The genus Vaccinium comprises hundreds of economically important berry crops, including blueberries, cranberries, and bilberries. Global market demand continues to expand as consumers increasingly recognize their nutritional value and health benefits. However, in stark contrast to the genus' growing economic importance, the development of biotechnological tools, particularly stable genetic transformation systems, remains severely lagging. Existing Agrobacterium-mediated transformation protocols are generally inefficient and exhibit strong genotype dependence, which has become a key bottleneck restricting Vaccinium functional genomics and genome editing research.
In March 2026, bioRxiv published the latest research progress from P Munoz's team at the University of Florida: "A robust and high-efficiency Rhizobium rhizogenes hairy root transformation platform for Vaccinium." This research on Vaccinium hairy root transformation systems provides a new solution to this challenge. The research team established a rapid and efficient transformation platform based on Rhizobium rhizogenes (formerly Agrobacterium rhizogenes), opening new technological pathways for molecular breeding and gene function research in Vaccinium species.
Compared to conventional Agrobacterium-mediated stable transformation, Rhizobium rhizogenes-mediated hairy root transformation offers advantages such as shorter cycle time and simpler operation. This system transfers T-DNA into plant cells via the Ri plasmid, inducing the formation of transgenic hairy roots, and has been widely used in gene function research and genome editing in various crops. For species like Vaccinium that are resistant to genetic transformation, the hairy root system provides a feasible alternative.
The study first conducted a systematic comparison of two commonly used strains, Ar. A4 and K599. The results showed that, in leaf explants of the blueberry variety ‘Albus’, Ar. A4 was significantly superior to K599 under all test conditions. On a half-strength woody plant medium, Ar. A4 achieved a hairy root induction rate of 56%, while K599 only reached 16%, a difference of 3.5 times.
Further analysis revealed that explant type had a decisive impact on transformation efficiency. Leaf explants responded significantly better than stem segments. Regarding wound treatment methods, both basal cutting and needle puncture could induce leaf rooting, but basal cutting was more efficient in operation and consistently produced a higher root quantity.
To accurately assess transformation efficiency, the study introduced the RUBY visual report system. This system achieves direct visual identification through the accumulation of betalain in transgenic roots, eliminating the need for expensive detection equipment or complex molecular detection. Under optimized conditions—using Ar. A4 strain, leaf explants, basal cut wounds, and half-strength WPM medium—the transformation efficiency reached 46.7% after 60 days of co-culture. More importantly, transgenic roots could be visually identified by the RUBY signal as early as day 16 after co-culture, a significantly shorter timeframe compared to traditional stable transformation systems.
Statistical analysis confirmed that strain type, explant type, and culture time had a significant impact on transformation efficiency, while culture medium concentration alone did not have a significant effect. This suggests that bacterial strain, plant genotype, and tissue type are the core variables determining the success or failure of hairy root transformation.
Considering the significant differences in strain dependence, the study further conducted a cross-sectional evaluation among six R. rhizogenes strains, including Ar.1193, Ar. A4, ATCC15834, C58C1, K599, and MSU440. By controlling bacterial overgrowth through various antibiotic regimes, the results showed that:
These results further confirm the importance of host-strain interaction compatibility for successful transformation.
To test the platform's universality, the study selected multiple Vaccinium genotypes representing different taxa for testing, including highbush blueberry varieties Sharper, Optimus, and Colossus, as well as wild or closely related species such as V. elliottii and V. staminium. Ar. A4 successfully induced RUBY expression roots in all tested genotypes, with transformation efficiencies ranging from 1.9% for Elliottii to 85% for Colossus. Despite significant differences among different genotypes, the broad applicability of Ar. A4 has been fully validated, demonstrating that this platform can cover diverse Vaccinium germplasm resources.
Figure 1. Hairy root transformation and regeneration in Vaccinium using Rhizobium rhizogenes. (Kumam, et al. 2026)
The ultimate goal of hairy root transformation is to obtain fully regenerated plants. The study first attempted a conventional plant hormone induction strategy, transferring RUBY-positive roots to callus induction media supplemented with different concentrations of auxin and cytokinin. Although the transgenic roots successfully dedifferentiated to form callus tissue and produced globular embryos, exhibiting good developmental plasticity, they consistently failed to differentiate into shoots.
To address this bottleneck, the research team employed a second strategy: overexpressing the developmental regulator WIND1 and isopentenyltransferase ipt, and using GFP as a visual marker for real-time screening. Under this strategy, the ‘Albus’ variety achieved a 7% shoot regeneration rate under standardized transformation conditions. Molecular detection and GFP fluorescence analysis confirmed that the target transgene was integrated in most regenerated shoots.
It is noteworthy that although the GFP signal was strong in the callus tissue, the fluorescence signal in the green aerial tissue was masked by red-chlorophyll accumulation, and GFP expression was only detected in the promeristematic tissue. This phenomenon is related to the quenching effect of chlorophyll on the green fluorescent protein signal, which is a technical artifact and does not affect the actual integration of transgenes.
The R. rhizogenes-mediated hairy root transformation platform established in this study has the following core advantages:
The establishment of this platform provides a practical tool for gene function verification, metabolic pathway analysis, and genome editing in Vaccinium plants, which will strongly promote the molecular breeding process of perennial berry crops in this genus. With further optimization of regeneration efficiency, this technology is expected to become one of the standardized solutions for Vaccinium biotechnology research, accelerating the transformation from laboratory results to field applications.
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