| 摘要： 番石榴(Psidium guajava L.)為熱帶美洲原產之桃金孃科（Myrtaceae）經濟果樹，其 多數的栽培品種為採後易轉黃變軟、不耐儲運的更年性果實；而幾種由『泰國拔』衍生的 非更年性品種，因具有果肉清脆鮮綠、耐儲運之特性，符合鮮銷市場的需求，已成為 台灣番石榴栽培的主流。雖然，導致這兩群番石榴果實後熟行為差異的原因，仍待試 驗釐清。本計劃的預試驗及先前的研究結果皆顯示，非更年性品種果實採收後僅能合 成微量乙烯，外加乙烯或丙烯皆可引起呼吸速率的上升，但並無法誘發其大量內生乙 烯的合成（即缺乏乙烯自動催化系統），ACC（1-aminocyclopropane-1-carboxylic acid） 合成.活性應是兩者差異的關鍵所在。所以本計劃擬將以RT-PCR（ reverse transcription-polymerase chain reaction）技術進行番石榴ACC 合成.的選殖，藉以比較 更年性與非更年性番石榴品種ACC 合成.基因結構或表現上的不同。研究結果將可提 供番石榴果實採後保鮮、儲運處理技術開發的基礎背景資料；並可供爾後雜交育種親 本選擇及子代後熟特性早期選拔之標誌。Two cDNA fragments, 1576 bp encoding 435 amino acids and 977 bp encoding 326 amino acids, respectively, were amplified from total mRNA prepared from guava fruit tissues by RT-PCR gene cloning strategy. Sequence analysis revealed that both of these two cDNA fragments contain all the seven regions that are highly conserved among ACC (1-Aminocyclopropane-1-carboxylic acid) synthases (ACS). Notably, the amino acid sequence (SLSKDMGLPGFR) in the active-site of ACS enzyme (the fifth conserved region) is almost identical with the cases studied before. Therefore, it is very like that the two cDNA clones isolated are guava ACS genes and were named Pg-ACS1 and Pg-ACS2, respectively. Gene expression patterns were investigated via RT-PCR differential display analysis in different fruit development stages of ‘Da-Di’, a climacteric guava variety, as well as ‘Jen-Ju Bar’, a nonclimacteric guava cultivar, by utilization of gene specific primers. The significant mRNA accumulation of Pg-ACS1 was detected only in green mature and ripening phases of ‘Da-Di’fruit tissues. In contrast, Pg-ACS2 gene was not expressed in any fruit development stage examined in this research, except in fruit set stage of ‘Jen-Ju Bar’. Base on the gene expression pattern obtained, Pg-ACS1 should belong to System II ACS isoform, and Pg-ACS2, on the other hand, will attribute to System I counterpart. The key factor that causes the different ripening behaviors between ‘Da-Di’and ‘Jen-Ju Bar’is attributed to the lack of Pg-ACS1 gene expression in the later variety during fruit ripening. |
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