Tobias Peri A, Christie Nanette, Naidoo Sanushka, Guest David I, Külheim Carsten
School of Life and Environmental Science, Sydney Institute of Agriculture, University of Sydney, Eveleigh, NSW 2015, Australia.
Department of Genetics, Forestry and Agricultural Biotechnology Institute (FABI), Genomics Research Institute, University of Pretoria, Pretoria 0028, South Africa.
Tree Physiol. 2017 May 1;37(5):565-582. doi: 10.1093/treephys/tpx010.
Eucalyptus grandis (W. Hill ex Maiden) is an Australian Myrtaceae tree grown for timber in many parts of the world and for which the annotated genome sequence is available. Known to be susceptible to a number of pests and diseases, E. grandis is a useful study organism for investigating defense responses in woody plants. Chitinases are widespread in plants and cleave glycosidic bonds of chitin, the major structural component of fungal cell walls and arthropod exoskeletons. They are encoded by an important class of genes known to be up-regulated in plants in response to pathogens. The current study identified 67 chitinase gene models from two families known as glycosyl hydrolase 18 and 19 (36 GH18 and 31 GH19) within the E. grandis genome assembly (v1.1), indicating a recent gene expansion. Sequences were aligned and analyzed as conforming to currently recognized plant chitinase classes (I-V). Unlike other woody species investigated to date, E. grandis has a single gene encoding a putative vacuolar targeted Class I chitinase. In response to Leptocybe invasa (Fisher & La Salle) (the eucalypt gall wasp) and Chrysoporthe austroafricana (Gryzenhout & M.J. Wingf. 2004) (causal agent of fungal stem canker), this Class IA chitinase is strongly up-regulated in both resistant and susceptible plants. Resistant plants, however, indicate greater constitutive expression and increased up-regulation than susceptible plants following fungal challenge. Up-regulation within fungal resistant clones was further confirmed with protein data. Clusters of putative chitinase genes, particularly on chromosomes 3 and 8, are significantly up-regulated in response to fungal challenge, while a cluster on chromosome 1 is significantly down-regulated in response to gall wasp. The results of this study show that the E. grandis genome has an expanded group of chitinase genes, compared with other plants. Despite this expansion, only a single Class I chitinase is present and this gene is highly up-regulated within diverse biotic stress conditions. Our research provides insight into a major class of defense genes within E. grandis and indicates the importance of the Class I chitinase.
巨桉（W. Hill ex Maiden）是一种澳大利亚桃金娘科树木，在世界许多地区作为木材树种种植，并且已有其注释基因组序列。已知巨桉易受多种病虫害影响，是研究木本植物防御反应的有用研究对象。几丁质酶广泛存在于植物中，可裂解几丁质的糖苷键，几丁质是真菌细胞壁和节肢动物外骨骼的主要结构成分。它们由一类重要的基因编码，已知这类基因在植物对病原体的反应中会上调。当前研究在巨桉基因组组装（v1.1）中从糖基水解酶18和19两个家族（36个GH18和31个GH19）中鉴定出67个几丁质酶基因模型，表明最近发生了基因扩张。对序列进行比对并分析，使其符合目前公认的植物几丁质酶类别（I - V）。与迄今为止研究的其他木本物种不同，巨桉有一个单一基因编码假定的液泡靶向I类几丁质酶。响应桉树枝瘿姬小蜂（Fisher & La Salle）和澳洲金梭孢（Gryzenhout & M.J. Wingf. 2004）（真菌性茎溃疡病病原体），这种IA类几丁质酶在抗性和感病植物中均强烈上调。然而，抗性植物在真菌侵染后比感病植物表现出更高的组成型表达和更强的上调。蛋白质数据进一步证实了真菌抗性克隆中的上调情况。假定的几丁质酶基因簇，特别是在第3和第8号染色体上的，在响应真菌侵染时显著上调，而第1号染色体上的一个基因簇在响应瘿姬小蜂时显著下调。本研究结果表明，与其他植物相比，巨桉基因组中有一组扩张的几丁质酶基因。尽管有这种扩张，但仅存在一个I类几丁质酶基因，并且该基因在多种生物胁迫条件下高度上调。我们的研究深入了解了巨桉中的一类主要防御基因，并表明了I类几丁质酶的重要性。
