Asymmetric cell division generates different cell types and is a feature of development in multicellular organisms. Prior to asymmetric cell division, cell polarity is established. Maize (Zea mays) stomatal development serves as an excellent plant model system for asymmetric cell division, especially the asymmetric division of the subsidiary mother cell (SMC). In SMCs, the nucleus migrates to a polar location after the accumulation of polarly localized proteins but before the appearance of the preprophase band. We examined a mutant of an outer nuclear membrane protein that is part of the LINC (linker of nucleoskeleton and cytoskeleton) complex that localizes to the nuclear envelope in interphase cells. Previously, maize linc kash sine-like2 (mlks2) was observed to have abnormal stomata. We confirmed and identified the precise defects that lead to abnormal asymmetric divisions. Proteins that are polarly localized in SMCs prior to division polarized normally in mlks2. However, polar localization of the nucleus was sometimes impaired, even in cells that have otherwise normal polarity. This led to a misplaced preprophase band and atypical division planes. MLKS2 localized to mitotic structures; however, the structure of the preprophase band, spindle and phragmoplast appeared normal in mlks2. Timelapse imaging revealed that mlks2 has defects in pre-mitotic nuclear migration towards the polarized site and unstable position at the division site after formation of the preprophase band. Overall, our results show that nuclear envelope proteins promote pre-mitotic nuclear migration and stable nuclear position and that the position of the nucleus influences division plane establishment in asymmetrically dividing cells.
Formative asymmetric divisions produce cells with different fates and are critical for development. We show the maize (Zea mays) myosin XI protein, OPAQUE1 (O1), is necessary for asymmetric divisions during maize stomatal development. We analyzed stomatal precursor cells before and during asymmetric division to determine why o1 mutants have abnormal division planes. Cell polarization and nuclear positioning occur normally in the o1 mutant, and the future site of division is correctly specified. The defect in o1 becomes apparent during late cytokinesis, when the phragmoplast forms the nascent cell plate. Initial phragmoplast guidance in o1 is normal; however, as phragmoplast expansion continues o1 phragmoplasts become misguided. To understand how O1 contributes to phragmoplast guidance, we identified O1-interacting proteins. Maize kinesins related to the Arabidopsis thaliana division site markers PHRAGMOPLAST ORIENTING KINESINs (POKs), which are also required for correct phragmoplast guidance, physically interact with O1. We propose that different myosins are important at multiple steps of phragmoplast expansion, and the O1 actin motor and POK-like microtubule motors work together to ensure correct late-stage phragmoplast guidance.
Both plants and animals rely on asymmetric cell division to generate new cell types and is a core characteristic of multicellular organisms. Prior to asymmetric cell division, cell polarity is established. Cell polarity establishment and asymmetric cell division are universally important, although proteins important for polarity differ in plants and animals. Zea mays stomatal development serves as an excellent plant model system for asymmetric cell division, especially the asymmetric division of the subsidiary mother cell (SMC). In SMCs, the nucleus migrates to a polar location, after the accumulation of polarly localized proteins but before the appearance of the preprophase band. We examined a mutant of the outer nuclear membrane protein, which is part of the LINC (linker of nucleoskeleton and cytoskeleton) complex that localizes to the nuclear envelope in interphase cells. Previously, mlks2 (maize linc kash sine-like2) were observed to have abnormal stomata. We confirmed these stomatal defects and used cell markers to pinpoint the precise defects that lead to abnormal asymmetric divisions. Proteins that are polarly localized in SMCs prior to division, including a polar accumulation of actin, polarize normally in milks2 and remain polarized throughout and after division. However, polar localization of the nucleus is impaired, even in cells that have otherwise normal polarity. This leads to a misaligned preprophase band and atypical division planes Localization of MLKS2 revealed it is able to associate with mitotic structures, however the structure of the preprophase band, spindle and phragmoplast appeared normal in mlks2. Time-lapse imaging in mlks2 revealed that the mutant had defects in pre-mitotic nuclear migration towards the polarized site, as well as an unstable position at the division site after formation of the preprophase band. Unpolarized nuclei lead to a misaligned preprophase band. Together, we show compelling genetic evidence that nuclear envelope proteins promote pre-mitotic nuclear migration and stable nuclear position, and that the position of the nucleus influences division plane establishment in asymmetrically dividing cells.
Stomata are epidermal pores that facilitate plant gas exchange. Grasses have fast stomatal movements, likely due to their dumbbell-shaped guard cells and lateral subsidiary cells. Subsidiary cells reciprocally exchange water and ions with guard cells. However, the relative contribution of subsidiary cells to stomatal closure is unresolved. We compared stomatal gas exchange and stomatal aperture dynamics in wild type andpan1, pan2, and pan1;pan2maize (Zea mays) mutants, which have varying percentages of aberrantly formed subsidiary cells. Stomata with 1 or 2 defective subsidiary cells cannot close properly, indicating that subsidiary cells are essential for stomatal function. Even though the percentage of aberrant stomata is similar inpan1 and pan2, pan2showed a more severe defect in stomatal closure. In pan1, only stomata with abnormal subsidiary cells fail to close normally. Inpan2,all stomata have stomatal closure defects, indicating that PAN2 has an additional role in stomatal closure. MaizePan2is orthologous toArabidopsis thaliana GHR1, which is also required for stomatal closure. PAN2 acts downstream of Ca2+ in maize to promote stomatal closure. This is in contrast to GHR1, which acts upstream of Ca2+, and indicates differently wired pathways in maize and Arabidopsis.
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