Spindle position dictates division site during asymmetric cell division in moss

Abstract

14Asymmetric cell division (ACD) underlies the development of multicellular organisms. 15The division site in plant cells is predetermined prior to mitosis and the localization of 16 the mitotic spindle is considered static, unlike in animal ACD, where the cell division site 17is defined by active spindle-positioning mechanisms. Here, we isolated a novel mutant of 18 the microtubule-associated protein TPX2 in the moss Physcomitrella patens and observed 19 abnormal spindle motility, which led to inverted asymmetric division during organ 20 development. This phenotype was rescued by restoring endogenous TPX2 function and, 21unexpectedly, by depolymerizing actin filaments. Thus, we identify an active spindle-22 positioning mechanism involving microtubules and actin filaments that sets the division 23 site in plants, which is reminiscent of the acentrosomal ACD in animals, and suggests the 24 existence of a common ancestral mechanism. 25 26 130 Alignment of TPX2 proteins. Conserved residues are boxed, whereas similar amino acids are hatched. (C) 131 Localization of endogenous TPX2-1-Citrine, mNeonGreen-TPX2-4 and TPX2-5-mNeonGreen. More 132 uniform spindle localization was detected for TPX2-5. Asterisks indicate autofluorescent chloroplasts. Bar, 133 10 µm. The full version of mitotic localization data is presented in Supplemental Figure 1.134

Phyllotaxis from a Single Apical Cell

“…Live imaging of shoot initials has revealed that the angle of the first three divisions is robust, indicating that division plane establishment is tightly controlled and crucial for phyllotaxis [9,32]. This is also suggested by mutants where apical cell formative divisions are defective and leafy shoot development cannot proceed normally [33], such as Defective Kernel1 (DEK1) [34,35], No Gametophores 1 (NOG1) [18], Targeting Protein for Xklp2 (TPX2) [36] or mutants in the CLAVATA (CLV) signaling pathway [37].…”

“…(B) In WT, following the first division of the shoot initial (i), the apical domain (marked by an asterisk) undergoes a series of oblique divisions (ii-iv) leading to the formation of the tetrahedral apical stem cell (coloured in green) at the origin of spiral phyllotaxis. (A) adapted from[9,36,37], (B) redrawn from[9] with permission from corresponding author.…”

Physcomitrium patens: A Single Model to Study Oriented Cell Divisions in 1D to 3D Patterning

“…In the study of these processes, the divisions in P. patens protonemata and buds have proven to be experimentally accessible models. For example, recent findings show that the microtubule-associated protein TPX2 is essential for maintenance of a central spindle position along the apical–basal axis in buds [ 62 ]. This defect could surprisingly be compensated by actin cytoskeleton disruption, which, under normal conditions, does not significantly interfere with spindle/phragmoplast positioning [ 21 , 62 ].…”

“…For example, recent findings show that the microtubule-associated protein TPX2 is essential for maintenance of a central spindle position along the apical–basal axis in buds [ 62 ]. This defect could surprisingly be compensated by actin cytoskeleton disruption, which, under normal conditions, does not significantly interfere with spindle/phragmoplast positioning [ 21 , 62 ]. These findings open new avenues for study on the mechanisms controlling “tugging” of the division apparatus during mitosis and its implications for division plane positioning.…”

Cell size: a key determinant of meristematic potential in plant protoplasts

“…Size control occurs at one or more checkpointsone that precedes the initiation of DNA synthesis (G1/S transition) and the other before nuclear division (G2/M transition). Critical cell size is obligatory for cell cycle progression and is entirely reliant on habituating conditions (García-Gó mez et al 2020;Kozgunova et al 2020;Winnicki 2020).…”