function of pectin HG in seed coat mucilage formation. In this study, we analyzed a novel mutant, designated things fall apart2 (tfa2), which contains a mutation in HG methyltransferase QUASIMODO2 (QUA2). Etiolated tfa2 seedlings display short hypocotyls and adhesion defects similar to qua2 and tumorous shoot development2 (tsd2) alleles, and show seed mucilage defects. The diminished uronic acid content and methylesterification degree of HG in mutant seed mucilage indicate the role of HG in the formation of seed mucilage. Cellulosic rays in mutant mucilage are collapsed. The epidermal cells of seed coat in tfa2 and tsd2 display deformed columellae and reduced radial wall thickness. Under polyethylene glycol treatment, seeds from these three mutant alleles exhibit reduced germination rates. Together, these data emphasize the requirement of pectic HG biosynthesis for the synthesis of seed mucilage, and the functions of different pectin domains together with cellulose in regulating its formation, expansion, and release.

Rhamnogalacturonan-I forms mucilage: behind its simplicity, a cutting-edge organization

Zhang Y, Yin Q, Qin W, Gao H, Du J, Chen J, Li H, Zhou G, Wu H, Wu A-M. 2022. The Class II KNOX family members KNAT3 and KNAT7 redundantly participate in Arabidopsis seed coat mucilage biosynthesis. Journal of Experimental Botany 73, 3477–3495.