Division polarity in developing stomata

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Stomata are generated via asymmetric cell division in both dicots and monocots. Intrinsic or extrinsic polarity cues are perceived and acted upon to generate mother cell polarity and determine asymmetric division planes. Arabidopsis employs both intrinsic and extrinsic cues to orient a variable series of asymmetric stomatal divisions, using novel proteins such as BASL and POLAR to generate polarity. In contrast, maize appears to employ only extrinsic cues to orient the polarities of divisions occurring in an invariant sequence to generate stomatal complexes. Although both plants use receptor-like kinases to generate or orient division polarity in developing stomata, there are few similarities in the proteins and pathway identified to date as regulators of these processes.

Highlights

Cell polarities must be generated and perceived to initiate asymmetric cell division. ► Cell polarities are generated in response to intrinsic or extrinsic cues. ► The sequence of asymmetric cell divisions that occur in stomatal lineages of Arabidopsis and maize differ. ► The molecules and mechanisms described so far leading to stomatal cell polarities also differ in Arabidopsis and maize.

Introduction

Asymmetric divisions, which generate daughters with distinct developmental fates, underlie pattern formation, establishment of new cell lineages, and formation of specialized cell types during plant development. Such divisions are often physically asymmetric as well, generating daughter cells with different sizes and/or shapes. Asymmetric division polarity is important not only for proper placement of different cell types within a developing tissue, but also for specification of daughter cell fates [1, 2, 3••]. To achieve physical division asymmetry, the mother cell becomes polarized such that the premitotic nucleus lies within the future division plane. The dividing nucleus is then maintained at the asymmetric location and the cell plate is ultimately attached there (Figure 1). Either intrinsic cues (already existing within the cell) or extrinsic cues (coming from outside the cell) provide the spatial information to orient mother cell polarity (Figure 1). Here we review recent work contributing insights into mechanisms that achieve and orient division polarity during stomatal development in both dicots and monocots (exemplified by Arabidopsis thaliana and maize, respectively). Excellent recent reviews provide a more comprehensive overview of stomatal development including aspects not discussed here [4••, 5••].

Section snippets

Dicots: variable division sequences

Stomata in dicots are produced via asymmetric divisions in which the smaller daughter is a stomatal precursor cell called a meristemoid (Figure 2a, [6]). A meristemoid can differentiate directly into a guard mother cell (GMC), which divides symmetrically to form a guard cell pair [6]. Alternatively, it can undergo additional asymmetric divisions (each producing a meristemoid and a nonstomatal cell) before differentiation of the meristemoid as a GMC (Figure 2a, [6]). The orientations of these

Monocots: invariant division sequences

Grass stomatal complexes are composed of four cells (a pair of guard cells flanked by a pair of subsidiary cells), which are generated by an invariant series of oriented divisions (Figure 3a). The grass leaf epidermis is organized into linear rows of cells; stomata are found in rows where they alternate with interstomatal cells, a pattern that is established when every cell in a file of stomatal precursors divides once asymmetrically in the same orientation to produce a GMC (Figure 3a).

Conclusion: asymmetries between dicots and monocots in stomatal symmetry breaking

The overall patterning of the leaf surface of monocots and dicots is fundamentally different, so it may not be surprising that mechanisms governing division polarity during stomatal development appear so far to be very different. While the orientations of Arabidopsis stomatal divisions appear to be determined by both intrinsic and extrinsic cues, it appears that maize stomatal divisions are oriented only by extrinsic cues. Although little is known at a mechanistic level about the asymmetric

References and recommended reading

Papers of particular interest, published within the period of review, have been highlighted as:

  • • of special interest

  • •• of outstanding interest

Acknowledgements

Research in the authors’ laboratory on the topic of this review is supported by NSF grants NSF IOS-0843704 and IOS-1147265 to LGS. We thank Dominique Bergmann for stimulating discussions about many issues discussed in this review.

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