Derr, J. | Search bioRxiv

bioRxiv2020

Hook Shape of Growing Leaves Results from an Active Regulation

The rachis of most growing compound leaves observed in nature exhibit a stereotyped hook shape. In this study, we focus on the canonical case of Averrhoa carambola. Combining kinematics and mechanical investigation, we characterize this hook shape and shed light on its establishment and maintenance. We show quantitatively that the hook shape is a conserved bent zone propagating at constant velocity and constant distance from the apex throughout development. A simple mechanical test first reveals non-zero spontaneous curvature profiles for the growing leaves, indicating that the hook shape is actively regulated. It then evidences the robust spatial organization of growth, curvature, rigidity and lignification and their interplay. Regulation processes appear to be specifically localized: in particular, differential growth occurs where the elongation rate drops. Finally, impairing the graviception of the leaf on a clinostat led to reduced hook curvatures but not to its loss. Altogether our results suggest a role for proprioception in the regulation of the apical hook, likely mediated via mechanical strain.

plant biology↗

bioRxiv2020

Emergence of ribozyme and tRNA-like structures from mineral-rich muddy pools on prebiotic earth

AO_SCPLOWBSTRACTC_SCPLOWThe RNA world hypothesis, although a viable one regarding the origin of life on earth, has so far failed to provide a compelling explanation for the synthesis of RNA molecules with catalytic functions, from free nucleotides via abiotic processes. To tackle this long-standing problem, we develop a realistic model for the onset of the RNA world, using experimentally determined rates for polymerization reactions. We start with minimal assumptions about the initial state that only requires the presence of short oligomers or just free nucleotides and consider the effects of environmental cycling by dividing a day into a dry, semi-wet and wet phases that are distinguished by the nature of reactions they support. Long polymers, with maximum lengths sometimes exceeding 100 nucleotides, spontaneously emerge due to a combination of non-enzymatic, non-templated polymer extension and template-directed primer extension processes. The former helps in increasing the lengths of RNA strands, whereas the later helps in producing complementary copies of the strands. Strands also undergo hydrolysis in a structure-dependent manner that favour breaking of bonds connecting unpaired nucleotides. We identify the most favourable conditions needed for the emergence of ribozyme and tRNA-like structures and double stranded RNA molecules, classify all RNA strands on the basis of their secondary structures and determine their abundance in the population. Our results indicate that under suitable environmental conditions, non-enzymatic processes would have been sufficient to lead to the emergence of a variety of ribozyme-like molecules with complex secondary structures and potential catalytic functions.

biophysics↗