Intrinsic Cell Polarity Coupled to Growth Axis Formation in Tobacco BY-2 Cells

Abstract

Summary Several plant proteins are preferentially localized to one end of a cell, allowing a polarity to be assigned to the cell. These cell polarity proteins often exhibit coordinated patterns between neighboring cells, termed tissue cell polarity. Tissue cell polarity is widespread in plants and can influence how cells grow, divide, and differentiate [ 1 , 2 , 3 , 4 , 5 ]. However, it is unclear whether cell polarity is established through cell-intrinsic or -extrinsic mechanisms and how polarity is coupled to growth. To address these issues, we analyzed the behavior of a tissue cell polarity protein BASL (BREAKING OF ASYMMETRY IN THE STOMATAL LINEAGE) in the simplifying context of cultured cell filaments and in protoplasts before and during regeneration. We show that BASL is polarly localized when ectopically expressed in tobacco BY-2 cell cultures. Ectopic BASL is found preferentially at the developing tips of cell filaments, likely marking a polarized molecular address. Polarity can shift during the cell cycle and is resistant to treatment with microtubule, actin or auxin transport inhibitors. BASL also exhibits polar localization in spherical protoplasts, in contrast to other polarity proteins so far tested. BASL polarity within protoplasts is dynamic and resistant to auxin transport inhibitors. As protoplasts regenerate, polarity remains dynamic in isotropically growing cells but becomes fixed in anisotropic cells and aligns with the axis of cell growth. Our findings suggest that plant cells have an intrinsic ability to polarize and that environmental or developmental cues may act by biasing the direction of this polarity and thus the orientation of anisotropic growth.

Formin-mediated bridging of cell wall, plasma membrane, and cytoskeleton in symbiotic infections of Medicago truncatula

“…Using the polarity marker BREAKING OF ASYMMETRY IN THE STOMATAL LINEAGE (BASL) in BY-2 protoplasts, it was recently shown that these isolated, spherical cells have an intrinsic polarity. 23 This fact additionally requires a polarization of the secretion machinery for delivering membrane material and proteins. To test the effect of polarized SYFO1 secretion in protoplasts, we Report generated a transgenic M. truncatula root organ culture (ROC) constitutively expressing SYFO1-GFP.…”

Mechanobiology of cell division in plant growth

“…However, ectopically expressed BASL or the binding partner BRXL shows alignment with global tissue polarity (Bringmann & Bergmann, 2017; Mansfield et al ., 2018) and with tissue scale mechanical stress (Bringmann & Bergmann, 2017). BASL polarity can also form spontaneously in tobacco BY2 cells and aligns with the axis of growth (Chan et al ., 2020). These results suggest that asymmetrical divisions could be sensitive to mechanical stress, however further work is needed to determine the mechanism and distinguish between the competing theories.…”

On the edge: Evolution of polarity protein BASL and the capacity for stomatal lineage asymmetric divisions

“…Previous efforts to reconstruct the evolutionary history of BASL have been hampered by low overall sequence conservation beyond close relatives of Arabidopsis (Chan et al, 2020;Vatén and Bergmann, 2012), although select protein domains could be found in more distantly related species (Zhang et al, 2016a). Using a reciprocal best hit heuristic, we found a rapid drop-off in sequence similarity of candidate BASL orthologues outside the Brassicaceae, with a plateau of around 60% amino acid similarity (30% identity) for eudicots followed by a sharp drop to ~30% similarity (10% identity) for non-eudicot candidates (Figure S1A).…”

“…In fact, BSLL gene expression is enriched in the stomatal precursor zone at the base of rice and maize leaves (Wang et al, 2014). Therefore, to characterize BASL and BSLL homologues, we began by assaying the feature that best defines BASL-its polarized subcellular localization, which can be replicated in non-native contexts (Chan et al, 2020;Dong et al, 2009;Zhang et al, 2015). We expressed BASL and/or BSLL coding sequences from Arabidopsis thaliana, Solanum lycopersicum, Aquilegia coerulea and the monocot outgroup Brachypodium distachyon in developing Nicotiana benthamiana leaves (Figure S2A-B).…”

“…Despite a pivotal role in Arabidopsis stomatal lineage development, and the fact that asymmetric divisions have been observed in the stomatal lineages of angiosperms, gymnosperms and ferns (Rudall et al, 2013), it is not known if BASL acts in diverse stomatal lineages. This is due, in part, to debates as to whether BASL homologues are even present in other plants (Chan et al, 2020;Vatén and Bergmann, 2012). Here we use a combination of phylogenetic analyses, cross-species complementation, and functional analysis in native contexts to identify likely BASL orthologues.…”