Expansin-mediated developmental and adaptive responses: A matter of cell wall biomechanics?

Debunking the idea of biological optimisation: quantitative biology to the rescue

The idea that plants would be efficient, frugal or optimised echoes the recurrent semantics of 'blueprint' and 'program' in molecular genetics. However, when analysing plants with quantitative approaches and systems thinking, we instead find that plants are the results of stochastic processes with many inefficiencies, incoherence or delays fuelling their robustness. If one had to highlight the main value of quantitative biology, this could be it: plants are robust systems because they are not efficient. Such systemic insights extend to the way we conduct plant research and opens plant science publication to a much broader framework.

Genome-Wide Identification of Expansins in Rubus chingii and Profiling Analysis during Fruit Ripening and Softening

Improving fruit size or weight, firmness, and shelf life is a major target for horticultural crop breeding. It is associated with the depolymerization and rearrangement of cell components, including pectin, hemicellulose, cellulose, and other structural (glyco)proteins. Expansins are structural proteins to loosen plant cell wall polysaccharides in a pH-dependent manner and play pivotal roles in the process of fruit development, ripening, and softening. Rubus chingii Hu, a unique Chinese red raspberry, is a prestigious pharmaceutical and nutraceutical dual-function food with great economic value. Thirty-three RchEXPs were predicted by genome-wide identification in this study, containing twenty-seven α-expansins (EXPAs), three β-expansins (EXPBs), one expansin-like A (EXPLA), and two expansin-like B (EXPLBs). Subsequently, molecular characteristics, gene structure and motif compositions, phylogenetic relationships, chromosomal location, collinearity, and regulatory elements were further profiled. Furthermore, transcriptome sequencing (RNA-seq) and real-time quantitative PCR assays of fruits from different developmental stages and lineages showed that the group of RchEXPA5, RchEXPA7, and RchEXPA15 were synergistically involved in fruit expanding and ripening, while another group of RchEXPA6 and RchEXPA26 might be essential for fruit ripening and softening. They were regulated by both abscisic acid and ethylene and were collinear with phylogenetic relationships in the same group. Our new findings laid the molecular foundation for improving the fruit texture and shelf life of R. chingii medicinal and edible fruit.

EXPANSIN15 is involved in flower and fruit development in Arabidopsis

KEY MESSAGE

EXPANSIN15 is involved in petal cell morphology and size, the fusion of the medial tissues in the gynoecium and expansion of fruit valve cells. It genetically interacts with SPATULA and FRUITFULL. Cell expansion is fundamental for the formation of plant tissues and organs, contributing to their final shape and size during development. To better understand this process in flower and fruit development, we have studied the EXPANSIN15 (EXPA15) gene, which showed expression in petals and in the gynoecium. By analyzing expa15 mutant alleles, we found that EXPA15 is involved in petal shape and size determination, by affecting cell morphology and number. EXPA15 also has a function in fruit size, by affecting cell size and number. Furthermore, EXPA15 promotes fusion of the medial tissues in the gynoecium. In addition, we observed genetic interactions with the transcription factors SPATULA (SPT) and FRUITFULL (FUL) in gynoecium medial tissue fusion, style and stigma development and fruit development in Arabidopsis. These findings contribute to the importance of EXPANSINS in floral and fruit development in Arabidopsis.

Hormone-regulated expansins: Expression, localization, and cell wall biomechanics in Arabidopsis root growth

Expansins facilitate cell expansion by mediating pH-dependent cell wall (CW) loosening. However, the role of expansins in controlling CW biomechanical properties in specific tissues and organs remains elusive. We monitored hormonal responsiveness and spatial specificity of expression and localization of expansins predicted to be the direct targets of cytokinin signaling in Arabidopsis (Arabidopsis thaliana). We found EXPANSIN1 (EXPA1) homogenously distributed throughout the CW of columella/lateral root cap, while EXPA10 and EXPA14 localized predominantly at 3-cell boundaries in the epidermis/cortex in various root zones. EXPA15 revealed cell-type-specific combination of homogenous vs. 3-cell boundaries localization. By comparing Brillouin frequency shift and AFM-measured Young's modulus, we demonstrated Brillouin light scattering (BLS) as a tool suitable for non-invasive in vivo quantitative assessment of CW viscoelasticity. Using both BLS and AFM, we showed that EXPA1 overexpression upregulated CW stiffness in the root transition zone (TZ). The dexamethasone-controlled EXPA1 overexpression induced fast changes in the transcription of numerous CW-associated genes, including several EXPAs and XYLOGLUCAN:XYLOGLUCOSYL TRANSFERASEs (XTHs), and associated with rapid pectin methylesterification determined by in situ Fourier-transform infrared spectroscopy in the root TZ. The EXPA1-induced CW remodeling is associated with the shortening of the root apical meristem, leading to root growth arrest. Based on our results, we propose that expansins control root growth by a delicate orchestration of CW biomechanical properties, possibly regulating both CW loosening and CW remodeling.

Cell wall dynamics: novel tools and research questions

Years ago, a classic textbook would define plant cell walls based on passive features. For instance, a sort of plant exoskeleton of invariable polysaccharide composition, and probably painted in green. However, currently, this view has been expanded to consider plant cell walls as active, heterogeneous, and dynamic structures with a high degree of complexity. However, what do we mean when we refer to a cell wall as a dynamic structure? How can we investigate the different implications of this dynamism? While the first question has been the subject of several recent publications, defining the ideal strategies and tools needed to address the second question has proven to be challenging due to the myriad of techniques available. In this review, we will describe the capacities of several methodologies to study cell wall composition, structure, and other aspects developed or optimized in recent years. Keeping in mind cell wall dynamism and plasticity, the advantages of performing long-term non-invasive live-imaging methods will be emphasized. We specifically focus on techniques developed for Arabidopsis thaliana primary cell walls, but the techniques could be applied to both secondary cell walls and other plant species. We believe this toolset will help researchers in expanding knowledge of these dynamic/evolving structures.

Integrated Transcriptome Analysis Identified Key Expansin Genes Associated with Wheat Cell Wall, Grain Weight and Yield

This research elucidates the dynamic expression of expansin genes during the wheat grain (Triticum aestivum L.) development process using comprehensive meta-analysis and experimental validation. We leveraged RNA-seq data from multiple public databases, applying stringent criteria for selection, and identified 60,852 differentially expressed genes across developmental stages. From this pool, 28,558 DEGs were found to exhibit significant temporal regulation in at least two different datasets and were enriched for processes integral to grain development such as carbohydrate metabolism and cell wall organization. Notably, 30% of the 241 known expansin genes showed differential expression during grain growth. Hierarchical clustering and expression level analysis revealed temporal regulation and distinct contributions of expansin subfamilies during the early stages of grain development. Further analysis using co-expression networks underscored the significance of expansin genes, revealing their substantial co-expression with genes involved in cell wall modification. Finally, qPCR validation and grain morphological analysis under field conditions indicated a significant negative correlation between the expression of select expansin genes, and grain size and weight. This study illuminates the potential role of expansin genes in wheat grain development and provides new avenues for targeted genetic improvements in wheat.

The root apoplastic pH as an integrator of plant signaling

Plant nutrition, growth, and response to environmental stresses are pH-dependent processes that are regulated at the apoplastic and subcellular levels. The root apoplastic pH is especially sensitive to external cues and can also be modified by intracellular inputs, such as hormonal signaling. Optimal crosstalk of the mechanisms involved in the extent and span of the apoplast pH fluctuations promotes plant resilience to detrimental biotic and abiotic factors. The fact that variations in local pHs are a standard mechanism in different signaling pathways indicates that the pH itself can be the pivotal element to provide a physiological context to plant cell regions, allowing a proportional reaction to different situations. This review brings a collective vision of the causes that initiate root apoplastic pHs variations, their interaction, and how they influence root response outcomes.