Initiation and characterization of a cotton (Gossypium hirsutum L.) photoautotrophic cell suspension culture

Abstract

A heterotrophic cotton (Gossypium hirsutum L. cv. Stoneville 825) cell suspension culture was adapted to grow photoautotrophically. After two years in continuous photoautotrophic culture at 5% CO₂ (balance air), the maximum growth rate of the photoautotrophic cell line was a 400% fresh weight increase in eight days. The Chl concentration was approximately 500 μg per g fresh weight.

Elevated CO₂ (1%–5%) was required for culture growth, while the ambient air of the culture room (600 to 700 ul CO₂ 1⁻¹) or darkness were lethal. The cell line had no net photosynthesis at 350 ul 1⁻¹ CO₂, 2% O₂, and dark respiration ranged from 29 to 44 μmol CO₂ mg⁻¹ Chl h⁻¹. Photosynthesis was inhibited by O₂. The approximate 1:1 ratio of ribulose 1,5-bisphosphate carboxylase (RuBPcase) to phosphoenolpyruvate carboxylase (PEPcase) (normally about 6:1 in mature leaves of C₃ plants) was due to low RuBPcase activity relative to that of C₃ leaves, not to high PEPcase activity. The PEPcase activity per unit Chl in the cell line was identical to that of spinach leaves, while the RuBPcase activity was only 15% of the spinach leaf RuBPcase activity. RuBPcase activity in the photoautotrophic cells was not limited by a lack of activation in vivo, since the enzyme in a rapidly prepared cell extract was 73% activated. No evidence of enzyme inactivation by secondary compounds in the cells was found as can be found with cotton leaves. Low RuBPcase activity and high respiration rates are most likely important factors in the low photosynthetic efficiency of the cells at ambient CO₂.