Ectopic expression of Arabidopsis CYCD2 and CYCD3 in tobacco has distinct effects on the structural organization of the shoot apical meristem

Abstract

Transgenic tobacco lines expressing Arath-CYCD2 or Arath-CYCD3 genes under a cauliflower mosaic virus 35S promoter are modified in the timing of their development, but not in the phenotype of their vegetative organs. They display an increased rate of leaf initiation, which is shown to be associated with distinct changes in the structural organization of their shoot apical meristem (SAM). Constitutive expression of Arath-CYCD2 leads to a progressive modification of the SAM structural organization with predominant periclinal divisions in the L3 layer and to the loss of the classical cytophysiological zonation, the central zone being reduced to the central cells of the L1 and L2 layers. These changes reveal a particular sensitivity of the corpus cells (L3) to Arath-CYCD2 over-expression and suggest a role for CYCD2 in controlling the planes of cell division in these cells. The SAM structural modifications in the Arath-CYCD3 over-expressing lines are less drastic; only an increased cell number together with a reduced cell size, particularly in the L1 layer, characterizes the peripheral zones. This could be related to the shortening of the G1-phase duration that renders cell growth incomplete between successive mitoses. Cell proliferation continues beyond the SAM in the developing internodes and confers a delayed senescence to Arath-CYCD3 over-expressing juvenile tissues. In addition, the ploidy levels of mature stem tissues in both types of transgenic lines are unaffected, suggesting that the studied G1 to S cell-cycle genes have no effect on the extent of endoreduplication in tobacco stem tissues.

Cytokinins and Their Possible Role in Seed Size and Seed Mass Determination in Maize

“…Specifically, in transgenic tobacco lines expressing Arath-CycD3 gene the ploidy levels of mature stem tissues were not affected, suggesting no effect on the extent of endoreduplication. Moreover, the Arath-CycD3 overexpressing lines have an increased cell number together with a reduced cell size (Boucheron et al, 2005). Similar effects were also observed in Arabidopsis that expressed very high levels of CycD3;1 (Dewitte et al, 2003).…”

The D-Type Cyclin CYCD3;1 Is Limiting for the G1-to-S-Phase Transition inArabidopsis

“…Ectopic expression of Arabidopsis CYCD2;1 in transgenic tobacco (Nicotiana tabacum) plants led to accelerated development and a faster growth rate attributable to a reduction in cell cycle length caused by a reduced G1-phase duration (Cockcroft et al, 2000). No externally visible accompanying effects on morphology were reported, although more detailed analysis has suggested subtle alterations in meristematic zonation (Boucheron et al, 2005).…”

Dual Effects of miR156-TargetedSPLGenes andCYP78A5/KLUHon Plastochron Length and Organ Size inArabidopsis thaliana

“…A direct link between cell division and plastochron length has been made with plants that overexpress Cyclin D2, which leads to faster leaf initiation correlated with higher cell division rates. At least early on, size and organization of the shoot apical meristem are normal in these plants (Cockcroft et al, 2000;Boucheron et al, 2005). However, while cell division can under certain circumstances be limiting for shortening plastochron length, slowing down cell division with a dominantnegative version of CDC2a, a cyclin-dependent kinase that interacts with Cyclin D2, did not affect leaf initiation rate, apparently because increased cell size compensated for lower cell numbers (Hemerly et al, 1995).…”