MICROTUBULAR CYTOSKELETON AND ROOT MORPHOGENESIS

Although the microtubular cytoskeleton of plant cells is important in maintaining the direction of cell growth, its natural lability can be harnessed in such a way that new growth axes are permitted. In these c ircumstances, the system which fabricates the cytoskeleton is presumab ly responsive to morphogenetic information originating from outside th e cell. Spatial patterns of hormonal and metabolic signals within the tissue or organ that house the responsive cells are one possible sourc e of this information. However, a contrasting source takes the form of biophysical information, such as the supracellular patterns of stress es and strains. We examined the microtubular cytoskeleton in roots of tomato and maize to test the assumption that the cortical microtubular array of each cell would have a particular orientation relative to th e cell's position within the growth field of the root apex. Accordingl y, each intracellular cortical array was mapped to the overall pattern of cells within the apex. In certain areas of the meristem, the array s seemed to be more variable than elsewhere. These are sites where mor phogenetic decisions are taken, usually involving a change in the plan e of cell division. Roots which have suffered disturbance to their phy sical structure (e.g. removal of the root cap), or which had been expo sed to low temperatures or treated with certain chemicals (e.g. inhibi tors of nuclear division or DNA synthesis), exhibited new patterns of microtubular arrays which in turn predicted novel patterns of cell div ision. In all these circumstances, the arrays showed consistent altera tions within distinct regions of the root - e.g. in the quiescent cent re and also in a group of cells just behind the quiescent centre, at t he boundary between cortex and stele. These altered arrays indicate th at there are supracellular domains in which the microtubules respond t o morphogenetic signals. Studies such as these reinforce the concept o f microtubule lability and the inherent responsiveness of the microtub ular system to external and internal stimuli. However, at present ther e is no indication of how the morphogenetic programme of the root is s et up in the first place. Probably, this is established and stabilized early in embryogenesis and is then perpetuated by the prevailing meta bolic and biophysical conditions. The microtubules of the cytoskeleton can be regarded as intracellular automata which not only participate in mitosis and cytokinesis but also ensure the realization of an organ ogenetic programme. Should the root confront circumstances which tempo rarily destabilize this programme, the prevailing growth field is suff iciently robust to ensure that the microtubular system is attracted ba ck to the stable, pre-existing state capable of reinstating normal mor phogenesis.