Aging later but faster: how StCDF1 regulates senescence in Solanum tuberosum.

In potato, maturity is assessed by leaf senescence, which, in turn, affects yield and tuber quality traits. Previously, we showed that the CYCLING DOF FACTOR1 (StCDF1) locus controls leaf maturity in addition to the timing of tuberization. Here, we provide evidence that StCDF1 controls senescence onset separately from senescence progression and the total life cycle duration. We used molecular-biological approaches (DNA-Affinity Purification Sequencing) to identify a direct downstream target of StCDF1, named ORESARA1 (StORE1S02), which is a NAC transcription factor acting as a positive senescence regulator. By overexpressing StORE1S02 in the long life cycle genotype, early onset of senescence was shown, but the total life cycle remained long. At the same time, StORE1S02 knockdown lines have a delayed senescence onset. Furthermore, we show that StORE1 proteins play an indirect role in sugar transport from source to sink by regulating expression of SWEET sugar efflux transporters during leaf senescence. This study clarifies the important link between tuber formation and senescence and provides insight into the molecular regulatory network of potato leaf senescence onset. We propose a complex role of StCDF1 in the regulation of potato plant senescence.

Genome sequencing of adapted diploid potato clones.

Cultivated potato is a vegetatively propagated crop, and most varieties are autotetraploid with high levels of heterozygosity. Reducing the ploidy and breeding potato at the diploid level can increase efficiency for genetic improvement including greater ease of introgression of diploid wild relatives and more efficient use of genomics and markers in selection. More recently, selfing of diploids for generation of inbred lines for F1 hybrid breeding has had a lot of attention in potato. The current study provides genomics resources for nine legacy non-inbred adapted diploid potato clones developed at Agriculture and Agri-Food Canada. De novo genome sequence assembly using 10× Genomics and Illumina sequencing technologies show the genome sizes ranged from 712 to 948 Mbp. Structural variation was identified by comparison to two references, the potato DMv6.1 genome and the phased RHv3 genome, and a k-mer based analysis of sequence reads showed the genome heterozygosity range of 1 to 9.04% between clones. A genome-wide approach was taken to scan 5 Mb bins to visualize patterns of heterozygous deleterious alleles. These were found dispersed throughout the genome including regions overlapping segregation distortions. Novel variants of the StCDF1 gene conferring earliness of tuberization were found among these clones, which all produce tubers under long days. The genomes will be useful tools for genome design for potato breeding.

BEL1-like protein (StBEL5) regulates CYCLING DOF FACTOR1 (StCDF1) through tandem TGAC core motifs in potato.

Tuberization in potato is governed by many intrinsic and extrinsic factors. Various molecular signals, such as red light photoreceptor (StPHYB), BEL1-like transcription factor (StBEL5), CYCLING DOF FACTOR1 (StCDF1), StCO1/2 (CONSTANS1/2) and StSP6A (Flowering Locus T orthologue), function as crucial regulators during the photoperiod-dependent tuberization pathway. StCDF1 induces tuberization by increasing StSP6A levels via StCO1/2 suppression. Although the circadian clock proteins, GIGANTEA (StGI) and FLAVIN-BINDING, KELCH REPEAT, F-BOX 1 (StFKF1), are reported as StCDF1 interactors, how the StCDF1 gene is regulated in potato is unknown. The BEL-KNOX heterodimer regulates key tuberization genes through tandem TGAC core motifs in their promoters. A recent study reported the presence of six tandem TGAC core motifs in the StCDF1 promoter, suggesting possible regulation of StCDF1 by StBEL5. In our study, we observed a positive correlation between StBEL5 and StCDF1 expression, whereas StCDF1 and its known repressor, StFKF1, showed a negative correlation for the tested tissue types. To investigate the StBEL5-StCDF1 interaction, we generated transgenic potato promoter lines containing a wild-type or mutated (deletion of six tandem TGAC sites) StCDF1 promoter fused to GUS. Wild-type promoter transgenic lines exhibited widespread GUS activity, whereas this activity was absent in the mutated promoter transgenic lines. Moreover, StBEL5 and StCDF1 transcript levels were significantly higher in the stolon-to-tuber stages under short-day conditions compared to long-day conditions. Using wild-type and mutated prStCDF1 as baits in Y1H assays, we further demonstrated that StBEL5 interacts with the StCDF1 promoter through tandem TGAC motifs, indicating direct regulation of StCDF1 by StBEL5 in potato.