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• Wheat is the most widely grown crop globally, providing 20% of all human calories and protein. Achieving step changes in genetic yield potential is crucial to ensure food security, but efforts are thwarted by an apparent trade-off between grain size and number. Expansins are proteins that play important roles in plant growth by enhancing stress relaxation in the cell wall, which constrains cell expansion. • Here, we describe how targeted overexpression of an α-expansin in early developing wheat seeds leads to a significant increase in grain size without a negative effect on grain number, resulting in a yield boost under field conditions. • The best-performing transgenic line yielded 12.3% higher average grain weight than the control, and this translated to an increase in grain yield of 11.3% in field experiments using an agronomically appropriate plant density. • This targeted transgenic approach provides an opportunity to overcome a common bottleneck to yield improvement across many crops.

Expansins are proteins without catalytic activity, but able to break hydrogen bonds between cell wall polysaccharides hemicellulose and cellulose. This proteins were reported for the first time in 1992, describing cell wall extension in cucumber hypocotyls caused particularly by alpha-expansins. Although these proteins have GH45 and CBM63 domains, characteristic of enzymes related with the cleavage of cell wall polysaccharides, demonstrating in vitro that they extend plant cell wall. Its participation has been associated to molecular processes such as development and growing, fruit ripening and softening, tolerance and resistance to biotic and abiotic stress and seed germination. Structural insights, facilitated by bioinformatics approaches, are highlighted, shedding light on the intricate interactions between alpha-expansins and cell wall polysaccharides. After more than thirty years of its discovery, we want to celebrate the knowledge of alpha-expansins and emphasize their importance to understand the phenomena of disassembly and loosening of the cell wall, specifically in the fruit ripening phenomena, with this state-of-the-art dedicated to them.Key messageAlpha-expansin are key proteins in fruit development and ripening. Bioinformatics strategies have allowed us to understand the molecular mechanisms behind the action of these proteins.

Background Microbial expansins (EXLXs) are non-lytic proteins homologous to plant expansins involved in plant cell wall formation. Due to their non-lytic cell wall loosening properties and potential to disaggregate cellulosic structures, there is considerable interest in exploring the ability of microbial expansins (EXLX) to assist the processing of cellulosic biomass for broader biotechnological applications. Herein, EXLXs with different modular structure and from diverse phylogenetic origin were compared in terms of ability to bind cellulosic, xylosic, and chitinous substrates, to structurally modify cellulosic fibrils, and to boost enzymatic deconstruction of hardwood pulp. Results Five heterogeneously produced EXLXs ( Clavibacter michiganensis; Cmi EXLX2, Dickeya aquatica; Daq EXLX1, Xanthomonas sacchari; Xsa EXLX1, Nothophytophthora sp.; Nsp EXLX1 and Phytophthora cactorum; Pca EXLX1) were shown to bind xylan and hardwood pulp at pH 5.5 and Cmi EXLX2 (harboring a family-2 carbohydrate-binding module) also bound well to crystalline cellulose. Small-angle X-ray scattering revealed a 20–25% increase in interfibrillar distance between neighboring cellulose microfibrils following treatment with Cmi EXLX2, Daq EXLX1, or Nsp EXLX1. Correspondingly, combining xylanase with Cmi EXLX2 and Daq EXLX1 increased product yield from hardwood pulp by ~ 25%, while supplementing the Tr AA9A LPMO from Trichoderma reesei with Cmi EXLX2, Daq EXLX1, and Nsp EXLX1 increased total product yield by over 35%. Conclusion This direct comparison of diverse EXLXs revealed consistent impacts on interfibrillar spacing of cellulose microfibers and performance of carbohydrate-active enzymes predicted to act on fiber surfaces. These findings uncover new possibilities to employ EXLXs in the creation of value-added materials from cellulosic biomass.

Plant-specific expansins were first discovered as pH-dependent cell-wall-loosening proteins involved in diverse physiological processes. No comprehensive analysis of the expansin gene family has yet been carried out at the whole genome level in Chinese jujube (Ziziphus jujuba Mill.). In this study, 30 expansin genes were identified in the jujube genome. These genes, which were distributed with varying densities across 10 of the 12 jujube chromosomes, could be divided into four subfamilies: 19 ZjEXPAs, 3 ZjEXPBs, 1 ZjEXLA, and 7 ZjEXLBs. Phylogenetic analysis of expansin genes in Arabidopsis, rice, apple, grape, and jujube classified these genes into 17 subgroups. Members of the same subfamily and subgroup shared conserved gene structure and motif compositions. Homology analysis identified 20 homologous gene pairs between jujube and Arabidopsis. Further analysis of ZjEXP gene promoter regions uncovered various growth, development and stress-responsive cis-acting elements. Expression analysis and transcript profiling revealed that ZjEXPs had different expression patterns in different tissues at various developmental stages. ZjEXPA4 and ZjEXPA6 were highly expressed in young fruits, ZjEXPA3 and ZjEXPA5 were significantly expressed in flowers, and ZjEXPA7 was specifically expressed in young leaves. The results of this study, the first systematic analysis of the jujube expansin gene family, can serve as a strong foundation for further elucidation of the physiological functions and biological roles of jujube expansin genes.

The growth of plant cells is inseparable from relaxation and expansion of cell walls. Expansins are a class of cell wall binding proteins, which play important roles in the relaxation of cell walls. Although there are many members in expansin gene family, the functions of most expansin genes in plant growth and development are still poorly understood. In this study, the functions of two expansin genes, AtEXPA4 and AtEXPB5 were characterized in Arabidopsis thaliana. AtEXPA4 and AtEXPB5 displayed consistent expression patterns in mature pollen grains and pollen tubes, but AtEXPA4 also showed a high expression level in primary roots. Two single mutants, atexpa4 and atexpb5, showed normal reproductive development, whereas atexpa4atexpb5 double mutant was defective in pollen tube growth. Moreover, AtEXPA4 overexpression enhanced primary root elongation, on the contrary, knocking out AtEXPA4 made the growth of primary root slower. Our results indicated that AtEXPA4 and AtEXPB5 were redundantly involved in pollen tube growth and AtEXPA4 was required for primary root elongation.

• Plants must rearrange the network of complex carbohydrates in their cell walls during normal growth and development. To accomplish this, all plants depend on proteins called expansins that nonenzymatically loosen noncovalent bonding between cellulose microfibrils. • Surprisingly, expansin genes have more recently been found in some bacteria and microbial eukaryotes, where their biological functions are largely unknown. • Here, we reconstruct a comprehensive phylogeny of microbial expansin genes. We find these genes in all eukaryotic microorganisms that have structural cell wall cellulose, suggesting expansins evolved in ancient marine microorganisms long before the evolution of land plants. We also find expansins in an unexpectedly high diversity of bacteria and fungi that do not have cellulosic cell walls. These bacteria and fungi inhabit varied ecological contexts, mirroring the diversity of terrestrial and aquatic niches where plant and/or algal cellulosic cell walls are present. • The microbial expansin phylogeny shows evidence of multiple horizontal gene transfer events within and between bacterial and eukaryotic microbial lineages, which may in part underlie their unusually broad phylogenetic distribution. Overall, expansins are unexpectedly widespread in bacteria and eukaryotes, and the contribution of these genes to microbial ecological interactions with plants and algae has probbaly been underappreciated.

Reducing the enzyme loadings for enzymatic saccharification of lignocellulose is required for economically feasible production of biofuels and biochemicals. One strategy is addition of small amounts of synergistic proteins to cellulase mixtures. Synergistic proteins increase the activity of cellulase without causing significant hydrolysis of cellulose. Synergistic proteins exert their activity by inducing structural modifications in cellulose. Recently, synergistic proteins from various biological sources, including bacteria, fungi, and plants, were identified based on genomic data, and their synergistic activities were investigated. Currently, an up-to-date overview of several aspects of synergistic proteins, such as their functions, action mechanisms and synergistic activity, are important for future industrial application. In this review, we summarize the current state of research on four synergistic proteins: carbohydrate-binding modules, plant expansins, expansin-like proteins, and Auxiliary Activity family 9 (formerly GH61) proteins. This review provides critical information to aid in promoting research on the development of efficient and industrially feasible synergistic proteins.

The discovery of microbial expansins emerged from studies of the mechanism of plant cell growth and the molecular basis of plant cell wall extensibility. Expansins are wall-loosening proteins that are universal in the plant kingdom and are also found in a small set of phylogenetically diverse bacteria, fungi, and other organisms, most of which colonize plant surfaces. They loosen plant cell walls without detectable lytic activity. Bacterial expansins have attracted considerable attention recently for their potential use in cellulosic biomass conversion for biofuel production, as a means to disaggregate cellulosic structures by nonlytic means (“amorphogenesis”). Evolutionary analysis indicates that microbial expansins originated by multiple horizontal gene transfers from plants. Crystallographic analysis of BsEXLX1, the expansin from Bacillus subtilis, shows that microbial expansins consist of two tightly packed domains: the N-terminal domain D1 has a double-ψ β-barrel fold similar to glycosyl hydrolase family-45 enzymes but lacks catalytic residues usually required for hydrolysis; the C-terminal domain D2 has a unique β-sandwich fold with three co-linear aromatic residues that bind β-1,4-glucans by hydrophobic interactions. Genetic deletion of expansin in Bacillus and Clavibacter cripples their ability to colonize plant tissues. We assess reports that expansin addition enhances cellulose breakdown by cellulase and compare expansins with distantly related proteins named swollenin, cerato-platanin, and loosenin. We end in a speculative vein about the biological roles of microbial expansins and their potential applications. Advances in this field will be aided by a deeper understanding of how these proteins modify cellulosic structures.

Background Expansins (EXPs) are important components of the plant cell wall. They are involved in plant growth and development and diverse environmental stress responses by promoting cell-wall loosening and cell enlargement. Although EXPs have been characterized in many plant species, little is known about the EXPs in Carya illinoinensis . Methods The CiEXP gene family was systematically analyzed using bioinformatics. RNA-seq data (both from our study and public databases) and qRT-PCR were employed to analyze the expression patterns of the CiEXP family in different tissues, under biotic and abiotic stress, and in persistent versus abscised fruit. Results In this study, a total of 39 EXP genes unevenly distributed on 14 chromosomes were identified in the C. illinoinensis genome, which were classified into four subfamilies (27 CiEXPAs, 3 CiEXPBs, 2 CiEXLAs, and 7 CiEXLBs), and the motif and gene structures were consistent with this subfamily classification. Thirty-six pairs of duplicated genes were identified, suggesting that gene duplication may have contributed to the expansion of the EXP gene family. Collinearity analysis provided further phylogenetic insights into the EXP gene family. Cis -acting element analysis revealed that the promoter regions of the CiEXPs gene were associated with hormone-responsive, plant growth and development, and stress-responsive, particularly ABA response element (ABRE) and MeJA-responsive element. The expression results showed that most CiEXPs exhibited tissue-specific expression patterns, and some CiEXPs were highly responsive to abiotic and biotic stresses. Additionally, most CiEXPA genes, CiEXPB1/2 , and CiEXLA2 were up-regulated in persistent fruit. Conclusions Our study findings enhance the understanding of the CiEXP gene family and facilitate the selection of suitable candidate genes for further study, which lays a foundation for future investigations into the functional roles of specific CiEXPs .

Key message VviWOX13C plays a key regulatory role in the expansin during fruit set. Expansins as a type of non-enzymatic cell wall proteins, are responsible for the loosening and extension in cell walls leading to the enlargement of the plant cells. However, the current studies are still lacking in expansin genes associated with promoting fruit set. Here, 29 members of the expansin gene family were identified in the whole genome of grapes ( Vitis vinifera L.), and the functional prediction of expansins was based on the gene annotated information. Results showed that the 29 members of grape expansin gene family could be mainly divided into four subfamilies (EXPA, EXPB, LIKE A, and LIKE B), distributed on 16 chromosomes. Replication analysis showed that there were four segmental duplications and two tandem duplications. Each expansins sequence contained two conserved domain features of grape EXPs (DPBB_1 and Expansin_C) through protein sequence analysis. The transcriptome sequencing results revealed that VviEXPA37 , VviEXPA38 , and VviEXPA39 were induced and upregulated by CPPU. Furthermore, transcriptional regulatory prediction network indicated that VviWOX13C targeted regulates VviEXPA37 , VviEXPA38 , and VviEXPA39 simultaneously. EMSA and dual luciferase assays demonstrated that VviWOX13C directly activated the expression of VviEXPA37 , VviEXPA38 , and VviEXPA39 by directly binding to its promoter. These results provide a basis for further studies on the function and regulatory mechanisms of expansin genes in fruit set.