Table 1.

Mean chloroplast plan area, mean mesophyll plan area, and plastid number for populations of mesophyll cells from the youngest fully expanded leaves of 40-d-old control and 35S::ATHB17 Arabidopsis plantsChloroplast plan areas were measured by image analysis of fixed isolated cells from three 35S::ATHB17-overexpressing lines (T2 generation) and an empty-vector control line (pMEN65) in two independent experiments. Plastid number per cell (±SE) was determined from a regression line of chloroplast number per cell on mesophyll cell plan area using a value of 1000 μm² for the mesophyll cell plan area (Fig. 5). The y-intercept of the regression line was used to estimate the number of proplastids (±SE) in very small, meristematic cells. For Experiments 1 and 2, mean chloroplast plan area is the mean ±SE of 600 chloroplasts from 30 or 60 different mesophyll cells, respectively. **P <0.05.

Genotype	Mean chloroplast plan area (μm2)	Plastids
		No. chloroplasts per 1000 μm2 mesophyll cell plan area	Estimated number proplastids per meristematic cell
Experiment 1
pMEN65	39.8±0.4	35±2	14±2
35S::ATHB17 line 306	35.9±0.3**	51±1**	33±1**
35S::ATHB17 line 314	35.0±0.3**	55±1**	37±1**
35S::ATHB17 line 319	38.3±0.4**	48±1**	25±1**
Experiment 2
pMEN65	36.7±0.4	44±1	22±2
35S::ATHB17 line 306	32.7±0.3**	70±1**	48±1**
35S::ATHB17 line 314	32.7±0.3**	67±1**	44±1**
35S::ATHB17 line 319	30.5±0.4**	57±1**	34±1**