Roles of cortical actin microfilament patterning in division plane orientation in plants

In land plant cells, division planes are precisely predicted by the microtubule preprophase band and cortical actin microfilament pattern called the actin-depleted zone or actin microfilament twin peaks. However, the function of cortical actin microfilament patterning is not clear. In this study, we report that treatment with the inhibitor 2,3,5-triiodobenzonic acid (TIBA) or jasplakinolide increased the amount of thick actin microfilaments in tobacco BY-2 cells at interphase. However, during the division of BY-2 cells, these inhibitors did not induce visible alteration of actin microfilament thickness but altered cortical actin microfilament patterning without significant disorganization of the microtubule preprophase band. TIBA treatment induced a single intensity peak of actin microfilament distribution around the cell center, whereas jasplakinolide caused the appearance of triple peaks relative to the distribution of actin microfilament around the cell center, in approximately one-third of the cells at metaphase. Dual observations of microtubules and actin microfilaments revealed that abnormal cortical actin microfilament patterning with single or triple peaks is correlated with oblique mitotic spindles in BY-2 cells. In addition, oblique cell plates were frequently observed in BY-2 cells and Arabidopsis thaliana root cells treated with TIBA or jasplakinolide. These results provide evidence for the critical roles of cortical actin microfilament patterning in spindle and cell plate orientation.