Photoperiodic inhibition of potato tuberization: an update

Photoperiodic induction of tuberization in potato (Solanum tuberosum L. Gp. Andigenum) under short days (SDs) is processed via miRNA (e.g. miR172)-mRNA (e.g. StBEL5) and gibberellin (GA) signaling pathways, with spatio-temporal activation of several transcription factors. There is good evidence that the photoreceptor phytochrome B (PHYB) inhibits tuberization under long days (LDs) by producing a graft-transmissible signal. Since it is mostly unknown how PHYB negatively regulates tuberization, the molecular identity of this PHYB-induced LD-inhibitory signal still continues to be elusive. A recent study reported PHYB-mediated photoperiodic regulation of GIGANTEA (GI), a flowering-control gene of Arabidopsis thaliana and rice, in the leaves of a potato plant. Although GI is long assumed to be involved in the photoperiodic control of potato tuberization as an upstream gene of both CONSTANS and FLOWERING LOCUS T, its exact role could not be elucidated. Thus, its preferential PHYB-dependent upregulation under LDs compared to SDs concomitant with an upregulation of the gene ENT-KAURENOIC ACID OXIDASE (KAO) that controls an early step in the gibberellin biosynthetic pathway provides an upstream molecular basis for tuberization inhibition in potato by LDs. The results are likely to revisit the roles of several signal molecules that are putatively implicated in the photoperiodic inhibition of tuberization. Given that GI constitutes an evolutionary conserved species-specific LD response pathway in the family Solanaceae, this review argues for an evolutionary basis of tuberization inhibition in potato inherent in its flowering response pathway.