台灣木瓜輪點病毒系統變異之現況研究

Abstract

木瓜輪點病 (papaya ring spot) 由木瓜輪點病毒 (Papaya ringspot virus，PRSV)所引起，是限制木瓜經濟生產的重要因子。前人研究依 PRSV 在木瓜上所產生不同的病徵可分成三個系統 (strains)，包括嚴重畸型 (deformation，簡稱 DF)、嚴重嵌紋壞疽型 (severe mottling with necrosis，簡稱 SMN) 和嚴重嵌紋型 (severe mottling，簡稱SM)。為了調查現今臺灣PRSV之現況，從屏東田間受感染之木瓜園中採集 22 個病葉樣本，以系統專一性引子對進行 RT-PCR 偵測與鑑別，結果顯示田間以 DF 為優勢系統。將採集之病葉樣本利用白藜 (Chenopodium quinoa) 進行三次單斑分離純化病毒，再重新接種回木瓜上觀察，最後得到 i1~ i8 共 8 個PRSV 分離株，再以 RT-PCR 偵測，結果 i1、i2、i3 和 i4 被偵測出 DF，其餘 4 株皆無法偵測出所屬之系統。在接種試驗中，8個分離株接種在木瓜上之病徵表現呈現多樣性: i1、i2、i3、i4、i5 和 i8 的病徵和 DF 相似；i6 則較相似於 SMN；i7的病徵和 DF 相似卻較嚴重。不同木瓜品種也呈現出不同的感病性，臺農二號和日陞相對較為感病，而臺大八號和紅妃則較耐病；然而也發現臺大八號對i2耐病性較差，紅妃對i5耐病性也較差。由 PRSV的 P1基因定序的分子資料可知在 8 個分離株序列間還是有一些差異，目前已將 PRSV i7 和 i8之全長序列完成定序，根據分析結果，除了原有的 DF、SMN 和 SM 三系統外，尚存其他序列有變的 PRSV，顯示臺灣PRSV的基因序列仍不斷在變異中。

Papaya ring spot caused by Papaya ringspot virus (PRSV) is a major limiting factor for economical papaya production. The previous studies demonstrated that PRSV could be categorized into 3 strains such as SM (severe mottling), SMN (severe mottling with necrosis) and DF (deformation) strains according to their incited symptoms. In order to investigate current strain-diversity of PRSV in Taiwan, totally 22 samples were collected from infected papaya orchards in the Pingtung area and they were tested by RT-PCR with PRSV strain-specific primers. The results show that DF has become a predominant strain in the field. Furthermore, 8 PRSV isolates (named i1~i8) were obtained from 3-time serial passages of a single lesion through Chenopodium quinoa (C. q.), and then they were mechanically transmitted into papaya plants. The results based on the RT-PCR detection showed that 4 isolates should belong to the DF strains whereas other 4 isolates could not be clearly identified. The symptoms caused by these 8 isolates were not similar each other. The induced symptoms on the papaya cultivar TN2 caused by i1, i2, i3, i4, i5 and i8 are similar to those caused by DF; symptoms caused by i6 is close to those caused by SMN; and symptoms caused by i7 is close to those caused by DF but more severe. In addition, various infected papaya cultivars showed different symptoms. The TN2 and SR papaya cultivars were more susceptible to PRSV while the NTU8 and RL cultivars are more tolerant. However, NTU8 seemed to be not so tolerant to i2 and RL was not so tolerant to i5. The genomic sequences of PRSV-P1 are not completely similar among the 8 isolates. For comparative genomics analysis, full length genomic sequences of i7 and i8 have been determined. The results showed variation of genomic sequences should be usually in the PRSV genome. The other special strains of PRSV probably exist beside DF, SMN and SM strains, which reveals the variation of PRSV is always undergoing in the field in Taiwan.