The Mechanism of Cell Cycle Arrest Front Progression Explained by a KLUH/CYP78A5-dependent Mobile Growth Factor in Developing Leaves of Arabidopsis thaliana

Abstract

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.

Computational Method for Quantifying Growth Patterns at the Adaxial Leaf Surface in Three Dimensions

“…This change in gradient shape over time may reflect interplay between a growth-promoting signal originating from the leaf base from early time points onward and a growth-inhibiting signal originating from the leaf tip at later time points. Interestingly, the KLUH gene has been shown to be expressed at the leaf base during early development and was proposed to generate a proximodistal concentration gradient of a mobile growth factor in the leaf (Anastasiou et al, 2007;Kazama et al, 2010). On the other hand, the cell division patterns in leaves of the snapdragon (Antirrhinum majus) cincinnata mutant support the existence of a growth-repressing signal originating from the leaf tip (Nath et al, 2003).…”

“…Those earlier stages correspond to periods when cell divisions are occurring throughout the leaf primordium (Kazama et al, 2010) and therefore would be particularly interesting to capture. The computational methods presented could be adapted to quantify growth in those earlier stages using other 3D microscopy systems, such as confocal microscopy, optical projection tomography, or selective plane illumination (for review, see Ntziachristos, 2010).…”

Combined Large-Scale Phenotyping and Transcriptomics in Maize Reveals a Robust Growth Regulatory Network

“…For instance, leaves of maize (Zea mays) plants with altered levels of GA are affected in their growth rates, and the size of the division zone (DZ) is changed correspondingly . Examples in Arabidopsis of genes that are regulators of final leaf size by influencing cell proliferation are AVP1, JAW, and BRI1 (Gonzalez et al, 2010), GROWTH-REGULATING FACTOR1 (GRF1) and GRF2 (Kim and Kende, 2004), GRF5 (Horiguchi et al, 2005), DA1 and ENHANCER OF DA1 , ANGUSTIFOLIA3 (AN3)/GRF-INTERACTING FACTOR (Lee et al, 2009), and KLUH (Kazama et al, 2010). These examples illustrate that cell proliferation seems to be a key contributing factor to final leaf size.…”

Ubiquitylation activates a peptidase that promotes cleavage and destabilization of its activating E3 ligases and diverse growth regulatory proteins to limit cell proliferation in Arabidopsis

“…After specification of boundaries and growth axes, the leaf lamina grows in an initial period of cell division in which cell size is relatively constant, followed by a transition to endoreduplication associated with cell expansion and differentiation (Breuer et al 2010;De Veylder et al 2011). The transition from cell proliferation to cell expansion is spatially and temporarily regulated during leaf growth and appears to progress from the tip to the base of the leaf as a cell division arrest front (Kazama et al 2010) accompanied by shifts in gene expression patterns (Efroni et al 2008;Andriankaja et al 2012). A key question is how the transition from cell proliferation to cell expansion and differentiation is coordinated to generate a correctly sized organ.…”