Polymorphisms in the Wheat AP2 Homoeologs Orthologous to the Barley Cleistogamy Gene Cly1

Fertilization in closed flowers, termed cleistogamy, is important in controlling pollen-mediated gene flow in transgenic crops. Cleistogamous flowering is also associated with improvements in the level of resistance against Fusarium head blight (FHB) in barley. Natural variants of barley carrying a single recessive gene at the cleistogamy 1 (cly1) locus have the cleistogamous flower, in which the palea and lemma remain tightly closed throughout the pollination period. The cleistogamy in barley is due to the failure of lodicules to develop and swell sufficiently for flower opening. We have shown that the Cly1 gene encodes an AP2 protein (HvAP2) and that the cleistogamous cly1 allele contains a synonymous nucleotide substitution at the microRNA (miR172) targeting site (Nair et al. 2010). In contrast, the hexaploid wheat commonly exhibits the non-cleistogamous type of flowering. Although a limited number of cleistogamous wheat varieties have been reported, they exhibit incomplete and unstable cleistogamy depending on environmental conditions, together with normal development of lodicules. To examine the role of the wheat AP2 gene(s) on the flower opening, we cloned the three wheat homoeologs (designated TaAP2-A, -B and -D) orthologous to the barley Cly1 gene. Sequence analysis showed that each AP2 homoeolog was highly conserved among wheat varieties irrespective of the cleistogamy. Further searches for natural variants of AP2 homoeologs using wheat relatives (Triticum and Aegilops species) revealed that there was no functional polymorphism particularly in the miR172 targeting site. Use of the TILLING approach for a functional TaAP2 modification at each homoeolog will be discussed for engineering of the hexaploid wheat exhibiting complete cleistogamy.