HLP1, a protein from the hnRNP A/B family, is crucial for regulating flowering time in Arabidopsis via an FLC-dependent pathway. Using BLAST searches with the amino acid sequence of Hrp1 (a known alternative polyadenylation (APA) factor in yeast), we identified a novel RNA-binding protein (RBP), At5g40490, in Arabidopsis. Phylogenetic analysis revealed that HLP1 is closely related to the human hnRNP A1 and yeast Hrp1, which led us to name it HLP1 (hnRNP A1-like protein 1 or Hrp1-like protein 1).

HLP1 shares significant homology with the RNA recognition motifs (RRMs) of both hnRNP A1 and Hrp1, although other regions show lower sequence conservation. This suggests that HLP1 may play a pivotal role in regulating flowering time and post-transcriptional gene regulation, similar to its mammalian and yeast counterparts.

To explore the function of HLP1, we created a T-DNA insertion mutant (hlp1-1). RT-PCR and immunoblotting analysis confirmed the absence of HLP1 in the hlp1-1 mutant. These mutants exhibited a delayed flowering phenotype accompanied by elevated levels of FLC gene expression. Notably, hlp1-1 mutants showed a delayed flowering phenotype under both long-day and short-day photoperiods, while retaining normal responses to vernalization and gibberellic acid (GA) treatments.

To confirm that HLP1 regulates flowering time through FLC, we crossed hlp1-1 with the flc-3 null allele of FLC. The late-flowering phenotype of the hlp1-1 mutant was suppressed in the flc-3 background, and the phenotype was reversed by introducing a GFP-fused full-length HLP1 construct. This indicates that HLP1 is a key regulator of the floral transition, acting through an FLC-dependent mechanism.