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本期作者：Rui Wang

本来今天按计划是继续我们生信小菜鸟的养成记，但是今天小菜鸟在窝里不想出来了，我们就来谈谈腐生病原菌的抗病和育种问题。其实真正的起因是昨天我们的主编在群里分享了一篇即将在NG上发表的Septoria tritici blotch (STB) 抗病文章，正好小编自己博士阶段的研究也是相关方面的，现在就借这个机会来谈谈腐生病原菌的致病和抗病机理。今天这一系列文章也算是我们公众号首次正式的跟大家介绍这方面的内容了！

在开始之前，先概述一下基本的研究背景：大家对于小麦抗病机理的理解大多来自于锈病和白粉病这种危害比较大的疾病，但一定要注意这些疾病都是由一些共生病原菌造成的。这些共生病原菌侵染植物的目的是跟宿主共生，然后宿主最普遍的一种抗病手段就是通过HR反应来杀死自己一些局部的细胞和组织，这样共生病原菌就失去了营养来源，也就被抑制在了这个局部的组织处。而经典的分子抗病模型就是gene-for-gene model，如下图，R gene 识别 Avr gene来诱导抗性。在这个模型中，要想成功侵入宿主，那就要求病原菌没有Avr，而宿主要想抗病，那就必须含有R蛋白，所以这种模型的抗病基因一般都是Dominant Resistance。

但今天介绍的腐生病原菌可就大不相同了，顾名思义，腐生菌就是要生活在死的组织上，所以像上边那种HR反应的手段不仅不能抗病，反而帮助病原菌更好地生存了。事实上，根据研究发现（植病经典文章：The Cysteine Rich Necrotrophic Effector SnTox1 Produced by Stagonospora nodorum Triggers Susceptibility of Wheat Lines Harboring Snn1），腐生菌也就是利用了类似HR的生理过程，在侵染过程中释放一些毒性蛋白，诱导更为激烈的HR，我们称之为细胞程序性死亡Programmed cell death (PCD) 的过程来达到让植物的细胞没有止境的坏死，然后它们从死细胞中吸取营养。这个过程用模型归纳起来有点类似于反着的gene-for-gene model （这个模型目前还不能广泛应用），就如下图所示，在这个模型中，要想植物感病，病原菌首先要有毒素基因，并且植物中必须有能被这个毒素基因结合的蛋白。所以这种病的抗病基因一般是Recessive Resistance（Dominant Susceptibility）

提到这，小编也来炫耀一下，以上这两个model都是我读博所在NDSU和相关美国农部实验室共同提出来的。

相关文献

1. The wheat Stb6 gene controlling agene-for-gene resistance to Zymoseptoria tritici encodes a wall-associated kinase-like protein

Nature Genetics Feb. 2018
由于系统问题，现在这篇文章在线查不到了，以下的简介是该文在2016年一次会议的摘要。

导读：接下来的前四篇文章都是围绕Septoria tritici blotch (STB)来展开的，第一篇发现抗病基因符合gene-for-gene模型，跟一般的腐生菌抗病“ reverse gene-for-gen”模型不一样，这也就是为什么能发NG的原因吧。

Z. tritici is an ascomycete fungus that causesSeptoria tritici blotch (STB), a globally economically damaging foliar disease of wheat. Resistance to STB is an important target in wheat breeding and during the past several decades at least 18 major resistance loci, most of which confer an isolate-specific resistance, have been identified and genetically mapped. However, none of these have so far been cloned. Stb6 is the most well-characterized resistance gene that controls Z. tritici isolates carrying a matching avirulence gene AvrStb6 via an unknown mechanism not involving HR. Here we report the isolation of Stb6 using a combination of map-based cloning, VIGS, TILLING and stable wheat transformation. We show that Stb6 encodes a transmembrane protein with an extracellular galacturonan-binding domain and a cytoplasmic kinase domain, and thus is a member of the wall-associated receptor kinase(WAK) gene family. This finding is in line with Z. tritici being a pathogen that colonizes leaf apoplast and does not penetrate host plant cells or develop appressoria. We also show that the common disease susceptibility alleles contain non-synonymous mutations corresponding to changes at the conserved amino acid residues in the kinase domain of the encoded protein, suggesting a mechanism of susceptibility which involves a loss of kinase activity. To our knowledge, this is the first report demonstrating control of gene-for-gene resistance by a WAK-like protein in plants.

https://www.ismpmi.org/congress/2016/abstracts/pages/abstractdetail.aspx?LID=689

2. A small secreted protein in Zymoseptoria tritici is responsible for avirulence on wheat cultivars carrying the Stb6 resistance gene

New Phytologist Feb. 2017

导读：第二篇到第四篇讲的是病原菌这个角度的基因克隆，AvrStb6正好是上文中跟Stb6对应的因子。

1. Zymoseptoria tritici is the causal agent of Septoria tritici blotch, a major pathogen of wheat globally and the most damaging pathogen of wheat in Europe. A gene-for-gene (GFG) interaction between Z. tritici and wheat cultivars carrying the Stb6 resistance gene has been postulated for many years, but the genes have not been identified.

2. We identified AvrStb6 by combining quantitative trait locus mapping in a cross between two Swiss strains with a genome-wide association study using a natural population of c. 100 strains fromFrance. We functionally validated AvrStb6 using ectopic transformations.

3. AvrStb6 encodes a small, cysteine-rich, secreted protein that produces an avirulence phenotype on wheat cultivars carrying the Stb6 resistance gene. We found 16 nonsynonymous single nucleotide polymorphisms among the tested strains, indicating that AvrStb6 is evolving very rapidly. AvrStb6 is located in a highly polymorphic subtelomeric region and is surrounded by transposable elements, which may facilitate its rapid evolution to overcome Stb6 resistance.

4. AvrStb6 is the first avirulence gene to be functionally validated in Z. tritici, contributing to our understanding of avirulence in apoplastic pathogens and the mechanisms underlying GFGinteractions between Z. tritici and wheat.

http://onlinelibrary.wiley.com/wol1/doi/10.1111/nph.14434/full

3. Have we finally opened the door to understanding Septoria tritici blotch disease in wheat?

New Phytologist Mar. 2017

这篇是第二篇的评论性文章

The ability of a fungal pathogen to colonize and infect a plant is a special and unique process. Nearly all plant-fungal interactions occur out of sight as plants have evolved complex systems to recognize and resist most pathogens. However, there is a low percentage of fungal species that have evolved tools to circumvent host defense and cause disease, and it is the basis of these interactions which is of intense interest amongst plant and microbe researchers. It is now recognized that pathogens facilitate disease by secreting virulence molecules (also known as effectors) that target and manipulate endogenous host processes. Consequently, the identification and characterization of these effectors has been the focus of the molecular plant pathology field now for several decades, not only to understand the basis of disease, but also to underpin novel disease management strategies. In the study by Zhong et al. in this issue of New Phytologist (pp.619–631), the authors have identified the first effector from the devastating wheat pathogen Zymoseptoria tritici involved in a gene-for-gene interaction with a host resistance protein and, in doing so,have opened the door to finally begin to understand how it inflicts devastating yield losses.

http://onlinelibrary.wiley.com/doi/10.1111/nph.14502/full

4. Characterization of an antimicrobial and phytotoxic ribonuclease secreted by the fungal wheat pathogen Zymoseptoria tritici

New Phytologist Sept. 2017

导读：第四篇仍然是病原菌角度克隆致病因子，effector Zt6，这个因子应该符合Reverse gene-for-gene model，因为它是一个毒素蛋白，不过这个研究的创新点在于发现这个因子有双重效用，除了做为毒素侵染宿主以外，还能做为antimicrobial因子来抑制其它病原菌的生长进而减少竞争（这些毒素要是能被提纯开发成广谱抗生素或者抗真菌药物，那可是了不得的成果！）

1. The fungus Zymoseptoria tritici is the causal agent of SeptoriaTritici Blotch (STB) disease of wheat leaves. Zymoseptoria tritici secretes many functionally uncharacterized effector proteins during infection. Here, we characterized a secreted ribonuclease (Zt6) with an unusual biphasic expression pattern.

2. Transient expression systems were used to characterize Zt6, and mutants thereof, in both host and non-hostplants. Cell-free protein expression systems monitored the impact of Zt6protein on functional ribosomes, and in vitro assays of cells treated with recombinant Zt6 determined toxicity against bacteria, yeasts, and filamentous fungi.

3. We demonstrated that Zt6 is a functional ribonuclease and that phytotoxicity is dependent on both the presence of a 22-amino-acid N-terminal ‘loop’ region and its catalytic activity.Zt6 selectively cleaves both plant and animal rRNA species and is toxic to wheat, tobacco, bacterial and yeast cells, but not to Z. tritici itself.

4. Zt6 is the first Z. tritici effector demonstrated to have a likely dual functionality. The expression pattern of Zt6 and potent toxicity towards microorganisms suggest that, although it may contribute to the execution of wheat cell death, it is also likely to have an important secondary function in antimicrobial competition and nice protection.

http://onlinelibrary.wiley.com/doi/10.1111/nph.14786/full

5. The hijacking of a receptor kinase-driven pathway bya wheat fungal pathogen leads to disease

Science Advances Oct. 2016

导读：这篇是经典的reversegene-for-gene model 的研究文章，出自我史大哥之手！

Necrotrophic pathogens live and feed on dying tissue, but their interactions with plants are not well understood compared to biotrophic pathogens. The wheat Snn1 gene confers susceptibility to strains of the necrotrophic pathogen Parastagonospora nodorum that produce the SnTox1 protein. We report the positional cloning of Snn1, a member of the wall-associated kinase class of receptors, which are known to drive pathways for biotrophic pathogen resistance. Recognition of SnTox1by Snn1 activates programmed cell death, which allows this necrotroph to gain nutrients and sporulate. These results demonstrate that necrotrophic pathogens such as P. nodorum hijack host molecular pathways that are typically involved in resistance to biotrophic pathogens, revealing the complex nature of susceptibility and resistance in necrotrophic and biotrophic pathogen interactions with plants.

    http://advances.sciencemag.org/content/2/10/e1600822

两篇综述性文章：

文章发的再牛，用到田里才算成功！宋老师前天的推送“抗病育种之我遐想”我真是感受颇深，我自身其实就是从分子植病的背景转到育种课题组，起初也是不能太理解育种实践中的一些思维和操作，但是现在越来越明白，在小麦研究领域中，理论和应用还是有很大的一个鸿沟的。不过鸿沟越大，我们这些小麦研究者们才越有奋斗的动力和价值!

6. Navigating complexity to breed disease-resistant crops

Nature Reviews Genetics Jan. 2018

Plant diseases are responsible for substantial crop losses each year and pose a threat to global food security and agricultural sustainability. Improving crop resistance to pathogens through breeding is an environmentally sound method for managing disease and minimizing these losses. However, it is challenging to breed varieties with resistance that is effective, stable and broad-spectrum. Recent advances in genetic and genomic technologies have contributed to a better understanding of the complexity of host-pathogen interactions and have identified some of the genes and mechanisms that underlie resistance. This new knowledge is benefiting crop improvement through better-informed breeding strategies that utilize diverse forms of resistance at different scales, from the genome of a single plant to the plant varieties deployed across a region.

https://www.nature.com/articles/nrg.2017.82

7. A review of wheat diseases—a field perspective （这篇我们在10/24推送过：小麦病害综述--除了白粉病）

Molecular Plant Pathology Dec. 2017

Wheat is one of the primary staple foods throughout the planet.Significant yield gains in wheat production over the past 40 years have resulted in a steady balance of supply versus demand. However, predicted global population growth rates and dietary changes mean that substantial yield gains over the next several decades will be needed to meet this escalating demand. Akey component to meeting this challenge is better management of fungal incited diseases, which can be responsible for 15%–20% yield losses per annum.Prominent diseases of wheat that currently contribute to these losses include the rusts, blotches and head blight/scab. Other recently emerged or relatively unnoticed diseases, such as wheat blast and spot blotch, respectively, also threaten grain production. This review seeks to provide an overview of the impact, distribution and management strategies of these diseases. In addition, the biology of the pathogens and the molecular basis of their interaction with wheat are discussed.

http://onlinelibrary.wiley.com/doi/10.1111/mpp.12618/full

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