chloroplasts, dual targeting, nucleus, transcription factor, Whirly proteins
Whirly proteins belong to a small family of proteins with a characteristic secondary structure and a conserved DNA binding domain that is found mainly in angiosperms. At least one member of the Whirly protein family, Whirly1, is dually targeted to the nucleus and to the chloroplasts and it was shown that apart from its initially described function as a transcriptional regulator of nuclear disease resistance genes, this protein comprises many more functions. It seems to fulfil roles in nuclear telomere homeostasis and possibly chloroplast rRNA metabolism as well as chloroplast intron splicing. A homologous protein with a mitochondrial presequence, Whirly2, in contrast, is presumably involved in the replication of the mitochondrial genome and in mitochondrial gene expression. In addition, it seems to affect the expression of a small subset of nuclear genes. Both Whirly proteins show an antagonistic effect on leaf senescence. Although direct evidence for a nuclear localisation of Whirly2 has yet to be obtained, we hypothesise that all members of the Whirly family are intriguing candidates for organelle-to-nucleus crosstalk with an intricate interaction between each other.