| A unique developmental aspect involved in the life cycle of Davidia involucrata Baill. is the formation of two or three large-sized attractive white bracts surrounding a relatively small-sized capitulum. It has long been believed that plant bract is one kind of metamorphism of or evolved directly from leaf. Nonetheless nothing has been done on its role and function in the evolutionary and reproductive biology and in the life cycle of this species, saying nothing of its developmental mechanism, since the species was found from West China in 1869. This species is one of perfect materials to examine the developmental mechanism of plant bract, to explore the developmental relationship between leaf and bract, as well as to study the role and essential function of bract on plant reproductive biology and ecology.In virtue of the technique of suppression subtractive hybridization, we had successfully constructed a subtractive library of D. involucrata sprouting bract by taking homochronous leaf as driver in this study. Differential screening for the library revealed that, 167 of 237 clones randomly selected from the library are positive and represent genes that were exclusively or intensively expressed in bract, which contain inserts ranging from 100 to 2000 bp in length. Moreover, except a few clones were expressed predominantly in bract, absolute majority of subtraclive clones were expressed at low level in all checked organs, i.e., bract, leaf and root.Among sequenced 16 positive clones randomly selected, two represent novel expression tag sequences, two are homologous to two unknown proteins in GenBank; the rest are homologous to known or putative proteins or enzymes with definite functions by searching the GenBank through Blast program, which are involved in various life activities of cell such as regulation of gene expression, plant secondary metabolism, signal transduction, adversity resistance, stress response and defense reaction. Significant changes of quantities of these gene fragments were observedbefore and after SSH, which indicated they were enriched after SSH.Sequence analyses for those limited amount of inserts indicated that, (1) A few particular gene populations are expressed in D. involucrata sprouting bract, e.g., gene population encoding proteins that involve in plant defensive reactions and adversity resistance, such as P1A5 gene encoding a non-specific lipid-transfer protein, P1B4 gene encoding a raucaffricine-O-p-D- glucosidase and P2D4, encoding a proline-rich protein; (2) Gene populations with antagonistic effects occurred simultaneously in sprouting bract, e.g., P1F4 gene encoding an ABA stress ripening protein versus P1 H11 gene encoding an auxin-responsive protein.Reverse Northern hybridization indicated that P1A5 gene was over-expressed in bract compared with that in leaf. This expression pattern was further conformed by RT-PCR. Sequence analyses revealed that it is a partial sequence of a lipid transfer protein gene. A complete cDNA of 1047 bp was obtained by means of 5'-RACE (5'-Rapid Amplification of cDNA End) techniques using gene specific primer P1A5-1. The deduced amino acid sequence of P1A5 was most homologous to the lipid transfer protein 3 precursor isolated from upland cotton, the lipid transfer protein SDi-9 isolated from common sunflower, and the nonspecific lipid-transfer protein precursor allergen pru av3 isolated from sweet cherry. It also had common features of plant nsLTPs (non-specific lipid transfer proteins) such as eight conserved cysteine positions, high isoelectric points (8.9) and lack of tryptophans. The deduced amino acid sequence had two strong transmembrane helices, i.e., the first from position 5 (Gly) to position 35 (Val) and the second from position 28 (Ala) to position 46 (Leu). The cleavage site of putative signal peptide was predicted to occur between 28(Ala) and 29(Ala) thus the putative mature form of the protein composed of 92 amino acids with a molecular weight of 9.2 kDa. All these provide compelling evidence that the P1A5 clone b...
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