ROTUNDIFOLIA4 regulates cell proliferation along the body axis in Arabidopsis shoot.

Molecular genetics has been successful in identifying leaf- size regulators such as transcription factors, phytohormones, and signal molecules. Among them, a ROTUNDIFOLIA4-LIKE/DEVIL (RTFL/DVL) family of Arabidopsis, genes encoding peptides with no secretion-signal sequence, is unique in that their overexpressors have a reduced number of leaf cells specifically along the proximodistal axis. However, because the RTFL/DVL lack any obvious homology with functionally identified domains, and because of genetic redundancy among RTFL/DVL, their molecular and developmental roles are unclear. In this study we focused on one member in the family, ROTUNDIFOLIA4 (ROT4), and identified the core functional region within it and we found no proteolytic processing in planta. Developmental analysis of leaf primordia revealed that ROT4 overexpression reduces the meristematic zone size within the leaf blade. Moreover, induced local overexpression demonstrated that ROT4 acts as a regulator of the leaf shape via a change in positional cue along the longitudinal axis. Similarly, ROT4 overexpression results in a protrusion of the main inflorescence stem, again indicating a change in positional cue along the longitudinal axis. These results suggest that ROT4 affects the positional cue and cell proliferation along the body axis.

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.

Reconstruction of active regular motion in amoeba extract: dynamic cooperation between sol and gel states.

Amoeboid locomotion is one of the typical modes of biological cell migration. Cytoplasmic sol-gel conversion of an actomyosin system is thought to play an important role in locomotion. However, the mechanisms underlying sol-gel conversion, including trigger, signal, and regulating factors, remain unclear. We developed a novel model system in which an actomyosin fraction moves like an amoeba in a cytoplasmic extract. Rheological study of this model system revealed that the actomyosin fraction exhibits shear banding: the sol-gel state of actomyosin can be regulated by shear rate or mechanical force. Furthermore, study of the living cell indicated that the shear-banding property also causes sol-gel conversion with the same order of magnitude as that of shear rate. Our results suggest that the inherent sol-gel transition property plays an essential role in the self-regulation of autonomous translational motion in amoeba.