contrib-uted equally to this work;designed the research contrib-uted equally to this work;designed the research contrib-uted equally to this work Hao-Fu Dai, Professor, Key Laboratory of Tropical Crop Biotechnology, Ministry of Agriculture, Insti- tute of Tropical Bioscience and Biotechnology, Chinese Acad- emy of Tropical Agricultural Sciences, Haikou 571101, Hainan Province, China. hf_dai@yahoo.cn