Cold-tolerant C4 photosynthesis in Miscanthus x giganteus

Abstract

The grass Miscanthus x giganteus is closely related to sugar cane and uses the same C4 (NADP-ME type) pathway, but in contrast to the vast majority of C4 species, it is cold-tolerant. The mechanism for cold-tolerant C4 photosynthesis in Miscanthus was examined. Miscanthus and Zea mays were grown at 25/20ºC and 14/10ºC (day/night) and the temperature response of photosynthesis measured from 5-38ºC. Miscanthus grown at 25ºC demonstrated a temperature optimum of photosynthesis at 35ºC, with similar maximum rates to corn which had a temperature optimum of 30ºC. Below 20ºC, Miscanthus had moderately higher rates of photosynthesis than corn. A dramatic difference is evident when grown at 14ºC. Here, corn shows a large reduction in photosynthesis at all measurement temperatures. In contrast, growth at 14°C causes no reduction in Miscanthus photosynthesis measured below 35ºC and only a slight reduction at 35-38ºC compared to rates for plants grown at 25ºC. Western blot analysis showed that growth at 14ºC results in large decreases of ribulose bisphosphate carboxylase/oxygenase (RUBISCO), phosphoenolpyruvate carboxylase (PEPCase) and pyruvate orthophosphate dikinase (PPDK) in corn. However, growth of Miscanthus at 14ºC did not affect amounts of RUBISCO or PEPCase, and caused a large increase in PPDK. It has been suggested that PPDK is the most likely limitation to C4 photosynthesis at low temperature; therefore, this increase is intriguing. One hypothesis for the observed accumulation of PPDK protein for Miscanthus grown at 14oC is elevated mRNA expression. This hypothesis is currently being investigated using RT-PCR analysis of Miscanthus PPDK genes.