Measuring spore settling velocity for an improved assessment of dispersal rates in mosses

The settling velocity of diaspores is a key parameter for the measurement of dispersal ability in wind-dispersed plants and one of the most relevant parameters in explicit dispersal models, but remains largely undocumented in bryophytes. The settling velocities of moss spores were measured and it was determined whether settling velocities can be derived from spore diameter using Stokes’ Law or if specific traits of spore ornamentation cause departures from theoretical expectations

Measuring spore settling velocity for an improved assessment of dispersal rates in mosses

[Background and Aims] The settling velocity of diaspores is a key parameter for the measurement of dispersal ability in wind-dispersed plants and one of the most relevant parameters in explicit dispersal models, but remains largely undocumented in bryophytes. The settling velocities of moss spores were measured and it was determined whether settling velocities can be derived from spore diameter using Stokes' Law or if specific traits of spore ornamentation cause departures from theoretical expectations.[Methods] A fall tower design combined with a high-speed camera was used to document spore settling velocities in nine moss species selected to cover the range of spore diameters within the group. Linear mixed effect models were employed to determine whether settling velocity can be predicted from spore diameter, taking specific variation in shape and surface roughness into account.[Key Results] Average settling velocity of moss spores ranged from 0·49 to 8·52 cm s. There was a significant positive relationship between spore settling velocity and size, but the inclusion of variables of shape and texture of spores in the best-fit models provides evidence for their role in shaping spore settling velocities.[Conclusions] Settling velocities in mosses can significantly depart from expectations derived from Stokes' Law. We suggest that variation in spore shape and ornamentation affects the balance between density and drag, and results in different dispersal capacities, which may be correlated with different life-history traits or ecological requirements. Further studies on spore ultrastructure would be necessary to determine the role of complex spore ornamentation patterns in the drag-to-mass ratio and ultimately identify what is the still poorly understood function of the striking and highly variable ornamentation patterns of the perine layer on moss spores.Many thanks are due to two referees for their comments on the manuscript. This work was supported by the Belgian Industrial and Agricultural Research Funds (FRIA) [grant no. 21653026].Peer Reviewe

Expression of cell-wall related genes is highly variable and correlates with sepal morphology

Control of organ morphology is a fundamental feature of living organisms. There is, however, observable variation in organ size and shape within a given genotype. Taking the sepal of Arabidopsis as a model, we investigated whether we can use variability of gene expression alongside variability of organ morphology to identify gene regulatory networks potentially involved in organ size and shape determination. We produced a dataset composed of morphological parameters and genome-wide transcriptome obtained from 27 individual sepals from wild-type plants with nearly identical genetic backgrounds, environment, and developmental stage. Sepals exhibited appreciable variability in both morphology and transcriptome, with response to stimulus genes and cell-wall related genes displaying high variability in expression. We additionally identified five modules of co-expressed genes which correlated significantly with morphology, revealing biologically relevant gene regulatory networks. Interestingly, cell-wall related genes were overrepresented in two of the top three modules. Overall, our work highlights the benefit of using coupled variation in gene expression and phenotype in wild-type plants to shed light on the mechanisms underlying organ size and shape determination. Although causality between gene expression and sepal morphology has not been established, our approach opens the way to informed analysis for mutant characterization and functional studies

Robustness of organ morphology is associated with modules of co-expressed genes related to plant cell wall

Reproducibility in organ size and shape is a fundamental trait of living organisms. Themechanisms underlying such robustness remain, however, to be elucidated. Taking the sepal ofArabidopsis as a model, we investigated whether variability of gene expression plays a role invariation of organ morphology. To address this question, we produced a dataset composed ofboth transcriptomic and morphological information obtained from 27 individual sepals fromwild-type plants. Although nearly identical in their genetic background, environment, anddevelopmental stage, these sepals exhibited appreciable variability in both morphology andtranscriptome. We identified modules of co-expressed genes in sepals, three of whichcorrelated significantly with morphology, revealing biologically relevant gene regulatorynetworks. Interestingly, cell-wall related genes were overrepresented in two of these threemodules. Additionally, we found that highly variable genes were unexpectedly enriched incell-wall related processes. We then analyzed sepal morphology from 16 cell-wall mutants andfound that the more a gene is expressed in wild-type, the more variable the morphology of thecorresponding mutant. Altogether, our work unravels the contribution of cell-wall related genesto the robustness of sepal morphology. More generally, we propose that canalizing traits duringdevelopment could rely on the modulation of highly expressed genes