PEPC‐mediated carbon fixation in transmitting tract cells reflects style–pollen tube interactions

Styles nurture and guide pollen tubes to the ovules. The styles of Nicotiana tabacum, a C₃ plant, contain a concentric strand of transmitting tract cells replete with well‐developed chloroplasts. It is shown that the chloroplasts have normal ultrastructure and electron transport ability. However, they were found to be devoid of Rubisco, the key enzyme responsible for carbon fixation in C₃ plants. Nevertheless, non‐invasive fluorescence techniques showed a light‐driven photosynthetic flux. Carbon fixation via phosphoenol pyruvate carboxylase (PEPC) into malate was demonstrated, the latter accumulating during stylar development. Characterization of stylar PEPC in vitro and in vivo revealed apparent K_m values consistent with bicarbonate as a rate limiting factor for photosynthetic flux. Presumably, in the closed confines of the intact style, respired CO₂ is the source of carbonate. Enhanced photosynthetic flux was detected following pollination, suggesting utilization of the additional respired bicarbonate and underlining metabolic interactions between the style and the elongating pollen tube.