Fig. 4: TraB family proteins are essential for normal mitochondrial structure and energy metabolism.

a–c In the trb1/trb2 T-DNA mutant Arabidopsis line stably expressing Mito-mCherry, the morphology of mitochondria is significantly altered, as indicated by the increased population of circular mitochondria (aspec ratio at 1-1.3, c). Mitochondrial aggregations (b, inset) were also frequently observed. In the trb1/trb2 Arabidopsis, mitochondrial structures are also disrupted at the ultrastructural level (e); their internal membrane cisternae are more fragmented and the matrix becomes more electron-transparent compared with that in the wild type (d). f Quantification of the mitochondrial cristae coverage of trb1/trb2 mutant and wild-type Arabidopsis. The levels of ATP, ADP and AMP are all significantly reduced in the trb1/trb2 mutant (g), and the ATP/ADP ratio is also lower in the mutant (h), both results suggested a disruption in energy metabolism and mitochondrial function. i–k The starch levels in Col-0, trb1/trb2 T-DNA plants (4–5 weeks old) were analysed by Lugol staining and the trb1/trb2 loss-of-function mutant accumulated high levels of starch by the end of the light cycle. Such results were confirmed by spectrophotometry (k). l Under dark conditions, starch consumption is reduced in the trb1/trb2 mutant, suggesting a reduction in mitochondrial respiration. n = 100 mitochondria from 10 cells for mitochondrial morphology analysis; n = 20 mitochondrial number over three biologically independent samples for TEM analysis; n = 9 biologically independent plants examined over three independent experiments for starch quantification; n = 8 biologically independent samples for ATP measurement, error bars are SEM, * 0.01 < P < 0.05, *** P < 0.001 in two-tailed unpaired t tests with Welch’s correction (scale bar = 10 μm for light microscopy; scale bar = 500 nm for TEM).