The mechanism of cell cycle arrest front progression explained by a KLUH/CYP78A5-dependent mobile growth factor in developing leaves of Arabidopsis thaliana.

The size and shape of leaves are influenced by the progression of the cell cycle arrest front (AF). However, the AF progression with leaf growth has not been characterized quantitatively. Moreover, the mechanism linking AF progression and genetic factors is not fully understood. Recently, it was proposed that a KLUH/CYP78A5 (KLU)-dependent signal acts as a mobile growth factor (MGF) for cell proliferation and controls the lateral organ size of Arabidopsis. This study examines this hypothesis under the assumption that the gradient field dynamics of the KLU-dependent MGF provide the mechanism of AF progression using molecular markers and computer simulations. First, we measured the exact AF position with leaf growth using the pCYCB1;1::CYCB1;1::GUS expression pattern, which visualizes mitotic cells. As a result, we found that the AF stayed at an almost constant distance from the leaf blade base (stage 1) and then progressed towards the base and disappeared relatively quickly (stage 2), which previously had not been identified. Secondly, we showed that KLU may generate a concentration gradient of MGF in leaves, if KLU really controls cell division via the biosynthesis of MGF, by comparing the expression patterns of pKLU::GUS and pCYCB1;1::CYCB1;1::GUS. Finally, we built a simulation model using a diffusion equation with a decay term, in which the rate of MGF production estimated from the KLU expression level was included in the boundary condition. Our simulation model successfully reproduced both stages 1 and 2 of the AF, suggesting that the proposed mechanism does explain the AF progression under some restricted conditions.

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