A Semi-Dominant Mutation in OsCESA9 Improves Salt Tolerance and Favors Field Straw Decay Traits by Altering Cell Wall Properties in Rice

OsWRKY12 negatively regulates the drought-stress tolerance and secondary cell wall biosynthesis by targeting different downstream transcription factor genes in rice

With the increasing occurrence of global warming, drought is becoming a major constraint for plant growth and crop yield. Plant cell walls experience continuous changes during the growth, development, and in responding to stressful conditions. The plant WRKYs play pivotal roles in regulating the secondary cell wall (SCW) biosynthesis and helping plant defend against abiotic stresses. qRT-PCR evidence showed that OsWRKY12 was affected by drought and ABA treatments. Over-expression of OsWRKY12 decreased the drought tolerance of the rice transgenics at the germination stage and the seedling stage. The transcription levels of drought-stress-associated genes as well as those genes participating in the ABA biosynthesis and signaling were significantly different compared to the wild type (WT). Our results also showed that less lignin and cellulose were deposited in the OsWRKY12-overexpressors, and heterogenous expression of OsWRKY12 in atwrky12 could lower the increased lignin and cellulose contents, as well as the improved PEG-stress tolerance, to a similar level as the WT. qRT-PCR results indicated that the transcription levels of all the genes related to lignin and cellulose biosynthesis were significantly decreased in the rice transgenics than the WT. Further evidence from yeast one-hybrid assay and the dual-luciferase reporter system suggested that OsWRKY12 could bind to promoters of OsABI5 (the critical component of the ABA signaling pathway) and OsSWN3/OsSWN7 (the key positive regulators in the rice SCW thickening), and hence repressing their expression. In conclusion, OsWRKY12 mediates the crosstalk between SCW biosynthesis and plant stress tolerance by binding to the promoters of different downstream genes.

Research Progress on Mechanical Strength of Rice Stalks

As one of the most important food crops in the world, rice yield is directly related to national food security. Lodging is one of the most important factors restricting rice production, and the cultivation of rice varieties with lodging resistance is of great significance in rice breeding. The lodging resistance of rice is directly related to the mechanical strength of the stalks. In this paper, we reviewed the cell wall structure, its components, and its genetic regulatory mechanism, which improved the regulatory network of rice stalk mechanical strength. Meanwhile, we analyzed the new progress in genetic breeding and put forward some scientific problems that need to be solved in this field in order to provide theoretical support for the improvement and application of rice breeding.

Vegetative cell wall protein OsGP1 regulates cell wall mediated soda saline-alkali stress in rice

Plant growth and development are inhibited by the high levels of ions and pH due to soda saline-alkali soil, and the cell wall serves as a crucial barrier against external stresses in plant cells. Proteins in the cell wall play important roles in plant cell growth, morphogenesis, pathogen infection and environmental response. In the current study, the full-length coding sequence of the vegetative cell wall protein gene OsGP1 was characterized from Lj11 (Oryza sativa longjing11), it contained 660 bp nucleotides encoding 219 amino acids. Protein-protein interaction network analysis revealed possible interaction between CESA1, TUBB8, and OsJ_01535 proteins, which are related to plant growth and cell wall synthesis. OsGP1 was found to be localized in the cell membrane and cell wall. Furthermore, overexpression of OsGP1 leads to increase in plant height and fresh weight, showing enhanced resistance to saline-alkali stress. The ROS (reactive oxygen species) scavengers were regulated by OsGP1 protein, peroxidase and superoxide dismutase activities were significantly higher, while malondialdehyde was lower in the overexpression line under stress. These results suggest that OsGP1 improves saline-alkali stress tolerance of rice possibly through cell wall-mediated intracellular environmental homeostasis.

A Tos17 transposon insertion in OsCesA9 causes brittle culm in rice

The mechanical strength of rice stalks plays an important role in rice growth and development. In this study, the brittle culm mutant bc26 was identified from a tissue culture-derived line of the japonica rice cultivar 'ZH15'. bc26 plants showed fragile leaves and stalks and reduced plant height and spike number compared with wild-type 'ZH15'. Analysis of cell wall components revealed that the cellulose content of stems, leaves, roots and spikes of bc26 plants was significantly lower than that of the wild type, while the hemicellulose content in these tissues of bc26 plants was significantly higher than that of the wild type. Further sequencing using a mixed pool bulked segregant analysis sequencing (BSA-Seq) of brittle plants from the F2 populations localized the bc26 gene to chromosome 9, and a Tos17 transposon insertion in the bc6 gene near the highest SNP-index point was associated with a loss of gene function. Therefore, the bc26 gene was tentatively identified as a new allele of the Bc6 gene, resulting in the brittle culm trait caused by the insertion of the Tos17 transposon.

Modification of lignin composition by ectopic expressing wheat TaF5H1 led to decreased salt tolerance in transgenic Arabidopsis plants

Lignin is an important cell wall component that provides plants with mechanical support and improved tolerance to pathogen attacks. Previous studies have shown that plants rich in S-lignin content or with a higher S/G ratio always exhibit higher efficiency in the utilization of lignocellulosic biomass. Ferulate 5-hydroxylase, or coniferaldehyde 5-hydroxylase (F5H, or CAld5H), is the critical enzyme in syringyl lignin biosynthesis. Some F5Hs have been characterized in several plant species, e.g., Arabidopsis, rice, and poplar. However, information about F5Hs in wheat remains unclear. In this study, a wheat F5H gene, TaF5H1, together with its native promoter (pTaF5H1), was functionally characterized in transgenic Arabidopsis. Gus staining results showed that TaF5H1 could be expressed predominantly in the highly lignified tissues in transgenic Arabidopsis plants carrying pTaF5H1:Gus. qRT-PCR results showed that TaF5H1 was significantly inhibited by NaCl treatment. Ectopic expression of TaF5H1 driven by pTaF5H1 (i.e., pTaF5H1:TaF5H1) could increase the biomass yield, S-lignin content, and S/G ratio in transgenic Arabidopsis plants, which could also restore the traces of S-lignin in fah1-2, the Arabidopsis F5H mutant, to an even higher level than the wild type (WT), suggesting that TaF5H1 is a critical enzyme in S lignin biosynthesis, and pTaF5H1:TaF5H1 module has potential in the manipulation of S-lignin composition without any compromise on the biomass yield. However, expression of pTaF5H1:TaF5H1 also led to decreased salt tolerance compared with the WT. RNA-seq analysis showed that many stress-responsive genes and genes responsible for the biosynthesis of cell walls were differentially expressed between the seedlings harboring pTaF5H1:TaF5H1 and the WT, hinting that manipulation of the cell wall components targeting F5H may also affect the stress adaptability of the modified plants due to the interference to the cell wall integrity. In summary, this study demonstrated that the wheat pTaF5H1: TaF5H1 cassette has the potential to modulate S-lignin composition without any compromise in biomass yield in future engineering practice. Still, its negative effect on stress adaptability to transgenic plants should also be considered.

The Fragile culm19 (FC19) mutation largely improves plant lodging resistance, biomass saccharification, and cadmium resistance by remodeling cell walls in rice

Cell wall is essential for plant upright growth, biomass saccharification, and stress resistance. Although cell wall modification is suggested as an effective means to increase biomass saccharification, it is a challenge to maintain normal plant growth with improved mechanical strength and stress resistance. Here, we reported two independent fragile culm mutants, fc19-1 and fc19-2, resulting from novel mutations of OsIRX10, produced by the CRISPR/Cas9 system. Compared to wild-type, the two mutants exhibited reduced contents of xylose, hemicellulose, and cellulose, and increased arabinose and lignin without significant alteration in levels of pectin and uronic acids. Despite brittleness, the mutants displayed increased breaking force, leading to improved lodging resistance. Furthermore, the altered cell wall and increased biomass porosity in fc19 largely increased biomass saccharification. Notably, the mutants showed enhanced cadmium (Cd) resistance with lower Cd accumulation in roots and shoots. The FC19 mutation impacts transcriptional levels of key genes contributing to Cd uptake, sequestration, and translocation. Moreover, transcriptome analysis revealed that the FC19 mutation resulted in alterations of genes mainly involved in carbohydrate and phenylpropanoid metabolism. Therefore, a hypothetic model was proposed to elucidate that the FC19 mutation-mediated cell wall remodeling leads to improvements in lodging resistance, biomass saccharification, and Cd resistance.

CEF3 is involved in membrane trafficking and essential for secondary cell wall biosynthesis and its mutation enhanced biomass enzymatic saccharification in rice

Background

As one of the most important staple food crops, rice produces large of agronomic biomass residues that contain lots of secondary cell walls (SCWs). Membrane trafficking plays key roles in SCWs biosynthesis, but information association membrane trafficking and SCWs formation in plants is limited.

Results

In this study, we report the function characterization of a rice mutant, culm easily fragile 3 (cef3), that exhibits growth retardation and fragile culm phenotype with significantly altered cell wall composition and reduced secondary wall thickness. Map-based cloning revealed that CEF3 encodes a homologous protein of Arabidopsis STOMATAL CYTOKINESIS DEFECTIVE2 (SCD2). The saccharification assays revealed that CEF3 mutation can improve biomass enzymatic saccharification. Expression pattern analysis indicated that CEF3 is ubiquitously expressed in many organs at different developmental stages. Subcellular localization revealed that CEF3 is a Golgi-localized protein. The FM4-64 uptake assay revealed CEF3 is involved in endocytosis. Furthermore, mutation of CEF3 not only affected cellulose synthesis-related genes expression, but also altered the abundance of cellulose synthase catalytic subunit 9 (OsCESA9) in the PM and in the endomembrane systems.

Conclusions

This study has demonstrated that CEF3 participates in the membrane trafficking that is essential for normal cellulose and other polysaccharides biosynthesis of the secondary cell wall, thereby manipulation of CEF3 could alter cellulose content and enhance biomass enzymatic saccharification in rice plants. Therefore, the study of the function of CEF3 can provide a strategy for genetic modification of SCWs in bioenergy crops.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13068-022-02205-y.

Identification and Fine Mapping of the Recessive Gene BK-5, Which Affects Cell Wall Biosynthesis and Plant Brittleness in Maize

The cellulose of the plant cell wall indirectly affects the cell shape and straw stiffness of the plant. Here, the novel brittleness mutant brittle stalk-5 (bk-5) of the maize inbred line RP125 was characterized. We found that the mutant displayed brittleness of the stalk and even the whole plant, and that the brittleness phenotype existed during the whole growth period from germination to senescence. The compressive strength was reduced, the cell wall was thinner, and the cellulose content was decreased compared to that of the wild type. Genetic analysis and map-based cloning indicated that bk-5 was controlled by a single recessive nuclear gene and that it was located in a 90.2-Kb region on chromosome 3 that covers three open reading frames (ORFs). Sequence analysis revealed a single non-synonymous missense mutation, T-to-A, in the last exon of Zm00001d043477 (B73: version 4, named BK-5) that caused the 951th amino acid to go from leucine to histidine. BK-5 encodes a cellulose synthase catalytic subunit (CesA), which is involved with cellulose synthesis. We found that BK-5 was constitutively expressed in all tissues of the germinating stage and silking stage, and highly expressed in the leaf, auricula, and root of the silking stage and the 2-cm root and bud of the germinating stage. We found that BK-5 mainly localized to the Golgi apparatus, suggesting that the protein might move to the plasma membrane with the aid of Golgi in maize. According to RNA-seq data, bk-5 had more downregulated genes than upregulated genes, and many of the downregulated genes were enzymes and transcription factors related to cellulose, hemicellulose, and lignin biosynthesis of the secondary cell wall. The other differentially expressed genes were related to metabolic and cellular processes, and were significantly enriched in hormone signal transduction, starch and sucrose metabolism, and the plant-pathogen interaction pathway. Taken together, we propose that the mutation of gene BK-5 causes the brittle stalk phenotype and provides important insights into the regulatory mechanism of cellulose biosynthesis and cell wall development in maize.

Updating Insights into the Catalytic Domain Properties of Plant Cellulose synthase (CesA) and Cellulose synthase-like (Csl) Proteins

The wall is the last frontier of a plant cell involved in modulating growth, development and defense against biotic stresses. Cellulose and additional polysaccharides of plant cell walls are the most abundant biopolymers on earth, having increased in economic value and thereby attracted significant interest in biotechnology. Cellulose biosynthesis constitutes a highly complicated process relying on the formation of cellulose synthase complexes. Cellulose synthase (CesA) and Cellulose synthase-like (Csl) genes encode enzymes that synthesize cellulose and most hemicellulosic polysaccharides. Arabidopsis and rice are invaluable genetic models and reliable representatives of land plants to comprehend cell wall synthesis. During the past two decades, enormous research progress has been made to understand the mechanisms of cellulose synthesis and construction of the plant cell wall. A plethora of cesa and csl mutants have been characterized, providing functional insights into individual protein isoforms. Recent structural studies have uncovered the mode of CesA assembly and the dynamics of cellulose production. Genetics and structural biology have generated new knowledge and have accelerated the pace of discovery in this field, ultimately opening perspectives towards cellulose synthesis manipulation. This review provides an overview of the major breakthroughs gathering previous and recent genetic and structural advancements, focusing on the function of CesA and Csl catalytic domain in plants.