Department of Plant Pathology, University of Wisconsin-Madison, Madison, Wisconsin, USA.
mBio. 2012 Aug 31;3(4). doi: 10.1128/mBio.00114-12. Print 2012.
Plant xylem fluid is considered a nutrient-poor environment, but the bacterial wilt pathogen Ralstonia solanacearum is well adapted to it, growing to 10(8) to 10(9) CFU/g tomato stem. To better understand how R. solanacearum succeeds in this habitat, we analyzed the transcriptomes of two phylogenetically distinct R. solanacearum strains that both wilt tomato, strains UW551 (phylotype II) and GMI1000 (phylotype I). We profiled bacterial gene expression at ~6 × 10(8) CFU/ml in culture or in plant xylem during early tomato bacterial wilt pathogenesis. Despite phylogenetic differences, these two strains expressed their 3,477 common orthologous genes in generally similar patterns, with about 12% of their transcriptomes significantly altered in planta versus in rich medium. Several primary metabolic pathways were highly expressed during pathogenesis. These pathways included sucrose uptake and catabolism, and components of these pathways were encoded by genes in the scrABY cluster. A UW551 scrA mutant was significantly reduced in virulence on resistant and susceptible tomato as well as on potato and the epidemiologically important weed host Solanum dulcamara. Functional scrA contributed to pathogen competitive fitness during colonization of tomato xylem, which contained ~300 µM sucrose. scrA expression was induced by sucrose, but to a much greater degree by growth in planta. Unexpectedly, 45% of the genes directly regulated by HrpB, the transcriptional activator of the type 3 secretion system (T3SS), were upregulated in planta at high cell densities. This result modifies a regulatory model based on bacterial behavior in culture, where this key virulence factor is repressed at high cell densities. The active transcription of these genes in wilting plants suggests that T3SS has a biological role throughout the disease cycle. IMPORTANCE Ralstonia solanacearum is a widespread plant pathogen that causes bacterial wilt disease. It inflicts serious crop losses on tropical farmers, with major economic and human consequences. It is also a model for the many destructive microbes that colonize the water-conducting plant xylem tissue, which is low in nutrients and oxygen. We extracted bacteria from infected tomato plants and globally identified the biological functions that R. solanacearum expresses during plant pathogenesis. This revealed the unexpected presence of sucrose in tomato xylem fluid and the pathogen's dependence on host sucrose for virulence on tomato, potato, and the common weed bittersweet nightshade. Further, R. solanacearum was highly responsive to the plant environment, expressing several metabolic and virulence functions quite differently in the plant than in pure culture. These results reinforce the utility of studying pathogens in interaction with hosts and suggest that selecting for reduced sucrose levels could generate wilt-resistant crops.
植物木质部液被认为是一种营养贫瘠的环境，但细菌枯萎病原体丁香假单胞菌能够很好地适应它，在番茄茎中生长到 10(8)到 10(9) CFU/g。为了更好地了解丁香假单胞菌如何在这种栖息地中成功生存，我们分析了两个在系统发育上有明显区别的丁香假单胞菌菌株的转录组，这两个菌株都能使番茄枯萎，菌株 UW551（II 型）和 GMI1000（I 型）。我们在培养物中或在番茄细菌性枯萎病发病早期的植物木质部中，以约 10(8)CFU/ml 的细菌密度对细菌基因表达进行了分析。尽管存在系统发育差异，但这两个菌株以通常相似的模式表达了它们的 3477 个共同的直系同源基因，其中约 12%的转录本在植物体内与在丰富培养基中的差异显著。一些主要的代谢途径在发病过程中高度表达。这些途径包括蔗糖的摄取和分解代谢，而这些途径的组成部分由 scrABY 簇中的基因编码。UW551scrA 突变体在抗性和敏感番茄以及马铃薯和流行病学上重要的杂草宿主 Solanum dulcamara 上的毒力显著降低。在番茄木质部的定殖过程中，蔗糖摄取和分解代谢等途径的表达水平较高，而 scrABY 簇中的基因编码的途径也表达较高。UW551scrA 突变体在抗性和敏感番茄以及马铃薯和流行病学上重要的杂草宿主 Solanum dulcamara 上的毒力显著降低。在番茄木质部的定殖过程中，蔗糖摄取和分解代谢等途径的表达水平较高，而 scrABY 簇中的基因编码的途径也表达较高。在番茄木质部的定殖过程中，蔗糖摄取和分解代谢等途径的表达水平较高，而 scrABY 簇中的基因编码的途径也表达较高。scrA 突变体在番茄木质部中的竞争适应性显著降低，而在富含蔗糖的番茄木质部中，scrA 的表达受到蔗糖的诱导，但在植物体内的诱导程度要大得多。出乎意料的是，直接受转录激活因子 HrpB 调控的 45%的基因在高细胞密度时在植物体内被上调。这一结果改变了基于细菌在培养物中行为的调控模型，在该模型中，这种关键毒力因子在高细胞密度时受到抑制。这些基因在萎蔫植物中的转录活性表明，T3SS 在整个疾病周期中都具有生物学功能。重要的是，丁香假单胞菌是一种广泛存在的植物病原体，它会引起细菌性枯萎病。它给热带农民造成了严重的作物损失，造成了重大的经济和人类后果。它也是许多破坏微生物的模型，这些微生物定植在低营养和低氧的输水植物木质部组织中。我们从感染的番茄植物中提取细菌，并对其在植物发病过程中的生物功能进行了全面鉴定。这揭示了意想不到的番茄木质部液中存在蔗糖，以及宿主蔗糖对番茄、马铃薯和常见杂草茄属植物的毒力作用。此外，丁香假单胞菌对植物环境高度敏感，在植物体内的代谢和毒力功能与纯培养物中的表达有很大不同。这些结果强化了在与宿主相互作用的情况下研究病原体的实用性，并表明选择降低蔗糖水平可能会产生抗枯萎病的作物。
