Potato plants detect shorter days and colder nights as signs of impending winter, so they begin to develop tubers as a means of surviving the cold. During the winter, potato tubers lie dormant in the ground, only to spring into life and sprout new plants.

“Shorter day lengths or longer nights are a strict requirement for tuber formation in wild potato landraces, whereas mutations in the earliness locus evade this strict day length control and lead to cultivars forming tubers in long days. These variants appeared after potato introduction into Europe, whilst they were selected during modern potato breeding, most potato cultivars including at least one of the earliness alleles,” according to a recent Europatat press release.

The earliness locus is associated with shortened versions of CYCLING DOF FACTOR1 (StCDF1), a DOF domain protein that has been shown to suppress the production of potato homologs for CONSTANS (StCOLs). StCOLs cause the potato SP5G FT-like gene to become active, which hurts the tuberigen’s expression (FT SP6A) and consequent tuberization. StCOLs are constitutively suppressed by the shorter StCDF1 versions, which are more stable than the full-length repressor. This suppresses the production of the SP5G gene and activates the SP6A tuberigen, even during non-inductive long days.

The direct targets of StCDF1 were discovered through a partnership between partners from the University of Wageningen and the CRAG in Barcelona. This was accomplished by combining RNA-Seq and DNA binding-affinity purification with high-throughput sequencing (DAP-Seq). These investigations revealed that StCDF1 not only functions as the conventional repressor of the day duration pathway but also regulates a wide range of targets linked to other biological processes, such as many genes associated with N metabolism.

“This clock output regulates diurnal expression of various nitrate and amino acid transporters and strongly suppresses NITRATE REDUCTASE (NR/NIA), a limiting enzyme in nitrate assimilation that in potato is encoded by a single locus. This observation reveals that the earliness locus, facilitating tuber formation in long days, negatively affects nitrogen use efficiency (NUE), thus making modern potato cultivars more dependent on nitrate fertilizers for high tuber yields,” the experts mentioned.

The results of this study suggest that increasing the number of NIA2 copies from other plants or eliminating the StCDF1-binding sites from the potato NIA1 promoter have great potential to improve the efficiency of potato nitrate consumption.

The Horizon 2020 EU project Accelerated Development of multiple-stress tolerAnt PoTato (ADAPT), in which Europatat is participating, aims to understand the signaling pathways for single and combined abiotic stresses to the selection of potato cultivars more resilient to climate change.