A bZIP transcription factor accelerates the transition to reproductive tuber growth and aging in Solanum tuberosum

Solanaceous plants, such as potato, tomato, and pepper, form key components of the global food supply. This is especially true for potato, the vegetative storage organs of which are a key food source for starch and vitamin C. Solanum tuberosum, or domesticated potato, falls under a category of plants known as geophytes. Geophytes are plants that form vegetative storage organs, or tubers, from modified stems known as stolons (reviewed in Khosa et al., 2021). Geophytes also employ two modes of reproduction: sexual reproduction involving flowers and vegetative reproduction through tubers. Switching between these two modes reproduction is a fitness strategy that allows geophytes to flourish in harsh climates, such as the mountainous South American Andes where potato was first cultivated. Typically, flowering is a process more appropriate for long summer days. In contrast, tuberization is a reproductive strategy better suited for short winter days (reviewed in Khosa et al., 2021).

Genetic regulation of these two modes of reproduction is complex but key flowering time genes play a significant role for these developmental processes. Perhaps, the best-known regulators of the transition to reproductive growth are members of the PHOSPHATIDYLETHANOLAMINE-BINDING PROTEIN gene family, the most famous of which is the florigen FLOWERING LOCUS T (FT; reviewed in Khosa et al., 2021). In many flowering plants, the expression of FT is regulated by photoperiod, with the switch from short days to long days. The perennial potato possesses two FT homologs that are each a master regulator of either long-day-induced flowering or short-day-induced tuberization. The florigen SELF-PRUNING 3D (StSP3D) promotes flowering under long-day conditions in distinction to the tuberigen SELF-PRUNING 6A (StSP6A), which promotes tuberization under short-day conditions (Navarro et al., 2011). Furthermore, the induction of StSP6A expression accelerates developmentally regulated aging (senescence; Lehretz et al., 2019). Despite growing knowledge about protein complexes related to potato FTs, the molecular mechanism of these genes synchronizing tuberization, flowering, and lifespan remains poorly understood. Here, (Jing et al., 2022) set out to discover and characterize a novel member of alternative tuberigen and florigen activation complexes. In their studies, the authors identify and characterize the bZIP transcription factor (TF) StABI5-like 1 (StABL1) as a novel regulator of maturity and reproductive growth in domesticated potato.

To identify novel molecular regulators of reproductive growth in potato, the authors performed a protein–protein BLAST search using sequences from the bZIP TF FLOWERING LOCUS D-like 1 (StFDL1), a protein known to form a complex with potato FTs (Teo et al., 2010). As such, StABL1 was identified as a candidate tuberization regulator. Protein interaction assays would later reveal that StABL1 physically interacts with the FT protein StSP6A, thus hinting that StABL1 plays a role in the transition to reproductive growth in potato.

Jing et al. used RNAi mutants to reduce StABL1 function as well as created plants overexpressing StABL1. The transgenic plants were assayed under short-day conditions for tuberization phenotypes in vitro. Compared to wild-type and StABL1-RNAi plants, StABL1-overexpression (OE) mutants displayed accelerated time to tuberization. On the other hand, StABL1-RNAi mutants showed increased microtuber size and weight compared to wild-type and StABL1-OE mutants. Once plants were transferred to in vivo short-day conditions, StABL1-OE plants exhibited more swollen tubers much earlier than the wild-type. In contrast, StABL1-RNAi plants showed no difference in time to tuberization compared to the wild-type, likely due to genetic redundancy of StABL1 and other group A bZIP TFs. StABL1-OE plants were also found to have accelerated aging phenotypes when grown in soil under long-day conditions. Because of these early maturity phenotypes, StABL1-OE also possessed lower tuber yield than the wild-type. This relationship makes sense because if plants initiate reproductive growth prior to having the optimal mount of nutrient stores, yields are often negatively impacted.

The authors performed chromatin immunoprecipitation sequencing (ChIP-seq) assay using transgenic plants with an StABL1 construct tethered to GFP. After performing ChIP-seq, the authors observed reasonably standard patterns on TF binding within the potato genome. Using their ChIP-seq data, the authors took advantage of a TF motif-discovery algorithm (HOMER) to identify enrichment of StABL1-associated sequences (Heinz et al., 2010). Known TF motif enrichment showed that basic leucine zipper/ basic helix–loop–helix (bZIP/bHLH) and TEOSINTE BRANCHED1/CYCLOIDEA/PCF (TCP) motifs were overrepresented in StABL1 ChIP-seq data. De novo motif enrichment yielded similar findings with ABI5/bHLH, TCP, TALE-type motifs being highly abundant. The algorithm also identified a core CACGTG motif that was consistent with a previous publication from the authors’ laboratory (Liu et al., 2019).

In tandem with their ChIP-seq experiment, the authors performed RNA sequencing (RNA-seq) on wild-type and StABL1-OE mutants. After identifying differentially expressed genes (DEGs), Jing and colleagues partitioned their DEGs in StABL1 targets and nontargets. StABL1 targets were defined as genes with a nearby StABL1 peak. Using these criteria, approximately 16,000 genes were classified as StABL1 targets. Gene ontology terms amongst differentially expressed targets were consistent with the processes required for initiation of tuber growth, phytohormone responses, and cell signaling.

Gibberellic acid (GA) is the best-characterized phytohormone known to block tuberization in potato (Navarro et al., 2011; Kloosterman et al., 2013). GA 2-oxidase 1 (StGA2ox1) catalyzes the hydroxylation of biologically active GA compounds to inactive Gas, thereby repressing the activity of GA (Navarro et al., 2011; Kloosterman et al., 2013). Adding support to the relationship between tuberization and GA signaling, the authors observed StABL1 interacting with the promoter StGA2ox1 in their ChIP-seq data. An expression time course for StGA2ox1 indicated that under short days, StGA2ox1 levels were increased in StABL1-OE leaves and stolon tips relative to the wild-type. When GA levels were profiled in StABL1-OE stolons, the authors saw that about half of the measured GAs were either absent or present in near trace amounts and four GA species showed largely decreased levels. Adding support to the authors’ findings, the GA profile of the StABL1-OE stolons closely matched that of the StGA2ox1 overexpression mutants (Kloosterman et al., 2013). When StABL1-OE plants were treated with exogenous GA, they displayed resistant phenotypes like initiation of tuberization. All together, these data support StABL1 as a direct regulator of StGA2ox1 (Liu et al., 2019).

In this article, Jing et al. describe additional complexity for the transition to reproductive growth in cultivated potato. The authors demonstrated that a novel FLOWERING LOCUS D-like protein, StABL1, can form an alternative tuberigen and florigen complexes. Furthermore, the authors observed that the overexpression of StABL1 was able to accelerate transition to tuberization as well as lead to rapid maturity and aging. The authors were also able to link the activity of StABL1 on a specific phytohormone signaling pathway, GA. Typically, GA represses the transition to tuberization. Most strikingly, the authors observed that StABL1 overexpression mutants were resistant to exogenous GA treatment, therefore, emphasizing the importance of StABL1 on the modulation of this signaling pathway. StABL1 is clearly important for the development of tubers and timing of developmental transitions; it will therefore be interesting to understand more of the mechanistic details of its effects on reproductive development.

Conflict of interest statement. The author states that they have no conflicts of interest.