日日春ARF17於植物抵抗日日春葉片黃化病植物菌質體所扮演之角色

Abstract

植物菌質體 (phytoplasma) 為寄生於植物篩管內無細胞壁的絕對寄生性原核生物。主要透過媒介昆蟲進行傳播，可引起葉片黃化、簇葉、花器綠化等病徵，並在世界各地造成多種經濟作物的損失。從日日春 (Catharanthus roseus) 的研究中發現，植物抗植物菌質體的機制可能與系統獲得性抗病(systemic acquired resistance, SAR) 有關。先前的研究發現，當抑制可能參與系統獲得性抗病的CrNPR1的表現，不但使CrPR1a在罹病植株中的表現量下降，亦加速由植物菌質體造成的日日春葉片黃化病 (periwinkle leaf yellowing, PLY) 之病程發展。因此推測調控CrPR1a表現的轉錄因子，可能參與在植物對抗植物菌質體的抗病途徑，進而影響日日春葉片黃化病之病程發展。先前我們已利用病毒誘導基因靜默技術（virus-induced gene silencing, VIGS） 對307個轉錄因子進行篩選可能影響CrPR1a表現的轉錄因子。結果篩選出3個ARF，1個Aux/IAA，以及1個bZIP類別之轉錄因子。因此，本研究將進一步探討以VIGS靜默這些轉錄因子的表現是否會對日日春葉片黃化病的病徵發展造成影響。在初步的兩次試驗中發現靜默與阿拉伯芥ARF17接近的ARF轉錄因子CrARF17 能降低CrPR1a 的表現量並也造成日日春葉片黃化病之病程發展加速。此結果與先前發現靜默CrARF17抑制CrPR1a表現的結果一致。由於在阿拉伯芥的研究中發現ARF一類的轉錄因子多參與在植物生長素的訊息傳遞上以及調控GH3和CalS基因群的表現，而植物生長素可能透過壓抑植物中水楊酸 (salicylic acid, SA) 的累積進而影響植物對病原菌的抗性。本研究證實當CrARF17靜默後會增加GH3基因表現和壓抑SA, IAA及IAA-Asp的累積。而GH3和大多的CalS基因在植株受到PLY感染後皆會被誘導表現。目前推測CrARF17參與植物的抗病反應，但並沒有直接參與在抗PLY的抗病路徑當中。往後我們將針對GH3和CalS基因在植物對日日春葉片黃化病的抗性上可能扮演的角色作更深入的探討。並進一步驗證callose的累積和IAA-Asp的含量對病程發展的影響。

Phytoplasmas are wall-less prokaryotic obligate plant pathogens that are restricted in phloem. These insect-transmittable pathogens cause symptoms including yellows, witches’-broom, virescence, and lead to worldwide economic losses. In periwinkle (Catharanthus roseus), plants may fight against phytoplasmas through systemic acquired resistance (SAR). Previously, we found suppression of CrNPR1, which may play a central role in SAR, result in reduction of CrPR1a induction and acceleration of symptom progression of periwinkle leaf yellowing (PLY), a disease caused by phytoplasma infection. Thus, we hypothesized that transcription factors that can affect the expression of CrPR1a may be involved in plant defense against PLY phytoplasmas and affect symptom progression. Previously, we have screened 307 transcription factors through virus-induced gene silencing (VIGS) and identified factors that regulate CrPR1a expression. These factors include three ARFs, one Aux/IAA, and one bZIP family members. In this proposed study, we aimed to understand whether these transcription factors are direct or indirectly involved in plant defense against PLY phytoplasma. In two preliminary tests, silencing of an Arabidopsis ARF17 ortholog, CrARF17, not only reduced the expression of CrPR1a, but also accelerated disease progression of PLY. The result is consistent with previous finding that silencing of CrARF17 resulted in repression of CrPR1a induction. Studies in Arabidopsis have shown that ARFs participate in auxin signaling and auxin can affect plant resistance through repressing of salicylic acid (SA) biosynthesis. In this study, we have clarified that silencing of CrARF17 resulted in increase of GH3 gene expression and repression of SA, IAA and IAA-Asp accumulation. Then, GH3 and most of CalS genes are induced in plants after PLY infection. We speculate that CrARF17 participates in plant resistance indirectly against PLY phytoplasmas. In the future, we will focus on clarifying the role of GH3 and CalS genes in plant resistance to PLY, and further clarify the effect of callose accumulation and the IAA-Asp level on symptom progression of PLY.