Changes in the pattern of cell arrangement at the surface of the shoot apical meristem in Hedera helix L. following gibberellin treatment

Abstract

The changes in the pattern of cell arrangement and surface topography at the shoot apical meristem of Hedera helix L., which occur during gibberellic acid (GA3)-induced transition from spiral to distichous phyllotaxis, were examined by scanning electron microscopy of rapidly frozen tissue. The technique preserves the original shape of the cells in their turgid state. It reveals distinct sets of radially oriented cell files, about four to eight cells wide, which extend from the central region of the meristem toward leaf primordia on the meristem flanks. In apices with spiral phyllotaxis, a new emerging primordium (0) appears as an acropetal bulge between the radial files adjacent to the third (3) and the second (2) older primordia. The bulging is associated with radial or oblique cell divisions while those located at the meristem flanks and in the radial files are oriented tangentially. As the displacement of existing primordia away from the central region increases following the GA3 treatment, radial and oblique divisions as well as acropetal bulging invade the radial files adjacent to the primordium 2; consequently the angular divergence of the emerging primordium from the youngest existing primordium (1) increases. In apices with distichous phyllotaxis, the earliest bulging appears on both sides of the radial files facing primordium 2, with a slight depression at the files. The radial files therefore correspond to regions of the meristem where acropetal bulging is generally delayed, although this effect apparently diminishes with increasing distance of existing primordia from the meristem center.

A role for gibberellic acid in orienting microtubules and regulating cell growth polarity in the maize root cortex

“…Moreover, the cellular organization of the gibberellin-deficient 9ib-l-tomato root apices underwent a profound reconstruction in the region of the quiescent centre, indicating that GAs are involved in the morphogenetic control of root development. Similar findings were reported from GA3-treated shoot apical meristems of Hedera (Marc and Hackett 1992). Here, the GAsinduced cellular reorganizations were closely related to effects of GA3 on distribution of CMTs (Marc and Hackett 1989).…”

A quantitative gibberellin signalling biosensor reveals a role for gibberellins in internode specification at the shoot apical meristem

“…5i). Taken together, these results suggest that GA signalling controls orientation of cell division in the SAM coherently with previous reports 35,36 , high GA signalling likely inducing a transverse orientation of cell divisions in the IPR.…”

“…Mechanical stress has also been suggested to regulate cell division plane orientation by acting on cortical microtubule 34,41,46 and our results are compatible with a combined effect of mechanical stress and GAs in this regulation. GA signalling could affect the orientation of the cell division plane also through an effect on cortical microtubule orientation 35,36,47 . Likewise, previous works have highlighted the importance of the DELLA interacting proteins TCP14 and 15 in the control of internode patterning 48,49…”

Shoot Meristem Formation in Vegetative Development.

“…This integration process is most obvious during the transition to flowering, when the shape of the meristem and the phyllotaxy of the shoot can change markedly. However, changes in phyllotaxy and meristem shape may also occur during vegetative phase change, when a plant makes the transition from a juvenile pattern of growth to an adult phase (Poethig, 1990(Poethig, , 1997Marc and Hackett, 1992). Moreover, aquatic or semiaquatic plants may display different patterns of growth when submerged than when growing in air, and at least some of these differences are likely to be regulated at the SAM.…”