Alpha-expansins: more than three decades of wall creep and loosening in fruits

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.

Functional characterization of MaEXPA11 and its roles in response to biotic and abiotic stresses in mulberry

Mulberry is a traditional economic tree with various values in sericulture, ecology, food industry and medicine. Expansins (EXPs) are known as cell wall expansion related proteins and have been characterized to involve in plant development and responses to diverse stresses. In present study, twenty EXP and expansin-like (EXL) genes were identified in mulberry. RNA-seq results indicated that three EXP and EXL genes showed up-regulated expression level under sclerotiniose pathogen infection in three independent RNA-seq datasets. The most significant upregulated EXPA11 was selected as key EXP involving in response to sclerotiniose pathogen infection in mulberry. Furthermore, a comprehensive functional analysis was performed to reveal subcellular location, tissue expression profile of MaEXPA11 in mulberry. Down-regulation of MaEXPA11 using virus induced gene silence (VIGS) was performed to explore the function of MaEXPA11 in Morus alba. Results showed that MaEXPA11 can positively regulate mulberry resistance to Ciboria shiraiana infection and negatively regulate mulberry resistance to cold or drought stress.

Genome-Wide Identification of Expansin Gene Family and Their Response under Hormone Exposure in Ginkgo biloba L

Expansins are pH-dependent enzymatic proteins that irreversibly and continuously facilitate cell-wall loosening and extension. The identification and comprehensive analysis of Ginkgo biloba expansins (GbEXPs) are still lacking. Here, we identified and investigated 46 GbEXPs in Ginkgo biloba. All GbEXPs were grouped into four subgroups based on phylogeny. GbEXPA31 was cloned and subjected to a subcellular localization assay to verify our identification. The conserved motifs, gene organization, cis-elements, and Gene Ontology (GO) annotation were predicted to better understand the functional characteristics of GbEXPs. The collinearity test indicated segmental duplication dominated the expansion of the GbEXPA subgroup, and seven paralogous pairs underwent strong positive selection during expansion. A majority of GbEXPAs were mainly expressed in developing Ginkgo kernels or fruits in transcriptome and real-time quantitative PCR (qRT-PCR). Furthermore, GbEXLA4, GbEXLA5, GbEXPA5, GbEXPA6, GbEXPA8, and GbEXPA24 were inhibited under the exposure of abiotic stresses (UV-B and drought) and plant hormones (ABA, SA, and BR). In general, this study expanded our understanding for expansins in Ginkgo tissues' growth and development and provided a new basis for studying GbEXPs in response to exogenous phytohormones.

Changes in the morphology traits, anatomical structure of the leaves and transcriptome in Lycium barbarum L. under salt stress

Salt stress directly affects the growth of plants. The limitation of leaf grow is among the earliest visible effects of salt stress. However, the regulation mechanism of salt treatments on leaf shape has not been fully elucidated. We measured the morphological traits and anatomical structure. In combination with transcriptome analysis, we analyzed differentially expressed genes (DEGs) and verified the RNA-seq data by qRT-PCR. Finally, we analyzed correlation between leaf microstructure parameters and expansin genes. We show that the leaf thickness, the width, and the leaf length significantly increased at elevated salt concentrations after salt stress for 7 days. Low salt mainly promoted the increase in leaves length and width, but high salt concentration accelerated the leaf thickness. The anatomical structure results indicated that palisade mesophyll tissues contribute more to leaf thickness than spongy mesophyll tissues, which possibly contributed to the increase in leaf expansion and thickness. Moreover, a total of 3,572 DEGs were identified by RNA-seq. Notably, six of the DEGs among 92 identified genes concentrated on cell wall synthesis or modification were involved in cell wall loosening proteins. More importantly, we demonstrated that there was a strong positive correlation between the upregulated EXLA2 gene and the thickness of the palisade tissue in L. barbarum leaves. These results suggested that salt stress possibly induced the expression of EXLA2 gene, which in turn increased the thickness of L. barbarum leaves by promoting the longitudinal expansion of cells of the palisade tissue. This study lays a solid knowledge for revealing the underlying molecular mechanisms of leaf thickening in L. barbarum in response to salt stresses.

Overexpression of AcEXPA23 Promotes Lateral Root Development in Kiwifruit

Kiwifruit is loved by consumers for its unique taste and rich vitamin C content. Kiwifruit are very sensitive to adverse soil environments owing to fleshy and shallow roots, which limits the uptake of water and nutrients into the root system, resulting in low yield and poor fruit quality. Lateral roots are the key organs for plants to absorb water and nutrients. Improving water and fertilizer use efficiency by promoting lateral root development is a feasible method to improve yield and quality. Expansin proteins plays a major role in lateral root growth; hence, it is important to identify expansin protein family members, screen key genes, and explore gene function in root development. In this study, 41 expansin genes were identified based on the genome of kiwifruit (‘Hongyang’, Actinidia chinensis). By clustering with the Arabidopsis thaliana expansin protein family, the 41 AcExpansin proteins were divided into four subfamilies. The AcExpansin protein family was further analysed by bioinformatics methods and was shown to be evolutionarily diverse and conserved at the DNA and protein levels. Based on previous transcriptome data and quantitative real-time PCR assays, we screened the candidate gene AcEXPA23. Overexpression of AcEXPA23 in kiwifruit increased the number of kiwifruit lateral roots.

A simple method for the application of exogenous phytohormones to the grass leaf base protodermal zone to improve grass leaf epidermis development research

BACKGROUND

The leaf epidermis functions to prevent the loss of water and reduce gas exchange. As an interface between the plant and its external environment, it helps prevent damage, making it an attractive system for studying cell fate and development. In monocotyledons, the leaf epidermis grows from the basal meristem that contains protodermal cells. Leaf protoderm zone is covered by the leaf sheath or coleoptile in maize and wheat, preventing traditional exogenous phytohormone application methods, such as directly spraying on the leaf surface or indirectly via culture media, from reaching the protoderm areas directly. The lack of a suitable application method limits research on the effect of phytohormone on the development of grass epidermis.

RESULTS

Here, we describe a direct and straightforward method to apply exogenous phytohormones to the leaf protoderms of maize and wheat. We used the auxin analogs 2,4-D and cytokinin analogs 6-BA to test the system. After 2,4-D treatment, the asymmetrical division events and initial stomata development were decreased, and the subsidiary cells were induced in maize, the number of GMC (guard mother cell), SMC (subsidiary mother cell) and young stomata were increased in wheat, and the size of the epidermal cells increased after 6-BA treatment in maize. Thus, the method is suitable for the application of phytohormone to the grass leaf protodermal areas.

CONCLUSIONS

The method to apply hormones to the mesocotyls of maize and wheat seedlings is simple and direct. Only a small amount of externally applied substances are needed to complete the procedure in this method. The entire experimental process lasts for ten days generally, and it is easy to evaluate the phytohormones' effect on the epidermis development.

The diverse roles of cytokinins in regulating leaf development

Leaves provide energy for plants, and consequently for animals, through photosynthesis. Despite their important functions, plant leaf developmental processes and their underlying mechanisms have not been well characterized. Here, we provide a holistic description of leaf developmental processes that is centered on cytokinins and their signaling functions. Cytokinins maintain the growth potential (pluripotency) of shoot apical meristems, which provide stem cells for the generation of leaf primordia during the initial stage of leaf formation; cytokinins and auxins, as well as their interaction, determine the phyllotaxis pattern. The activities of cytokinins in various regions of the leaf, especially at the margins, collectively determine the final leaf morphology (e.g., simple or compound). The area of a leaf is generally determined by the number and size of the cells in the leaf. Cytokinins promote cell division and increase cell expansion during the proliferation and expansion stages of leaf cell development, respectively. During leaf senescence, cytokinins reduce sugar accumulation, increase chlorophyll synthesis, and prolong the leaf photosynthetic period. We also briefly describe the roles of other hormones, including auxin and ethylene, during the whole leaf developmental process. In this study, we review the regulatory roles of cytokinins in various leaf developmental stages, with a focus on cytokinin metabolism and signal transduction processes, in order to shed light on the molecular mechanisms underlying leaf development.

Genome-wide identification, characterization, and expression analysis of the expansin gene family in watermelon (Citrullus lanatus)

Expansins are plant cell-wall loosening proteins involved in cell enlargement, adaptive responses to environmental stimuli, and various developmental processes. Although expansins have been characterized in many plant species, little is reported on this family in watermelon. In this study, 30 expansin genes in the watermelon genome (ClEXPs) were identified. These genes which were divided into four subfamilies (7 ClEXLAs, 2 ClEXLBs, 18 ClEXPAs, and 3 ClEXPBs) are unevenly distribute on 10 of 11 watermelon chromosomes. Chromosome mapping suggested that tandem duplication events may have played important roles in the expanding of watermelon expansins. Gene structure and motif identification revealed that same subfamily and subgroup have conserved gene structure and motif. Detection of cis-acting elements revealed that ClEXPs gene promoter regions were enriched with light-responsive elements, hormone-responsive, environmental stress-related, and development-related elements. Expression patterns of ClEXPs were investigated by qRT-PCR. The results showed that expression patterns of 15 ClEXP genes differed in three tissues. Through our own and public RNA-seq analysis, we found that ClEXPs had different expression patterns in fruit flesh, fruit rind, and seed at various developmental stages, and most of ClEXPs were highly responsive to abiotic and biotic stresses. Remarkably, 7 ClEXPs (ClEXLA1, ClEXLA6, ClEXLB1, ClEXLB2, ClEXPA5, ClEXPA10, and ClEXPA16) exhibited positive response to at least three kinds of stresses, suggesting that they might play important roles in the crosstalk of stress signal pathways. The results of this study provide useful insights for the functional identification of expansin gene family in watermelon.