Searching for the Genetic Basis of Cold Tolerance in Maize’s Sister Genus Tripsacum

Grass species in the two genera Tripsacum and Zea differ in their ability to withstand freezing temperatures. Tripsacum genotypes from northern climates can overwinter in a dormant state, while freezing temperatures kill maize leaves. The Tripsacum genus diverged from the Zea genus fewer than 1.2 million years ago, and the two genera share an ancestral allopolyploid event (Ross-Ibarra et al., 2009; Chia et al., 2012). Tripsacum and maize share most of their gene content, yet the basis for freezing tolerance in Tripsacum and freezing sensitivity in maize remains unknown. As a first step toward finding cold tolerance genes in Tripsacum, we sequenced the genome of Tripsacum dactyloides cultivar “Pete”, which is adapted to the northern United States, and performed mRNA-seq on “Pete” and Tripsacum floridanum, which is adapted to subtropical and tropical climates. We searched for sequence variation in known cold tolerance genes between these two species. Additionally, we performed a preliminary freezing tolerance screen using 161 Tripsacum offspring from crosses between Tripsacum accessions from the northern and southern United States. A pedigree-based kinship matrix explained 14.5% of the total variation in RNA degradation scores three days after freezing stress, indicating that genetic variation within the population partly explains differing levels of freezing tolerance. Identifying genes that confer freezing tolerance in Tripsacum could enable the genetic engineering of a freezing-tolerant, high-yielding maize variety. Freezing-tolerant maize could be planted earlier in the spring or could be developed into an overwintering line tailored to the southern United States.