Multidimensional Gene Regulatory Landscape of Motor Organ Pulvinus in the Model Legume Medicago truncatula

Haplotype-resolved genome of Mimosa bimucronata revealed insights into leaf movement and nitrogen fixation

BACKGROUND

Mimosa bimucronata originates from tropical America and exhibits distinctive leaf movement characterized by a relative slow speed. Additionally, this species possesses the ability to fix nitrogen. Despite these intriguing traits, comprehensive studies have been hindered by the lack of genomic resources for M. bimucronata.

RESULTS

To unravel the intricacies of leaf movement and nitrogen fixation, we successfully assembled a high-quality, haplotype-resolved, reference genome at the chromosome level, spanning 648 Mb and anchored in 13 pseudochromosomes. A total of 32,146 protein-coding genes were annotated. In particular, haplotype A was annotated with 31,035 protein-coding genes, and haplotype B with 31,440 protein-coding genes. Structural variations (SVs) and allele specific expression (ASE) analyses uncovered the potential role of structural variants in leaf movement and nitrogen fixation in M. bimucronata. Two whole-genome duplication (WGD) events were detected, that occurred ~ 2.9 and ~ 73.5 million years ago. Transcriptome and co-expression network analyses revealed the involvement of aquaporins (AQPs) and Ca2+-related ion channel genes in leaf movement. Moreover, we also identified nodulation-related genes and analyzed the structure and evolution of the key gene NIN in the process of symbiotic nitrogen fixation (SNF).

CONCLUSION

The detailed comparative genomic and transcriptomic analyses provided insights into the mechanisms governing leaf movement and nitrogen fixation in M. bimucronata. This research yielded genomic resources and provided an important reference for functional genomic studies of M. bimucronata and other legume species.

Nyctinastic movement in legumes: Developmental mechanisms, factors and biological significance

In legumes, a common phenomenon known as nyctinastic movement is observed. This movement involves the horizontal expansion of leaves during the day and relative vertical closure at night. Nyctinastic movement is driven by the pulvinus, which consists of flexor and extensor motor cells. The turgor pressure difference between these two cell types generates a driving force for the bending and deformation of the pulvinus. This review focuses on the developmental mechanisms of the pulvinus, the factors affecting nyctinastic movement, and the biological significance of this phenomenon in legumes, thus providing a reference for further research on nyctinastic movement.

Tapping into the plasticity of plant architecture for increased stress resilience

Plant architecture develops post-embryonically and emerges from a dialogue between the developmental signals and environmental cues. Length and branching of the vegetative and reproductive tissues were the focus of improvement of plant performance from the early days of plant breeding. Current breeding priorities are changing, as we need to prioritize plant productivity under increasingly challenging environmental conditions. While it has been widely recognized that plant architecture changes in response to the environment, its contribution to plant productivity in the changing climate remains to be fully explored. This review will summarize prior discoveries of genetic control of plant architecture traits and their effect on plant performance under environmental stress. We review new tools in phenotyping that will guide future discoveries of genes contributing to plant architecture, its plasticity, and its contributions to stress resilience. Subsequently, we provide a perspective into how integrating the study of new species, modern phenotyping techniques, and modeling can lead to discovering new genetic targets underlying the plasticity of plant architecture and stress resilience. Altogether, this review provides a new perspective on the plasticity of plant architecture and how it can be harnessed for increased performance under environmental stress.

Pulvinar slits: Cellulose-deficient and de-methyl-esterified pectin-rich structures in a legume motor cell

The cortical motor cells (CMCs) in a legume pulvinus execute the reversible deformation in leaf movement that is driven by changes in turgor pressure. In contrast to the underlying osmotic regulation property, the cell wall structure of CMCs that contributes to the movement has yet to be characterized in detail. Here, we report that the cell wall of CMCs has circumferential slits with low levels of cellulose deposition, which are widely conserved among legume species. This structure is unique and distinct from that of any other primary cell walls reported so far; thus, we named them "pulvinar slits." Notably, we predominantly detected de-methyl-esterified homogalacturonan inside pulvinar slits, with a low deposition of highly methyl-esterified homogalacturonan, as with cellulose. In addition, Fourier transform infrared spectroscopy analysis indicated that the cell wall composition of pulvini is different from that of other axial organs, such as petioles or stems. Moreover, monosaccharide analysis showed that pulvini are pectin-rich organs like developing stems and that the amount of galacturonic acid in pulvini is greater than in developing stems. Computer modeling suggested that pulvinar slits facilitate anisotropic extension in the direction perpendicular to the slits in the presence of turgor pressure. When tissue slices of CMCs were transferred to different extracellular osmotic conditions, pulvinar slits altered their opening width, indicating their deformability. In this study, we thus characterized a distinctive cell wall structure of CMCs, adding to our knowledge of repetitive and reversible organ deformation as well as the structural diversity and function of the plant cell wall.

Mechanics of Reversible Deformation during Leaf Movement and Regulation of Pulvinus Development in Legumes

Plant cell deformation is a mechanical process that is driven by differences in the osmotic pressure inside and outside of the cell and is influenced by cell wall properties. Legume leaf movements result from reversible deformation of pulvinar motor cells. Reversible cell deformation is an elastic process distinct from the irreversible cell growth of developing organs. Here, we begin with a review of the basic mathematics of cell volume changes, cell wall function, and the mechanics of bending deformation at a macro scale. Next, we summarize the findings of recent molecular genetic studies of pulvinar development. We then review the mechanisms of the adaxial/abaxial patterning because pulvinar bending deformation depends on the differences in mechanical properties and physiological responses of motor cells on the adaxial versus abaxial sides of the pulvinus. Intriguingly, pulvini simultaneously encompass morphological symmetry and functional asymmetry along the adaxial/abaxial axis. This review provides an introduction to leaf movement and reversible deformation from the perspective of mechanics and molecular genetics.