Transformed Callus of Panax vietnamensis: Promising In Vitro Platform for Majonoside R2 Production alongside Hairy Roots

Technical Summary: Transformed Callus of Panax vietnamensis as an In Vitro Platform for Majonoside R2 Production

Problem Statement
Panax vietnamensis (Vietnamese ginseng) is a valuable medicinal plant known for producing Majonoside R2 (MR2), a rare ocotillol-type saponin with significant pharmacological properties, including anti-tumor, chemopreventive, and hepatoprotective activities. However, natural resources of this species are severely limited due to slow growth, restricted distribution, and overexploitation. Furthermore, the plant requires several years to reach harvestable maturity and accumulate sufficient ginsenosides. While Agrobacterium rhizogenes-mediated hairy root cultures have been established in Panax species as effective platforms for secondary metabolite production, the potential of callus-derived transformed cultures in P. vietnamensis remains largely unexplored. Additionally, there is limited data on the scalability of these systems and the efficacy of methyl jasmonate (MeJA) elicitation in enhancing MR2 production within these specific culture types.

Methodology
The study established both hairy root and transformed callus cultures from P. vietnamensis callus explants using A. rhizogenes strain KCTC 2744.

• Transformation and Selection: Callus segments were infected with the bacterial suspension and co-cultivated. Putative transformed tissues (hairy roots and callus) were selected on hormone-free medium containing cefotaxime to eliminate bacteria. These cultures were maintained for over two years without exogenous plant growth regulators.
• Molecular Confirmation: PCR analysis verified the presence of rolA, rolB, rolC, and rolD genes and the absence of the bacterial virD2 gene, confirming successful transformation without residual contamination.
• Elicitation Studies: Hairy roots and transformed callus were treated with varying concentrations of MeJA (100, 120, 150 µM) for different durations (8, 10, 12 days). Biomass growth (Fresh Weight/Initial Weight ratio), dry matter content, and MR2 accumulation were measured.
• Scale-Up: Both culture systems were cultivated in 2-L bioreactors with continuous aeration for 8 weeks, followed by a 10-day MeJA elicitation period (150 µM for hairy roots; 100 µM for callus).
• Analysis: MR2 content was quantified using High-Performance Liquid Chromatography (HPLC) with ELSD detection.

Key Results

• Culture Stability: Both hairy roots and transformed callus were successfully established as stable, hormone-independent culture lines. The transformed callus remained undifferentiated and could be maintained independently for over two years.
• Effect of MeJA on Biomass: MeJA elicitation significantly suppressed biomass growth in both systems. The Fresh Weight/Initial Weight (FW/IW) ratios were markedly lower in MeJA-treated cultures compared to controls. Conversely, dry matter content increased in MeJA-treated cultures.
• MR2 Accumulation: MeJA significantly enhanced MR2 production.
  • Hairy Roots: MR2 content reached a maximum of 0.91 mg g⁻¹ DW (150 µM MeJA, 10 days), an approximately 8-fold increase over the control.
  • Transformed Callus: MR2 content reached a maximum of 1.77 mg g⁻¹ DW (100 µM MeJA, 10 days), an approximately 17-fold increase over the control.
  • Under flask conditions, transformed callus accumulated substantially higher levels of MR2 than hairy roots.
• Bioreactor Performance: Both systems were scalable in 2-L bioreactors.
  • Hairy Roots: Achieved a biomass increase of 25-fold, with a final MR2 yield of 22 mg L⁻¹ (productivity: 0.33 mg L⁻¹ day⁻¹).
  • Transformed Callus: Achieved a biomass increase of 21-fold, with a final MR2 yield of 19 mg L⁻¹ (productivity: 0.29 mg L⁻¹ day⁻¹).
  • While biomass accumulation was robust in both, the MR2 content in transformed callus decreased relative to flask cultures during scale-up, likely due to tissue aggregation restricting oxygen and nutrient transfer.

Significance and Claims
The authors claim that transformed callus derived from A. rhizogenes transformation represents a stable, hormone-independent culture system that serves as a promising alternative platform for sustainable MR2 production in P. vietnamensis.

• Novelty: The study highlights that transformed callus, often a byproduct of hairy root induction, can be maintained as an independent line with high metabolic potential, specifically showing higher maximum MR2 accumulation than hairy roots under flask conditions.
• Feasibility: The successful scale-up of both systems in a bioreactor demonstrates their potential for large-scale production, although the authors note that the current bioreactor configuration may be more suitable for hairy roots than for callus, which tends to aggregate.
• Sustainability: These in vitro systems offer a viable solution to the limitations of wild harvesting, providing a sustainable source of the rare MR2 saponin.

The paper concludes that while hairy roots remain a robust system, transformed callus offers a distinct and potentially superior metabolic profile for MR2 production, warranting further optimization of bioreactor conditions and elicitation strategies to maximize its industrial potential.