Quantitative predictions on auxin-induced polar distribution of PIN proteins during vein formation in leaves

The dynamic patterning of the plant hormone auxin and its efflux facilitator the PIN protein are the key regulator for the spatial and temporal organization of plant development. In particular auxin induces the polar localization of its own efflux facilitator. Due to this positive feedback auxin flow is directed and patterns of auxin and PIN arise. During the earliest stage of vein initiation in leaves auxin accumulates in a single cell in a rim of epidermal cells from which it flows into the ground meristem tissue of the leaf blade. There the localized auxin supply yields the successive polarization of PIN distribution along a strand of cells. We model the auxin and PIN dynamics within cells with a minimal canalization model. Solving the model analytically we uncover an excitable polarization front that triggers a polar distribution of PIN proteins in cells. As polarization fronts may extend to opposing directions from their initiation site we suggest a possible resolution to the puzzling occurrence of bipolar cells, such we offer an explanation for the development of closed, looped veins. Employing non-linear analysis we identify the role of the contributing microscopic processes during polarization. Furthermore, we deduce quantitative predictions on polarization fronts establishing a route to determine the up to now largely unknown kinetic rates of auxin and PIN dynamics.Comment: 9 pages, 4 figures, supplemental information included, accepted for publication in Eur. Phys. J.

A genetic perspective on the association between exercise and mental health.

Regular exercise is associated with better mental health. This association is widely assumed to reflect causal effects of exercise. In this paper we propose that two additional mechanisms contribute to the association between exercise and mental health in the population-at-large: genetic pleiotropy and gene-by-exercise interaction. Both mechanisms assume heritability of exercise behavior and a partial overlap between the genes influencing exercise behavior and mental health. We review a number of large-scale studies in monozygotic and dizygotic twins that support these assumptions. Based on the importance of genetic factors in exercise behavior we develop a model for gene-by-exercise interaction that explains differences in voluntary exercise behavior by differential genetic sensitivity to the mental health benefits of exercise. We focus on the genetic modulation of acute mood effects of exercise and longer-term effects on self-esteem through genetic effects on exercise ability. If correct, our model calls for a change from 'population-based' to 'personalized' intervention strategies. © 2008 Elsevier Ltd. All rights reserved