Engineering PHB (polyhydroxybutyrate) biosynthesis in plants In transgenic <i>Arabidopsis thaliana</i>, which bacterial enzyme gene(s) are introduced to enable synthesis of PHB biopolymer in plastids or cytosol?

Introduction / Context:
PHB is a biodegradable polyester produced naturally by bacteria. To synthesize PHB in plants, researchers express a minimal pathway encoded by bacterial genes, typically including acetoacetyl-CoA reductase (phbB) and PHB synthase (phbC), together with beta-ketothiolase (phbA) in many constructs.

Given Data / Assumptions:

• Target is to assemble a functional PHB biosynthetic route in plant cells.
• Enzymes must convert acetyl-CoA to PHB via acetoacetyl-CoA → 3-hydroxybutyryl-CoA → PHB.
• Option list focuses on two key enzymatic steps.

Concept / Approach:
Acetoacetyl-CoA reductase reduces acetoacetyl-CoA to 3-hydroxybutyryl-CoA, and PHB synthase polymerizes the monomer to PHB. Thus, both are necessary components of the engineered pathway in many plant systems.

Step-by-Step Solution:

Verification / Alternative check:
Transgenic plants expressing phbA, phbB, and phbC accumulate PHB granules detectable by microscopy and biochemical assays.

Why Other Options Are Wrong:

• Single-enzyme choices are insufficient for polymer formation.
• Pyrophosphorylase and glycolytic enzymes are unrelated to PHB polymerization.

Common Pitfalls:
Incorrect gene targeting (organelle localization) limits substrate availability; balanced expression of all pathway genes is required for efficient polymer accumulation.

Final Answer:
Both (a) and (b)