Understanding the physiological and molecular basis of chilling tolerance across species of the C4 genera Miscanthus and Spartina

C4 species of the genera Miscanthus and Spartina have successfully colonized cool temperate and cold climatic regions, with Spartina having the most northerly distribution of any known C4 genus. Within these genera, Miscanthus x giganteus, Miscanthus sinensis and Spartina pectinata have already been evaluated for their potential as biofuel crops, and M. x giganteus has been found to possess significant chilling tolerance relative to other C4 species. This dissertation aims to understand this apparently unique tolerance of C4 photosynthesis with three investigations (Chapters 2-4).;Chapter 2 analyses the transcriptome of M. x giganteus on acclimation to chilling temperatures in comparison to that of chilling-intolerant Zea mays using microarray and qPCR. Notably, in response to 14 days of chilling (14 °C) M. x giganteus up-regulated 30 transcripts encoding chloroplast and light-reaction proteins. Validation and comparison of a portion of the key transcripts to Z. mays confirmed these results and found that the opposite response in transcription is seen in Z. mays during chilling.;Chapter 3 examines whether chilling tolerance in Miscanthus is constitutive or has evolved progressively as species migrated northward through Japan. M. sinensis, one of the parent species of the M. x giganteus hybrid, is found throughout Japan. Three accessions of M. sinensis spanning from the sub-tropical southern-most tip to the cold northern-most tip of the major islands of Japan were grown in a common controlled environment and were then acclimated to chilling (14 °C) over 14 days. Following acclimation, the population from the northern-most location showed significantly higher photosynthetic rate of leaf CO2 uptake (A) and quantum yield of photosystem II (phiPSII), and elevated contents of the rate-limiting enzymes PPDK and Rubisco (western blot analysis).;Chapter 4 compares the chilling response of Spartina pectinata cv. 'Red River', a C4-PEP-CK sub-type, to two species of the C4-NAPD-ME sub-type, one chilling tolerant, M. x giganteus, and one chilling intolerant, Z. mays. After acclimation with 14 days of chilling at 14 °C, S. pectinata maintained significantly higher A than M. x giganteus and Z. mays during chilling, and there was also a significant decrease in whole chain electron transport rate and photochemical quenching in M. x giganteus but not in S. pectinata. Surprisingly, S. pectinata and M. x giganteus had similar amounts of PPDK per unit leaf area at 25 °C, and both exhibited large and significant increases in PPDK during chilling. M. x giganteus showed a significant increase in Rubisco (33%), while Rubisco content was maintained in S. pectinata, and declined significantly (30%) in Z. mays. PEP-CK was also maintained during chilling in S. pectinata. These results show for the first time that PPDK is not only present in Spartina , but that ability to acclimate photosynthetic capacity to chilling conditions also corresponds to an increase in photosynthesis.;Collectively, the results of this dissertation show that chilling tolerance in C4 photosynthesis is not species or sub-type specific and that chilling tolerance within C4 photosynthesis may have adapted through similar means in two very separate clades and biochemical types of C 4 grasses. In M. x giganteus, key transcriptional responses in chloroplast-specific genes correspond to the maintenance of higher rates of A and phiPSII. The three species exhibiting the greatest chilling tolerance, measured as higher rates of A and phiPSII, all showed the same response to chilling of an increase in PPDK content with no reduction in Rubisco content. (Abstract shortened by UMI.).