Cell-to-cell connectivity and stomatal patterning

CHOR is required to restrict stomatal cell-lineage divisions. (A-C) Promoter activity of TMM(proTMM::GUS-GFP) in 6-day-old cotyledon epidermis of wild type (A) and chor (B,C). In chor,clusters of small cells (dots) show high GFP signals. Pavement cells in chor occasionally show incomplete cytokinesis (arrowhead). (D-F) Expression of MUTE (proMUTE::MUTE-GFP) in 6-day-old cotyledon epidermis of wild type (D) and chor (E, F). Clusters of small stomatal-lineage cells are in brackets. MUTE expression in adjacent meristemoids (asterisks) predicts the eventual formation of clustered stomata. (G-I) Expression of SPCH (proSPCH::SPCH-GFP) in 3-day-old cotyledon epidermis of wild type (G) and chor (H, I). More cells express GFP signals in chor than in wild type. (J-L) Effects of spch on the excessive stomatal-lineage divisions in chor. Shown is 10-day-old cotyledon epidermis of spch (J) and spch chor (K,L). Small, highly divided cells are no longer produced in spch chor (K,L), whereas incomplete cytokinesis of pavement cells is still evident (K,L, arrowheads). Cell peripheries were highlighted by propidium iodide (PI; A-F,J-L) or FM4-64 (G-I). A-F, G-I, and J-L were taken under the same magnification. Scale bars: 20 μm.

Dysregulation of cell-to-cell connectivity and stomatal patterning by loss-of-function mutation in Arabidopsis CHORUS (GLUCAN SYNTHASELIKE 8).

by Guseman J. M., Lee J. S., Bogenschutz N. L., Peterson K. M., Virata R. E., Xie B., Kanaoka M. M., Hong Z., Torii K. U. (2010)

in Development 137:1731–1741. – doi: 10.1242/dev.049197 –

http://dev.biologists.org/content/137/10/1731 

Fig. 9. Model of phenotypical consequences of the loss of CHOR (GSL8) and signaling receptors. (A) In wild type, stomatal cell-fate determinants (cyan) are contained in a precursor cell, and inhibitory signals (red T bar) prevent its neighbors from adopting a stomatal-lineage fate. (B) In chor, cell-fate determinants (cyan) may leak through plasmodesmata, which results in excessive stomatal entry divisions. The functional inhibitory signals, however, are able to prevent the stomatal differentiation of neighbors at some level. This results in modest stomatal clusters. Incomplete cytokinesis also occurs in chor. (C) In the absence of both CHOR (GSL8) and signaling receptors (such as TMM), the loss (or reduction at the threshold level in er erl1) of inhibitory signals together with the leakage of cell-fate determinants leads to catastrophic stomatal patterning defects. – http://d2qiws50qrj9uc.cloudfront.net/content/develop/137/10/1731/F9.medium.gif

Summary

Patterning of stomata, valves on the plant epidermis, requires the orchestrated actions of signaling components and cell-fate determinants.

To understand the regulation of stomatal patterning, we performed a genetic screen using a background that partially lacks stomatal signaling receptors.

Genetic interactions of chor with stomatal patterning mutants. (A-L) Abaxial epidermis of rosette leaves (first pairs) from 12-day-old seedlings of the following genotypes: wild type (wt, A), chor (B), tmm (C), tmm chor (D), er erl1 erl2 (E), er erl1 erl2 chor (F), er (G), er chor (H), er erl1/+ (I), er erl1/+ chor (J), er erl2 (K) and er erl2 chor (L). chor in a wild-type background exhibits weak stomatal patterning defects and islands of small cells (dotted bracket; B), but the chormutation severely enhances stomatal clustering in tmm (C,D) and in the er erl1 erl2 triple mutant (E,F). er erl1/+ and er erl2 seedling leaves do not exhibit stomatal patterning defects (I,K); however, introduction of chor confers severe stomatal clustering defects (J,L). The er chordouble-mutant phenotype appeared additive, with increased small cells (dotted bracket) and incomplete cytokinesis (arrowheads, G,H). Images are taken under the same magnification. Scale bar: 20 μm.

Here, we report the isolation and characterization of chorus (chor), which confers excessive proliferation of stomatal-lineage cells mediated by SPEECHLESS (SPCH). chor breaks redundancy among three ERECTA family genes and strongly enhances stomatal patterning defects caused by loss-of-function in TOO MANY MOUTHS. chor seedlings also exhibit incomplete cytokinesis and growth defects, including disruptions in root tissue patterning and root hair cell morphogenesis.

CHOR encodes a putative callose synthase, GLUCAN SYNTHASE-LIKE 8 (GSL8), that is required for callose deposition at the cell plate, cell wall and plasmodesmata. Consistently, symplastic macromolecular diffusion between epidermal cells is significantly increased in chor, and proteins that do not normally move cell-to-cell, including a fluorescent protein-tagged SPCH, diffuse to neighboring cells.

Such a phenotype is not a general trait caused by cytokinesis defects.

Our findings suggest that the restriction of symplastic movement might be an essential step for the proper segregation of cell-fate determinants during stomatal development.