The Role of Fructan for Cold Stress Tolerance in Grasses

Fructans are soluble fructose polymers derived from sucrose that are synthesized and assimilated as storage carbohydrates during photosynthesis in many plant species, such as in members of the C₃ grass subfamily Pooideae that includes Triticeae species and forage grasses.   It is well known that fructans can act as cryoprotectants and contribute to freezing tolerance in plant species.  Transgenic Lolium perenne, which normally accumulates fructans, that overexpressed wft1 (sucrose: fructan 6-fructosyl transferase (6-SFT) cDNA) and wft2 (sucrose: sucrose 1-fructosyltransferase (1-SST) cDNA) from wheat showed an increase in fructan accumulation and freezing tolerance at the cellular level.   Transgenic plants in the model grass species Brachypodium distachyon that expressed cDNAs for 6-SFTs with different enzymatic properties: one cDNA encoded PpFT1 from Phleum pratense, an enzyme that produces high-degree of polymerization (DP) levans; a second cDNA encoded wft1 from Triticum aestivum, an enzyme that produces low-DP levans.  Transgenic lines expressing PpFT1 accumulated greater amounts of mono- and disaccharides than wild type and wft1 expressing lines.  PpFT1 overexpressed plants increased tolerance to freezing; whereas the freezing tolerance of the wft1 expressing lines was the same as that of wild type plants.   These provide new insights into the relationship of the composition of water soluble carbohydrates and the DP of fructans to freezing tolerance in plants.  On the other hand, transgenic Miscanthus sinsensis, a potential bioenergy C₄ grass that does not produce fructan, that overexpressed prft4 (1-SST) derived from perennial ryegrass also showed reduced leaf wilting and senescence, indicating a better growth behavior under cold environment.  Transgenic plants had acquired tolerance to a range of freezing temperature down to −8°C by test of leaf electrolyte leakages. The production of fructans counteracted chlorophyll losses and an increase in reactive oxygen species (ROS) under cold, indicating that fructans act as ROS scavengers and signals contributing to ROS homeostasis.  The development of transgenic fructan-accumulating Miscanthus plants with improved cold and freezing tolerance might contribute way to grow C₄ bioenergy grasses in colder regions.