Nuclear import of LIKE HETEROCHROMATIN PROTEIN1 is redundantly mediated by importins α-1, α-2 and α-3

LIKE HETEROCHROMATIN PROTEIN1 (LHP1) encodes the only plant homologue of the metazoan HETEROCHROMATIN PROTEIN1 (HP1) protein family. The LHP1 protein is necessary for proper epigenetic regulation of a range of developmental processes in plants. LHP1 is a transcriptional repressor of flowering-related genes, such as FLOWERING LOCUS T (FT), FLOWERING LOCUS C (FLC), AGAMOUS (AG) and APETALA 3 (AP3). We found that LHP1 interacts with importin alpha-1 (IMP alpha-1), importin alpha-2 (IMP alpha-2) and importin alpha-3 (IMP alpha-3) both in vitro and in vivo. A genetic approach revealed that triple mutation of imp alpha-1, imp alpha-2 and imp alpha-3 resulted in Arabidopsis plants with a rapid flowering phenotype similar to that of plants with mutations in lhp1 due to the upregulation of FT expression. Nuclear targeting of LHP1 was severely impaired in the imp alpha triple mutant, resulting in the de-repression of LHP1 target genes AG, AP3 and SHATTERPROOF 1 as well as FT. Therefore, the importin proteins IMP alpha-1, -2 and -3 are necessary for the nuclear import of LHP1.