Scientists reveal sugar transporter gene regulation in peony by ABA signals

Tree peony, one of the most celebrated ornamental flowers in the world and a plant of immense commercial value in horticulture, has long posed a stubborn challenge for growers: when cultivated in containers, it tends to become stunted and its flower buds often abort before they ever open. A new study published in BMC Plant Biology offers a detailed molecular explanation for this phenomenon and, more importantly, a genetic pathway toward solving it. Researchers at Henan Agricultural University in Zhengzhou, China, have mapped the regulatory circuitry controlling a key sucrose transporter gene in tree peony, revealing how the plant hormone abscisic acid orchestrates sugar distribution throughout the plant through a family of transcription factors.

The central problem the team set out to address is one of supply and demand. In container cultivation, tree peony roots are confined and the plant’s access to carbohydrates becomes restricted. Sucrose, the primary form in which photosynthetically fixed carbon travels through the phloem from source tissues such as leaves to sink tissues such as stems, roots, and developing buds, is critical for vegetative growth and flowering. When sucrose delivery falters, growth slows and buds wither. Previous work by the same group had established that a sucrose transporter gene called PsSUT1 plays a pivotal role in facilitating sucrose movement within the plant, thereby promoting vegetative growth and flowering. The new study digs one level deeper into the regulatory hierarchy, asking not just what PsSUT1 does, but how its expression is controlled.

To answer that question, the researchers first needed to characterize the stretch of DNA upstream of the PsSUT1 gene that acts as its promoter, the molecular control panel that determines when, where, and how strongly the gene is switched on. Using chromosome walking, a technique that allows researchers to sequentially identify DNA adjacent to a known sequence, they cloned a 1,606-base-pair promoter region. A close examination of this sequence revealed the presence of ABRE motifs, short DNA elements recognized by proteins involved in abscisic acid signaling. This was a significant clue, because abscisic acid, or ABA, is a hormone best known for coordinating plant responses to stress, but which also influences sugar transport and developmental transitions.

With the promoter in hand, the team turned to expression analysis. They found that PsSUT1 is expressed most abundantly in petioles, the stalks that connect leaves to stems, and least in roots, suggesting the transporter is particularly active in the channels through which leaf-produced sucrose exits toward the rest of the plant. When the researchers treated tree peony tissues with ABA, transcript levels of PsSUT1 rose across all tissues examined, with the most dramatic response in petioles, where expression increased approximately fivefold. This hormone responsiveness, combined with the ABRE motifs in the promoter, pointed toward a regulatory mechanism mediated by ABA-responsive transcription factors.

To confirm that the cloned promoter truly behaves as predicted, the researchers constructed reporter systems. In one set of experiments, they fused the PsSUT1 promoter to the GUS reporter gene and introduced it into the model plant Arabidopsis thaliana. These promoter::GUS lines showed staining in seedlings, roots, leaves, and siliques, mirroring the expression pattern observed in tree peony itself, and crucially, the reporter responded clearly to ABA treatment. In parallel, transient expression assays in Nicotiana benthamiana, a tobacco relative widely used in plant molecular biology for rapid functional testing, confirmed that the promoter drives robust reporter activity. Together, these results established that the 1,606-base-pair promoter captures the essential regulatory features of PsSUT1.

The next step was to identify which proteins bind to this promoter and activate the gene. The ABA signaling pathway in plants converges on a family of transcription factors known as ABFs, or ABA-responsive element binding factors, which recognize ABRE motifs in the promoters of their target genes. The team cloned four ABF genes from tree peony, designated PsABF1, PsABF2, PsABF3, and PsABF7. Expression profiling showed that all four are highly expressed in petioles, the same tissue where PsSUT1 expression peaks, and their expression patterns correlated strongly and positively with PsSUT1 transcript levels. This co-expression pattern provided circumstantial evidence that these transcription factors might be natural regulators of the sucrose transporter.

Circumstantial evidence, however, was not enough. The researchers employed yeast one-hybrid assays, a technique in which candidate transcription factors are tested for their ability to bind a DNA sequence and activate reporter transcription in yeast cells, to verify that the PsABF proteins physically attach to the PsSUT1 promoter. Binding was confirmed. They then moved to a plant system, using transient dual-luciferase assays in tobacco, in which the PsSUT1 promoter drives one luciferase enzyme while a constitutive promoter drives a second, allowing the ratio of the two signals to quantify transcriptional activation. When PsABF proteins were supplied, the PsSUT1 promoter-driven signal rose, demonstrating that these transcription factors act as genuine positive regulators of the sucrose transporter gene in a plant cellular environment.

The most compelling demonstration came from overexpression studies in Arabidopsis. The researchers generated lines in which PsABF7 was expressed at elevated levels and observed that these plants were noticeably more vigorous than wild-type controls, exhibiting significantly greater height and larger rosette diameters. When the team measured sucrose distribution, they found an intriguing pattern: overexpression lines had lower sucrose levels in rosette leaves but higher levels in stems and siliques. This redistribution is precisely what one would expect if the transporter system were more efficiently moving sucrose out of source leaves and into sink organs. Consistent with this interpretation, the PsABF7 overexpression lines showed enhanced growth under high-sucrose conditions, suggesting they could make more effective use of exogenously supplied sugar.

Taken together, the study sketches a coherent regulatory pathway. ABA, whether produced during stress or supplied exogenously, activates ABF transcription factors in petioles. These factors bind ABRE motifs within the PsSUT1 promoter and upregulate the transporter, which in turn accelerates sucrose loading and transport from leaves toward stems and reproductive structures. For a plant like tree peony, whose ornamental value depends on strong vegetative growth and intact flower buds, this pathway represents the molecular logic by which sugar supply is matched to demand, and by which its disruption in containers leads to dwarfism and bud abortion.

The practical implications extend in several directions. On the breeding side, the identification of PsABF7 as a growth-enhancing regulator offers a candidate gene for marker-assisted selection or genetic engineering aimed at producing tree peony cultivars that maintain vigor under the constrained root systems of container production. On the cultivation side, the finding that ABA treatment boosts PsSUT1 expression and that overexpression lines exploit exogenous sucrose more efficiently hints at management strategies, potentially combining hormone application with optimized sugar availability, that could alleviate sucrose limitation in nursery settings. The work also adds to a growing body of research showing that the intersection of hormone signaling and sugar transport is a fertile area for improving crop and ornamental performance.

The study, conducted by Jutang Jiang, Mengjuan Chai, Xiaojing Cao, Renxuan Xue, Jiuxing Lu, Yonghua Li, and Yan Li at the College of Landscape Architecture of Henan Agricultural University, was funded by the Key Technology R&D Program of He’nan Province. By dissecting a promoter, hunting down its binding partners, and validating the biological consequences of manipulating the pathway, the team has transformed a horticultural puzzle into a defined molecular target. For a flower that has been cultivated for centuries yet still resists modern container production, the sugar transport machinery governed by PsSUT1 and its ABA-responsive masters may prove to be the key that finally lets tree peony thrive in a pot as it does in the ground.