WHIRLY2 plays a key role in mitochondria morphology, dynamics, and functionality in .

WHIRLY2 is a single-stranded DNA binding protein associated with mitochondrial nucleoids. In the mutant of , a major proportion of leaf mitochondria has an aberrant structure characterized by disorganized nucleoids, reduced abundance of cristae, and a low matrix density despite the fact that the macroscopic phenotype during vegetative growth is not different from wild type. These features coincide with an impairment of the functionality and dynamics of mitochondria that have been characterized in detail in wild-type and mutant cell cultures. In contrast to the development of the vegetative parts, seed germination is compromised in the mutant. In line with that, the expression level of in seeds of wild-type plants is higher than that of , whereas in adult plant no difference is found. Intriguingly, in early stages of shoots development of the mutant, although not in seeds, the expression level of is enhanced. These results suggest that WHIRLY3 is a potential candidate to compensate for the lack of WHIRLY2 in the mutant. Such compensation is possible only if the two proteins are localized in the same organelle. Indeed, protein transport experiments using intact mitochondria and chloroplasts revealed that WHIRLY3 can be dually targeted into both, chloroplasts and mitochondria. Together, these data indicate that the alterations of mitochondria nucleoids are tightly linked to alterations of mitochondria morphology and functionality. This is even more evident in those phases of plant life when mitochondrial activity is particularly high, such as seed germination. Moreover, our results indicate that the differential expression of and predetermines the functional replacement of WHIRLY2 by WHIRLY3, which is restricted though to the vegetative parts of the plant.