Biosynthesis of polyhydroxybutyrate polymer in sink-constrained tobacco plants

Overview

This research addresses the urgent need for renewable, bio-based plastics by engineering tobacco plants to produce polyhydroxyalkanoates (PHAs), a family of biodegradable polymers that can replace petroleum-derived plastics. The approach builds on prior work demonstrating that a cyanobacterial carbon concentrating mechanism (CCM) can boost photosynthesis in chloroplast transgenic tobacco. However, in wild-type backgrounds, CCM expression created sink-constrained plants with reduced photosynthetic efficiency. The proposed solution introduces an inducible chloroplast-expressed polyhydroxybutyrate operon (PhbA, PhbB, and PhbC) into CCM-expressing lines, creating a metabolic sink that converts excess carbon into commercially relevant quantities of PHB biopolymer.

This technology offers a sustainable pathway to produce high-quality renewable plastics directly in plant biomass, reducing dependence on fossil fuel-derived materials while leveraging enhanced photosynthetic carbon fixation.

Technical specifications

• Carbon concentrating mechanism (CCM): A cyanobacterial alpha-carboxysome system previously engineered into tobacco chloroplasts to increase photosynthetic carbon influx
• Polyhydroxybutyrate operon: Three genes (PhbA, PhbB, and PhbC) expressed in the chloroplast to utilize acetyl-CoA pools for PHB synthesis
• Inducible expression system: Allows controlled activation of the PHB operon to direct excess carbon toward polymer production
• Dual transformation strategy: Both chloroplast and nuclear transformations are available, with plastid targeting enabling direct use of acetyl-CoA pools
• Sink relief mechanism: PHB production is expected to relieve feedback inhibition caused by sink constraints in CCM-expressing wild-type plants
• Validated plant system: Tobacco serves as the model crop with established transformation protocols and demonstrated CCM-enhanced photosynthesis

Technology readiness level

This technology is at an early-to-mid stage of development. Prior validation has confirmed that CCM expression in Rubisco-depleted tobacco backgrounds increases photosynthesis and produces visibly larger plants. However, CCM expression in wild-type backgrounds revealed sink constraints characterized by elevated minimum fluorescence, compromised Photosystem II efficiency, and a pre-reduced plastoquinone pool. The next phase involves engineering CCM-expressing wild-type and Rubisco-depleted tobacco lines with the PHB operon genes, targeting them to the plastid or chloroplast genome to produce PHAs at commercially relevant concentrations. Transgenic lines will be characterized to quantify PHB production levels. Both chloroplast and nuclear transformation capabilities are established in the principal investigator's program, supporting ongoing development toward commercial-scale biopolymer production.