The intracellular and intercellular cross-talk during subsidiary cell formation in Zea mays: existing and novel components orchestrating cell polarization and asymmetric division

Abstract

Background: Formation of stomatal complexes in Poaceae is the outcome of three asymmetric and one symmetric cell division occurring in particular leaf protodermal cells. In this definite sequence of cell division events, the generation of subsidiary cells is of particular importance and constitutes an attractive model for studying local intercellular stimulation. In brief, an induction stimulus emitted by the guard cell mother cells (GMCs) triggers a series of polarization events in their laterally adjacent protodermal cells. This signal determines the fate of the latter cells, forcing them to divide asymmetrically and become committed to subsidiary cell mother cells (SMCs).

Scope: This article summarizes old and recent structural and molecular data mostly derived from Zea mays, focusing on the interplay between GMCs and SMCs, and on the unique polarization sequence occurring in both cell types. Recent evidence suggests that auxin operates as an inducer of SMC polarization/asymmetric division. The intercellular auxin transport is facilitated by the distribution of a specific transmembrane auxin carrier and requires reactive oxygen species (ROS). Interestingly, the local differentiation of the common cell wall between SMCs and GMCs is one of the earliest features of SMC polarization. Leucine-rich repeat receptor-like kinases, Rho-like plant GTPases as well as the SCAR/WAVE regulatory complex also participate in the perception of the morphogenetic stimulus and have been implicated in certain polarization events in SMCs. Moreover, the transduction of the auxin signal and its function are assisted by phosphatidylinositol-3-kinase and the products of the catalytic activity of phospholipases C and D.

Conclusion: In the present review, the possible role(s) of each of the components in SMC polarization and asymmetric division are discussed, and an overall perspective on the mechanisms beyond these phenomena is provided.

Fig. 1.

Diagram illustrating the development of Z. mays stomatal complexes.

Fig. 2.

Diagram showing the organization of the MTs (green lines) and the AFs (red lines) in prophase SMCs of Z. mays (from Panteris et al., 2006).

Fig. 3.

Immunolocalization of PAN1 in developing Z. mays stomatal complexes of seedlings grown under physiological conditions (A–C) and in others treated with phospholipase modulators (D–F). The nuclei are shown in red. Immunodetection of PAN1 was performed following the procedure described in Cartwright et al. (2009). The anti-PAN1 antibody was kindly provided by Professor L. G. Smith (University of California San Diego, USA). (A) GMCs on both sides of which SCs are or will be formed. The arrowheads point to local PAN1 aggregations. (B and C) Young stomatal complexes in successive developmental stages. The arrowheads indicate local PAN1 aggregations in differentiating SCs. (D and E) SMCs treated with 1-butanol (But-1). The arrowheads point to PAN1 aggregation in the SMC polar site. Note that in all the SMCs the nucleus has not occupied its polar position. Treatment: 20 mm 1-butanol for 24 h. (F) Part of a stomatal row of a seedling treated with neomycin. The arrowheads point to local PAN1 aggregations. Treatment: 100 mm neomycin for 24 h. Scale bars = 10 μm.

Fig. 4.

Early markers of polarization in SMCs of Z. mays (BRK1–CFP in A, 2F4 in B). (A) SMCs in contact with young GMCs. The arrowheads point to the SMC–GMC interface where the BRK1–CFP signal is brighter than the adjacent portions of the cell surface. BRK1–CFP becomes polarized at the young GMC (Photo courtesy of Yeri Park and Laurie Smith, Division of Biological Sciences, UC San Diego, USA). (B) Immunolocalization of the 2F4-HGA epitope in SMCs that are in contact with young GMCs. The 2F4-HGA epitope is strictly located at the common cell wall between the SMC and GMC (as marked by the arrowheads). Scale bars = 10 μm.

Fig. 5.

AF organization in Z. mays GMCs and SMCs. AF staining was performed according to Panteris et al. (2006, 2007). (A and B) Optical sections passing through the surface (A) and the middle (B) of a young GMC. The AFs mainly line the transverse cell walls (arrowheads). (C) Optical section passing through the middle of an advanced GMC. Intense AF aggregations line the whole surface of the lateral cell walls of the GMC (arrowheads). The arrow points to the AF patch of the neighbouring SMC. (D) Advanced GMC and SMC treated with 20 mm 1-butanol (But-1) for 24 h. The AF organization in them is disturbed compared with the respective cell types of the untreated seedlings (compare with C). Scale bars = 10 μm.

Fig. 6.

Diagram illustrating the possible mechanism of auxin and ROS accumulation in the apoplast between the SMC and inducing GMC. Briefly, in the advanced GMC, PIN1 auxin carriers are redistributed presumably in a cortical AF-dependent manner, whereas PI3K and phospholipases are thought to be directly or indirectly involved in the activation of PIN1 carriers. Afterwards, auxin is transported from the GMC to the apoplast between the GMC and SMC. The auxin accumulation in the apoplast is likely to induce, in turn, ROS production catalysed by NADPH oxidases, mediated by PI3K.

Fig. 7.

Diagrammatic representation of the possible steps of the SMC protrusion towards the inducing GMC and the formation of the AF-patch. (1) ROS and auxin accumulation in the apoplast between the GMC and SMC might modulate the activity of pectin methylesterases which catalyse the de-esterification of HGAs. (2) The latter de-esterified HGAs are localized exclusively at the common cell wall between the two cells and contribute to its deformation. Consequently, SMC grows locally towards the GMC. (3) It is also likely that the de-esterified HGAs at the cell wall in the proximity of the polar SMC site are involved in ROS generation as well as in the recruitment of the SWRC in the SMC plasmalemma lining this region. (4) SWRC recruits PAN2 at the polar SMC site, which in turn recruits PAN1. (5) PAN1 facilitates local accumulation and activation of ROPs. (6) Activated ROPs interact with the SWRC, triggering the activation of the ARP2/3 complex which leads in the end to the formation of the AF patch. (7) The presence of the latter is implicated in the assembly of a respective ER patch at the polar SMC site.

Fig. 8.

Diagram depicting the possible mechanism of the induction of nuclear division of the SMC. Auxin accumulated in the apoplast between the GMC and SMC seems to be transferred to the cytoplasm of the polar SMC site. The presence of auxin leads to the accumulation of ROS in the SMC cortical cytoplasm, presumably via the activation of PI3K. Oligogalacturonide molecules originating from de-esterified HGA breakdown at the cell wall in the vicinity of the polar SMC site might also stimulate ROS production. Further, the AGPs localized at the same site probably contribute to the accumulation of Ca²⁺. Higher calcium concentrations, auxin and ROS modulating the function of the actomyosin system mediate the polar migration of the nucleus close to the inducing GMC. Auxin, ROS and phospholipases at the polar SMC site might be involved in the induction of nuclear division in multiple ways, i.e. triggering the activation of MAPK cascades or the expression of auxin-responsive genes.

Fig. 9.

Diagram showing the possible mechanism of MT-PPB assembly in SMCs. The division plane is delineated by the MT-PPB at the cortical cytoplasm lining the contacts between expanding and non-expanding SMC cell wall regions. Maximum mechanical stress generated at these regions triggers the reorganization of cortical MTs in these sites and their bundling, leading to the generation of the MT-PPB.