Increased brassinolide accumulation and increased growth in low-light-grown transgenic tobacco.

Transgenic tobacco, demonstrating bacterial cholesterol oxidase (CO) activity in the chloroplast, grown at PAR ~280 μmol(photon) m^-2s^-1 (low light), contained thylakoid membranes that include a greater variety of steroids and had reduced contents of sterol and sterol-esters per milligram chlorophyll in comparison to controls. The mature transgenic plants, grown at low light, demonstrate a ~3× larger root dry mass, ~3× larger stem dry mass, ~2× larger leaf dry mass, ~2× increased leaf number, and ~5× increased flower number than controls. Mature transgenic flowering plants, develop to first flower ~2× faster and grow ~30% taller than control flowering plants. The transgenic seedlings contain approximately 2× higher amounts of brassinolide (BR) per g fresh mass than controls. We propose that since the CO enzyme produces increased contents of oxidized steroids in the thylakoid membrane, this encourages enhanced photosynthesis, enhanced BR contents, and increased biomass accumulation.