Connecting gene and hormone action to form, pattern and organogenesis: biophysical transductions

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Abstract
Morphological effects of mutations and hormones in shoots and flowers are common. Hence the causal chain between genome and phenotype must contain biophysical steps which link soluble control compounds to the production of specific solid tissue configuration. Extension of cylindrical form results from the iterated directional expansion of cell and organ surface. This is thought to be based on transverse microfibril and microtubule (MT) alignment in cells. It is proposed that the key biophysical link is a dynamic ‘self-cinching’ process involving shear between adjacent cortical MTs in a ring. The MT array is maximally stable when the MTs are transverse. Gene products and hormones could act on form by influencing this minimal energy process. Production of pattern is the iterated emergence of new primordia in specific relation to pre-existing ones. It is proposed that the key biophysical step here is the minimal energy buckling (folding) of the tunica layer as it develops excess surface adjacent to older primordia. This physical response can also initiate regular undulations, primordia, in originally uniform tissue. Gene products and hormones could influence meristem size or tissueyielding properties and thereby govern both pattern and organ number. Organogenesis, connecting pattern to form, is proposed to involve the destabilization and restabilization of MT alignment controls within, and among, cells. All three processes are analysed in terms of minimal energy configurations of tissue and cell components.

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