番茄核因子次單元蛋白SlNF-YC3正向調控叢枝菌根菌共生

Abstract

叢枝菌根菌共生（arbuscular mycorrhizal (AM) symbiosis）是一種古老且廣泛存在於植物與叢枝菌根真菌之間的生命現象，許多重要的作物皆可與菌根菌建立共生關係，例如水稻、小麥、玉米、番茄等等。菌根菌的菌絲在共生過程中會在根內沿著植物細胞間隙（apoplast）生長，並且藉由特化之菌絲結構arbuscule進行營養互換。在叢枝菌根菌共生過程中，植物細胞的轉錄體會進行一系列大規模的改變，前人研究已在多種植物中發現上百個差異表現之基因，但是大多數的功能都還未知。本篇研究發現了一個保守存在於菌根菌宿主植物的轉錄因子基因NF-YC3，其蛋白質產物為核因子C型次單元蛋白（C subunit of nuclear factor Y），在番茄中正向調控叢枝菌根菌共生。此基因在番茄根部會被菌根菌大幅誘導表現，無真菌共生時則幾乎不表現。此基因之表現可能被順式作用元件（cis-regulatory element）GCC-box調控，大部分表現在含有arbuscule的皮層（cortex）細胞中。在nf-yc3基因靜默（gene silencing）之根部中，所有根內菌根菌結構的量會顯著地降低，且大型arbuscule出現的比率也減少了，顯示NF-YC3可能藉由支持根內菌絲（intraradical hyphae）生長以及arbuscule生成，進而正向調控叢枝菌根菌共生。相對地，過量表現NF-YC3-GFP並不會影響菌根菌的型態與多寡，顯示內源NF-YC3已足夠完全支持叢枝菌根菌共生。我們接著發現四個可能位於NF-YC3下游的基因：NF-YB3a（核因子B型次單元蛋白基因）、Exo70I（EXOCYST complex次單元蛋白基因）、RAD1（GRAS轉錄因子基因）以及Solyc02g088310 （AP2/ERF轉錄因子基因）。Exo70I和RAD1在苜蓿中的功能皆與arbuscule生成有正相關，和nf-yc3基因靜默的結果相符。總結來說，番茄中的NF-YC3可能藉由誘導NF-YB3a、Exo70I、RAD1和Solyc02g088310表現，支持根內菌絲生長以及arbuscule生成，進而正向調控叢枝菌根菌共生。

Arbuscular mycorrhizal (AM) symbiosis is an ancient and widespread plant-fungi association, and many agronomically important crops can be colonized by AM fungi, such as rice, maize, wheat, tomato, etc. AM fungi hyphae can extend through root apoplast and promote bidirectional nutrient exchange through a specialized hyphal structure called arbuscule. During AM symbiosis, plant cells undergo a set of large-scale transcriptional reprogramming. Hundreds of differentially expressed genes have been identified in plenty of plant species, while most of them are functionally unknown. Here, we demonstrated that a phylogenetically conserved transcription factor gene Nuclear Factor YC3 (SlNF-YC3), which encodes a C subunit of nuclear factor Y (NF-Y), positively regulates AM symbiosis in tomato. Its expression was barely detectable in roots without AM colonization but was strongly induced following the progression of AM symbiosis. The spatial expression of NF-YC3 was observed in cortical cells containing arbuscule, probably via the cis-regulatory element GCC-boxes. The abundance of all intraradical AM fungal structures, including arbuscule, vesicle and intraradical hyphae, was significantly reduced in nf-yc3 RNAi roots. Formation of larger arbuscules in nf-yc3 RNAi roots was also hindered. These results suggest that NF-YC3 positively regulates AM symbiosis, probably via supporting AM intraradical fungal growth and arbuscule formation. By contrast, no phenotype was observed in NF-YC3-GFP overexpression roots compared to the control groups, suggesting that endogenous NF-YC3 is able to fully support AM symbiosis. Subsequently, we identified four candidate genes, NF-YB3a (B subunit of NF-Y), Exo70I (EXOCYST complex subunit), RAD1 (GRAS transcription factor) and Solyc02g088310 (AP2/ERF transcription factor), which may act downstream of NF-YC3. Evidence supports that both Exo70I and RAD1 are responsible for arbuscule formation in Medicago truncatula, which coincides with the phenotype of nf-yc3 RNAi roots. Taken together, SlNF-YC3 was found positively regulating AM symbiosis in tomato, probably via inducing NF-YB3a, Exo70I, RAD1 and Solyc02g088310 to support AM intraradical fungal growth and arbuscule formation.