漫谈小麦赤霉病

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本期作者：Rui Wang and 胖丫

漫谈小麦赤霉病

今年的PAGXXVI马上就要召开了，相信国内有不少老师要去参加。后面我们陆续介绍一些与小麦相关的摘要或报告。欢迎参会的老师或同学在我们这里分享参会的照片和心得，也希望去参会的小伙伴有心收集一些报告回来分享给我们，会议之后，参会的老师也可以将报告发给我们，我们发在这里。这样分享给大家之后，参会的金钱成本和时间成本就会降低很多很多，也算是变相的为国家做贡献了

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现在就让我们来说一说今天的主题。上图展现的就是田间赤霉病的发病情况。小麦赤霉病症状又称麦穗枯、烂麦头、红麦头。主要引起苗枯、穗腐、茎基腐、秆腐和穗腐，从幼苗到抽穗都可受害，其中影响最严重是穗腐，其产生的毒素还威胁人畜安全，堪称小麦界的癌症。随着气候的变化，近些年来有加重的趋势，所以小麦抗赤霉病育种也受到空前的重视。

赤霉病由菌源量、寄主感病、生育期和气候条件的相互配合程度，决定年度间、地区间的流行程度。在黄淮海平原麦区，一般只要初侵染菌源量大，小麦抽穗扬花期间降雨多、雨日多、湿度大，病害就能够流行。有利于发病的环境持续时间愈长，则病害流行愈重。菌源量少与感病期气候干旱、少雨是限制病害流行的主要因素。但是，在部分麦区和个别年份，也会因小麦抽穗扬花期间温度特别低，不适合发病，所以，温度低也是限制病害流行的主要因素之一^[1]。我国中、南部稻麦两作区，病菌除在病残体上越夏外，还在水稻、玉米、棉花等多种作物病残体中营腐生生活越冬。翌年在这些病残体上形成的子囊壳是主要侵染源。子囊孢子成熟正值小麦扬花期。借气流、风雨传播，溅落在花器凋萎的花药上萌发，先营腐生生活，然后侵染小穗，几天后产生大量粉红色霉层(病菌分生孢子)。在开花至盛花期侵染率最高。

目前在普通小麦里还未发现完全免疫的材料，即使抗性最好的望水白和苏麦3号在适宜发病条件下也会感病。下面我们结合有限的几篇文献谈一谈关于赤霉病的几个方面。

第一个方面我想谈的是FHB1。众所周知，FHB1已经于2016年被克隆，相关成果发表在Nature Genetics上，题目是'Wheat Fhb1 encodes a chimeric lectin with agglutinin domains and a pore-forming toxin-like domain conferring resistance to Fusarium head blight'，摘要如下。

Fusarium head blight (FHB), caused by Fusarium graminearum, is a devastating disease of wheat and barley that leads to reduced yield and mycotoxin contamination of grain, making it unfit for human consumption. FHB is a global problem, with outbreaks in the United States, Canada, Europe, Asia and South America. In the United States alone, total direct and secondary economic losses from 1993 to 2001 owing to FHB were estimated at $7.67 billion. Fhb1 is the most consistently reported quantitative trait locus (QTL) for FHB resistance breeding. Here we report the map-based cloning of Fhb1 from a Chinese wheat cultivar Sumai 3. By mutation analysis, gene silencing and transgenic overexpression, we show that a pore-forming toxin-like (PFT) gene at Fhb1 confers FHB resistance. PFT is predicted to encode a chimeric lectin with two agglutinin domains and an ETX/MTX2 toxin domain. Our discovery identifies a new type of durable plant resistance gene conferring quantitative disease resistance to plants against Fusarium species.

本文并没有涉及FHB1的抗性机制研究,而且本文发表后争议渐起，日前马正强小组发表在The Crop Journal上的一篇综述阐述了争议的部分根源，题目是'A journey to understand wheat fusarium head blight resistance in the Chinese wheat landrace Wangshuibai'，摘要如下。

文中共列出了4点原因，PFT基因序列在感病材料PH691和望水白、苏麦3号之间完全一致；另外在重组自交系里，一个具有WSB PFT类型的家系却是感病；44个地方品种材料的PFT基因序列与望水白和苏麦3号一致，然而却只有12个表现为抗病类型；PFT基因在望水白和南大2419中的表达较低，侵染之后表达下调。下面引用了原文描述的4点原因。

Firstly, in our work the PFT gene sequence of PH691 was identical to that of WSB and Sumai 3, but PH691 was highly susceptible to F.graminearum.

Secondly, in the RIL population derived from Nanda 2419 × WSB, a recombinant with the WSB PFT allele alone was susceptible in multiple trials (下图-A). Recombinants with the WSB PFT allele alone obtained in secondary F2 populations derived from the NILs showed similar results.

Thirdly, among 151 cultivars used in an association analysis 44 had the WSB/Sumai 3 PFT allele, but only 12 of them were resistant (下图-B).

Fourthly, the expression levels of PFT in WSB,and particularly in Nanda 2419, without F. graminearum infection were extremely low and became negligible after infection (下图-C).

We conclude that association of PFT with FHB resistance in some germplasms is due to its tight linkage to the resistance gene.

显然如果这些质疑成立，那么PFT基因就不是真正的FHB1候选基因，但是文中敲除和过表达PFT之后赤霉病抗性确实也发生了变化，所以这还需要一些时间解决这些矛盾。接下来要说的是PAG上的一篇摘要，题目是“NAC Transcription Factor and Laccase Gene: Key Players in Deciphering FHB Resistance Mechanism in Wheat QTL-Fhb1”，作者是来自加拿大麦吉尔大学的Nancy Soni，摘要如下。

Fusarium head blight (FHB) is one of the most devastating and alarming diseases of wheat around the globe. In addition to causing a loss in wheat crop yield, it also reduces grain quality with mycotoxin contamination. Among 121 quantitative trait loci (QTLs) associated with FHB resistance, QTL-Fhb1 is considered to have major resistance effects. Wheat near isogenic lines (NILs), derived from Sumai 3 and Thatcher cross, were sequenced using Illumina HiSeq technology to capture the genes localized within the fine mapped QTL-Fhb1 region located within a 1.27cM interval. A total of 26 genes were putatively identified, of which, wheat NAC transcription factor (TaNAC), which is also known as a master regulator of plant secondary cell wall biosynthesis, was found polymorphic. Also, a laccase gene (TaLAC) which catalyzes cell wall lignification was also found polymorphic. Associated semi-comprehensive metabolomics study revealed a few important metabolites related to phenylpropanoid and flavonoid pathway with high fold change in pathogen inoculated samples. When the TaNAC or TaLAC silenced, the fungal biomass and the disease severity increased. However, no significant change in RR metabolites observed. In-silico analysis revealed secondary wall NAC binding element (SNBE) site in the promoter region of TaLAC, which suggest the regulation of laccase gene by NAC transcription factor, thus, unveiling the mechanism of FHB resistance associated with QTL-Fhb1.

首先也是通过遗传定位将基因定在一个1.27cM的区间内，包含26个基因，其中TaNAC和TaLAC被认为是潜在的候选基因，NAC是转录因子，通过生信分析表明NAC可能调控TaLAC。他们也与PFT基因不同，除了TaNAC和TaLAC在两个基因之外，其它基因是否还存在差异，这里并没有提。看来FHB1并没有我们想象的这么简单，FHB1究竟是不止一个基因还是区间内存在很多变异在影响某个基因？这也许是迄今为止为啥只有少数材料抗赤霉病的原因之一。总结来看，FHB1的研究还需要进一步跟进，特别是FHB1抗病机制的研究。

第二方面我想谈的赤霉病的抗病机制问题。上文我们已经交代小麦赤霉病特别是FHB1的抗病机制还不清楚，所以下面所谈的内容只是基于现象来推测。因为我对机制这一块也不是特别熟悉，只是有一些自己的想法，本来想在这里简单的说一说，但是通过检索发现2016年的时候刘易科等发表的综述（小麦赤霉病抗性机理研究进展）较好的描述了赤霉病抗性机制的一些研究进展，而且我所能想到的在这篇文章里都有体现，并且人家比我还说的详细还有参考文献，所以这里我就不再详细描述了，大家也可以参阅马正强组的那篇综述文章。

第三个方面其实上边的综述也涉及了，就是小麦近源物种中抗赤霉病基因的挖掘以及从病原菌角度研究赤霉病。所以，对赤霉病感兴趣的小伙伴可以以上述几篇文献为基础进一步拓展阅读。

第四个方面想谈一谈图位克隆，小麦基因组约88%是重复序列，那么在基因定位时有多大概率定位到这些重复区域？区间多态率太高也是一个头疼的事，卡在重复序列里走不出去也是常有的事，区间内有结构变异，参考序列不起作用。运气不好，每走一步都是坑。参考基因组已经释放，很多人都摩拳擦掌准备大干一番，可是小麦这一块真是不容易啊，再加上在功能基因组研究领域积累较少，哪个环节稍微一耽误，五六年就下去了，这还不考虑同行的竞争，很多事情能做到什么程度，其实在源头就被决定了。图位克隆这么多年，很多都是套路了，没有大量的数据以及冲击三观的点存在，还真是路漫漫其修远兮，吾将上下而求索。

我们并不是这方面的专家，有些观点也是基于文献有感而发，也许有些问题还需要具体问题具体分析，所以欢迎指导或拍砖。