Lin Nai-Chun, Martin Gregory B
Boyce Thompson Institute for Plant Research, Tower Rd., Ithaca, NY 14853-1801, USA.
Mol Plant Microbe Interact. 2007 Jul;20(7):806-15. doi: 10.1094/MPMI-20-7-0806.
The molecular basis underlying the ability of pathogens to infect certain plant species and not others is largely unknown. Pseudomonas syringae is a useful model species for investigating this phenomenon because it comprises more than 50 pathovars which have narrow host range specificities. Tomato (Solanum lycopersicum) is a host for P. syringae pv. tomato, the causative agent of bacterial speck disease, but is considered a nonhost for other P. syringae pathovars. Host resistance in tomato to bacterial speck disease is conferred by the Pto protein kinase which acts in concert with the Prf nucleotide-binding lucine-rich repeat protein to recognize P. syringae pv. tomato strains expressing the type III effectors AvrPto or AvrPtoB (HopAB2). The Pto and Prf genes were isolated from the wild tomato species S. pimpinellifolium and functional alleles of both of these genes now are known to exist in many species of tomato and in other Solanaceous species. Here, we extend earlier reports that avrPto and avrPtoB genes are widely distributed among pathovars of P. syringae which are considered nonhost pathogens of tomato. This observation prompted us to examine the possibility that recognition of these type III effectors by Pto or Prf might contribute to the inability of many P. syringae pathovars to infect tomato species. We show that 10 strains from presumed nonhost P. syringae pathovars are able to grow and cause pathovar-unique disease symptoms in tomato leaves lacking Pto or Prf, although they did not reach the population levels or cause symptoms as severe as a control P. syringae pv. tomato strain. Seven of these strains were found to express avrPto or avrPtoB. The AvrPto- and AvrPtoB-expressing strains elicited disease resistance on tomato leaves expressing Pto and Prf. Thus, a gene-for-gene recognition event may contribute to host range restriction of many P. syringae pathovars on tomato species. Furthermore, we conclude that the diverse disease symptoms caused by different Pseudomonas pathogens on their normal plant hosts are due largely to the array of virulence factors expressed by each pathovar and not to specific molecular or morphological attributes of the plant host.
病原体感染某些植物物种而不感染其他物种的分子基础在很大程度上尚不清楚。丁香假单胞菌是研究这一现象的有用模式物种，因为它包含50多个致病型，每个致病型都有狭窄的宿主范围特异性。番茄（Solanum lycopersicum）是丁香假单胞菌番茄致病型的宿主，该致病型是细菌性斑点病的病原体，但对于其他丁香假单胞菌致病型而言，番茄被视为非宿主。番茄对细菌性斑点病的宿主抗性由Pto蛋白激酶赋予，该激酶与Prf核苷酸结合富含亮氨酸重复序列蛋白协同作用，以识别表达III型效应子AvrPto或AvrPtoB（HopAB2）的丁香假单胞菌番茄致病型菌株。Pto和Prf基因是从野生番茄物种潘那利番茄（S. pimpinellifolium）中分离出来的，现在已知这两个基因的功能等位基因存在于许多番茄物种和其他茄科物种中。在这里，我们扩展了早期的报道，即avrPto和avrPtoB基因广泛分布于被认为是番茄非宿主病原体的丁香假单胞菌致病型中。这一观察结果促使我们研究Pto或Prf对这些III型效应子的识别是否可能导致许多丁香假单胞菌致病型无法感染番茄物种。我们表明，来自假定的非宿主丁香假单胞菌致病型的10个菌株能够在缺乏Pto或Prf的番茄叶片中生长并引起致病型特有的疾病症状，尽管它们没有达到对照丁香假单胞菌番茄致病型菌株的种群水平或引起同样严重的症状。其中7个菌株被发现表达avrPto或avrPtoB。表达AvrPto和AvrPtoB的菌株在表达Pto和Prf的番茄叶片上引发了抗病性。因此，基因对基因的识别事件可能有助于限制许多丁香假单胞菌致病型在番茄物种上的宿主范围。此外，我们得出结论，不同的假单胞菌病原体在其正常植物宿主上引起的各种疾病症状，很大程度上是由于每个致病型表达的一系列毒力因子，而不是由于植物宿主的特定分子或形态特征。
