Figure 1.

Simplified models and microscopy evidence for the control of chloroplast number in stomatal guard cells (GCs). (A) A typical stoma (GC pair) in abaxial epidermis of the Arabidopsis leaf blade expressing a stoma-targeted fusion of the transit peptide (TP) with cyan fluorescent protein (CFP; TP-CFP). (B) Two models of chloroplast number determination in GCs, involving either chloroplast partitioning (model 1) or both chloroplast proliferation and partitioning (model 2) during GC development from guard mother cells (GMCs). (C) A GC pair in adaxial epidermis of Arabidopsis leaf petiole expressing TP-CFP and FtsZ1 fused to the green fluorescent protein (GFP; FtsZ1-GFP). (D–F) GC pairs in abaxial epidermis of Arabidopsis leaf blade with (D, E) or without (F) the expression of TP fused to the yellow fluorescent protein (YFP; TP-YFP). (F) Chlorophyll autofluorescence (Chl) was used as a chloroplast marker. (G) Extended model 2, representing the involvement of equal and unequal chloroplast partitioning following GMC division and subsequent division of GC chloroplasts with equal (blue line) or selective (red line) division competency, which would result in four types of chloroplast number determination (Fates 1–4) during late stomatal development of Arabidopsis leaves. (A, C–F) Epifluorescence microscopy was performed with an Olympus IX71 inverted microscope using plant materials as previously described (Fujiwara et al., 2017, Fujiwara et al., 2018). Fluorescence signals of chlorophyll, CFP, GFP, and YFP are pseudo-colored in magenta, cyan, green (in merged image only), and green, respectively. Indications in panels are as follows: arrowhead, the FtsZ1 ring; arrow, enlarged GC chloroplast; dashed line, cell shape. Scale bar = 10 µm.