Abstract:

GIGANTEA gene plays an important role in several molecular regulations in plant，such as flowering and circadian rhythm oscillation．A full-length cDNA sequence of homologous GIGANTEA gene was cloned by employing homology gene cloning and RACE-PCR from sugar apple（Annona squamosa L.），which was named as AsGI（GenBank accession KR095281）. Sequence analysis showed that the AsGI gene has a 3 465 bp open reading frame（ORF）encoding 1 154 amino acids. Sequence alignment displayed that it was a similarity of 77% and 76% with GIGANTEA of Vitis vinifera，Elaeis guineensis，respectively. The phylogenetic tree showed that sugar apple AsGI and Musa acuminata GIGANTEA are the closest in molecular evolution distance，followed by Phoenix dactylifera GIGANTEA and Elaeis guineensisGIGANTEA. It was speculated that AsGI protein is transcription factor，located in the nucleus and it is a non-secretory protein. It do not have a signal peptide. The plant expression vector was transformed into tobacco via Agrobacterium tumefaciens mediated transformation. GFP expression was observed in the nucleus of the epidermis cells of tobacco by fluorescence microscopy. The real-time PCR results suggested that AsGI gene showed a high transcription level in leaf，flower bud and mature flower. Interestingly，the expression patterns of AsGI gene are circadian rhythms expression. Under long-day conditions，the expression levels of AsGI gene are higher than that under short-day conditions. And the the expression levels of AsGI gene are relatively high in April，June，and August. This study provides some references for further exploration of photoperiod pathway of the flower in sugar apple molecular control mechanism. Meanwhile，AsGI gene could be used as the target gene of molecular breeding of flowering phase.

Key words: sugar apple, GIGANTEA, circadian clock, subcellular localization, expression pattern