Mapping of TaHd1-1, a wheat homologue of a rice gene that controls heading date (Hd1)

D.H. Xu and T. Ban*

Biological Resources Division, Japan International Research Center for Agriculture Sciences (JIRCAS), 1-1 Ohwashi, Tsukuba, Ibaraki 305-8686, Japan

The Hd1 gene is a major photoperiod sensitivity gene in rice (Yamamoto et al. 1998; Yano et al. 2000). An ortholog of Hd1 in wheat, designated as TaHd1, has been isolated and has been functionally analyzed by Nemoto et al. (2003). They found that there are three homoeologous TaHd1 genes (TaHd1-1 , TaHd1-2, and TaHd1-3) corresponding to the three homoeologous genomes (A, B and D) in wheat. By using nullisomic-tetrasomic (NT) and ditelosomic (DT) lines of a common wheat cultivar, Chinese Spring, the three homoeologous TaHd1 genes were assigned to the group-6 chromosomes. An expression analysis indicated that, of the three homoeologous genes, only TaHd1-1 and TaHd1-3 were expressed in the seedling stage of Chinese Spring. However, no photoperiod response gene has been assigned so far to the group-6 chromosomes. When TaHd1-1 was transferred into a rice line deficient in Hd1 function, the transgenic plants complemented the function of Hd1; namely, TaHd1-1 promoted heading under the short-day-length condition and suppressed heading under the long-day-length condition. It is necessary to examine the genetic effects of TaHd1-1 under different photoperiod conditions and in different wheat genetic backgrounds. Toward this goal, we developed a PCR-based DNA marker for the direct targeting of TaHd1-1 and mapped the TaHd1-1 gene in a segregating wheat mapping population in the present study.

A common wheat cultivar Gamenya from Australia was used to identify the sequence polymorphism of the TaHd1-1 gene in wheat. A doubled-haploid (DH) population derived from an F₁ cross between two common wheat cultivars Sumai #3 and Gamenya, developed by the wheat x maize system (Ban and Suenaga 2000), was used to map TaHd1-1 with the developed DNA marker as well as the SSR, AFLP, and RFLP markers. To verify the variation in different wheat cultivars and to evaluate the potential of the application of the developed DNA marker in different wheat genetic backgrounds, the marker was tested for 14 common wheat cultivars derived from different countries.

The TaHd1 genes were amplified with a primer pair 5'-AGCTCACTTAGGATACAAGATGC-3' and 5'-CCTCCATTGATGAGAAAGATAT-3' under the same PCR conditions described by Nemoto et al. (2003). When PCR products were separated on a 1.5% agarose gel, a DNA fragment corresponding to the TaHd1-1 gene was excised from the gel. DNA that was purified from the gel was used as a template for a second PCR with the same primer pair. The second-round PCR products were cloned into vectors with the pGEM-T Easy kit (Promega, Madison, WI, USA). Cloned fragments were subjected to sequencing with the CEQ^TM sequencing kit (Beckman Coulter, Fullerton, CA, USA) on the CEQ^TM 2000 DNA Analysis System following the manufacturer's instructions. The obtained DNA sequences were aligned with the sequence of TaHd1-1 in Chinese Spring (AB094487) using the program GENETYX-MAC (Version 8.0, SOFTWARE DEVELOPMENT CO., LTD.).

*Corresponding author, E-mail: tomohiro@affrc.go.jp