A mutation affecting etiolation and cell elongation in Nicotiana plumbaginifolia causes abnormal division plane alignment and pattern formation in the root meristem

A mutant of Nicotiana plumbaginifolia, hyp2, which is abnormal in the regulation of post-embryonic cell elongation is described. All cells in the plant are affected by the mutation, and, in general, cells are shorter and thicker than in wild-type seedlings. Not only is the mutant perturbed in cell expansion, but it also shares a number of characteristics with some of the de-etiolated mutants of Arabidopsis. In the dark it has spread cotyledons and it expresses the photosynthetic gene RBCS. In the light it shows accumulation of anthocyanins and cells in the upper part of the root develop chloroplasts. An important characteristic of the mutant is that not only cell shape but also pattern formation in the root meristem is perturbed. First, the mutant has an extra cortical cell layer because the cells that usually give rise to the endodermis go through an additional round of longitudinal, periclinal cell divisions. In addition, the amount of cells per cell layer is increased when compared with the wild-type, showing that the mutation causes extra anticlinal cell divisions in all cell types. Experiments using gibberellic acid (GA) show that the gibberellin GA3 can partially compensate for abnormal HYP2 function. In the presence of this growth factor the mutant shows a hypocotyl which is even longer than in the untreated wild-type, and root meristem structure is normal, combined with a reduced anthocyanin content. However, a detailed analysis of meristem structure using the hormone and the inhibitor paclobutrazol shows that both HYPZ and gibberellic acid independently control cell elongation and pattern formation in the root meristem. The results also show that gibberellins have a more direct effect on the decision of a cell to divide in a particular plane than is usually proposed in the literature.