Our key developmental questions were how are gibberellin levels controlled, what is their relationship to cell growth, and how does environmental change affect gibberellin and plasticity?
We studied in depth the mechanisms directing gibberellin levels in hypocotyls and roots. We have determined the dominant biochemical and light signalling steps determining how hypocotyl gibberellin gradients are drawn in darkness and redrawn in the light. During these studies, we also uncovered that a spatial correlation between gibberellin levels and cellular growth in the dark grown hypocotyl can be disrupted in certain mutant backgrounds, indicating that gibberellins alone are not responsible for setting cell growth patterns.
In roots, we previously observed a gibberellin gradient correlating with cell length and have now deciphered which enzymatic steps are most important for setting up this gradient. Interestingly, root meristematic cells and elongating cell types exhibited distinct rate-limiting biochemical steps. An unexplained gibberellin pattern in a third cell type led us to a new mode of regulation of gibberellin biosynthesis. Our results conflicted with previous multiscale mathematical models of gibberellin patterning and thus we collaborated the modellers that generated the models to develop new multiscale models that better capture in vivo hormone dynamics in roots. Finally, we are continuing to probe which gibberellin accumulations control which growth phenomena, including testing an emergent hypothesis that cell elongation requires two distinct phases of GA accumulation.
We further investigated in high resolution the hormonal and biophysical dynamics in hypocotyls exposed to light for the first time, i.e. apical hook opening, which is a model for differential growth. We have characterised how light triggers dynamics in growth, pH, microtubule orientations and gibberellin and auxin hormone levels at the cellular level and also developed a morphodynamic mathematical model describing apical hook opening. Working iteratively between experiments and modelling, we found evidence for several phenomena being critical for differential cell growth during apical hook opening. Some of the answer lies in innate physical stress patterns that emerge during the opening of a pressurised hooked object with interior expansion and exterior restriction. The model is improved with the addition of hormone differential mediated mechanical loosening along with dynamic and differential microtubule reorientations in response to stress. Another fascinating discovery is that the light induced hormone dynamics are themselves dependent on changes in physical mechanics such that a mechanochemical feeback loop is operating in apical hook opening.
