探討SlCEP2胜肽荷爾蒙在叢枝菌根菌促進蕃茄側根生長中的角色

Abstract

前人發現，當植物與土壤中有益真菌-叢枝菌根菌(Arbuscular mycorrhizal fungi)共生時，植物可發展出較多的側根，同時，根部可累積較多磷及生長素(Auxin)。雖然前人推測磷和生長素對於菌根菌促進側根生長的過程中或許扮演重要角色，其詳細的分子機制仍然未知。我們發現，番茄和叢枝菌根菌共生在低磷狀態時，確實有較多側根。同時， SlCEP2 (C-terminally encoded peptides)基因表現於菌根菌接種後會持續下降，並與菌根菌共生提升的根部磷含量呈現負相關，顯示菌根菌造成的磷累積可能是SlCEP2表現下降的原因。處理幼苗合成的SlCEP2胜肽可造成側根密度及長度減少，IBA (生長素IAA的前驅物) 處理則可回復被SlCEP2影響的側根密度和長度。此外，受到SlCEP2胜肽影響的生長素在根尖和側根原基 (lateral root primordia) 的強度和極性分布，以及IBA轉換成IAA的酵素基因、生長素運輸載體和下游訊息傳遞的基因表現，都可因為 IBA 處理而恢復正常，顯示SlCEP2影響側根發育可能是藉由調控IBA轉換成IAA的途徑。SlCEPR1 (CEP receptor 1)基因靜默可提升番茄側根密度，同時，SlCEP2的胜肽處理不會影響SlCEPR1基因靜默植物的側根發育，顯示SlCEP2的受體可能為SlCEPR1。最重要的是，在SlCEP2過表現或是基因靜默的番茄中，側根生長無法進一步被菌根菌共生誘導，顯示SlCEP2確實為菌根菌促進植物根部發育機制中的重要基因。我們的研究揭示了，菌根菌共生或許可透過胜肽賀爾蒙以連結磷和生長素的訊號，並藉此調控側根發育。

In response to arbuscular mycorrhizal (AM) fungi colonization, lateral root (LR) development is usually stimulated, accompanied with increased root phosphate content and auxin accumulation. Though both phosphate and auxin signaling has been postulated to be involved in AM-induced LR growth, the underlying molecular mechanism is still unclear. Here, we found tomato colonized with AM fungi indeed had increased LR. In addition, C-terminally encoded peptides (CEP) SlCEP2 was significantly downregulated in mycorrhizal roots, and this downregulation was negatively correlated with the accumulation of AM-derived phosphate, suggesting that repression of SlCEP2 might be due to the higher root phosphate derived from AM. Both LR density and length was significantly reduced under synthetic SlCEP2 peptide treatment; however, application of the IAA precursor, IBA, could rescue the defective LR density and length caused by SlCEP2 treatment. Furthermore, the impaired expression of genes involved in IBA to IAA conversion, auxin polar transport and LR-related signaling pathway in CEP2-treated roots could all be rescued by IBA application, strongly indicating that SlCEP2 might mediate LR growth through regulating IBA to IAA conversion. In SlCEPR1 (CEP receptor 1) knockdown plants, LR density was higher and even under SlCEP2 treatment, LR growth was unaffected, showing that SlCEPR1 may be the receptor of SlCEP2. Most importantly, LR density of SlCEP2 overexpression or knockdown plants cannot be further increased by AM inoculation, suggesting that SlCEP2 was critical for AM-induced LR growth. This new discovery may help us to clarify how AM regulates plants root development through modulating peptide hormone to connect phosphate and auxin signaling.