Trehalose Biosynthesis Promotes Pseudomonas aeruginosa Pathogenicity in Plants

Pseudomonas aeruginosa strain PA14 is a multi-host pathogen that infects plants, nematodes, insects, and vertebrates. Many PA14 factors are required for virulence in more than one of these hosts. Noting that plants have a fundamentally different cellular architecture from animals, we sought to identify PA14 factors that are specifically required for plant pathogenesis. We show that synthesis by PA14 of the disaccharide trehalose is required for pathogenesis in Arabidopsis, but not in nematodes, insects, or mice. In-frame deletion of two closely-linked predicted trehalose biosynthetic operons, treYZ and treS, decreased growth in Arabidopsis leaves about 50 fold. Exogenously co-inoculated trehalose, ammonium, or nitrate, but not glucose, sulfate, or phosphate suppressed the phenotype of the double ΔtreYZΔtreS mutant. Exogenous trehalose or ammonium nitrate does not suppress the growth defect of the double ΔtreYZΔtreS mutant by suppressing the plant defense response. Trehalose also does not function intracellularly in P. aeruginosa to ameliorate a variety of stresses, but most likely functions extracellularly, because wild-type PA14 rescued the in vivo growth defect of the ΔtreYZΔtreS in trans. Surprisingly, the growth defect of the double ΔtreYZΔtreS double mutant was suppressed by various Arabidopsis cell wall mutants that affect xyloglucan synthesis, including an xxt1xxt2 double mutant that completely lacks xyloglucan, even though xyloglucan mutants are not more susceptible to pathogens and respond like wild-type plants to immune elicitors. An explanation of our data is that trehalose functions to promote the acquisition of nitrogen-containing nutrients in a process that involves the xyloglucan component of the plant cell wall, thereby allowing P. aeruginosa to replicate in the intercellular spaces in a leaf. This work shows how P. aeruginosa, a multi-host opportunistic pathogen, has repurposed a highly conserved “house-keeping” anabolic pathway (trehalose biosynthesis) as a potent virulence factor that allows it to replicate in the intercellular environment of a leaf.

铜绿假单胞菌（Pseudomonas aeruginosa）PA14菌株是一类可侵染植物、线虫、昆虫及脊椎动物的多宿主病原菌。该菌株的众多毒力因子在其侵染一种及以上宿主时均为致病所必需。鉴于植物与动物的细胞结构存在本质差异，本研究旨在筛选出铜绿假单胞菌PA14中特异性参与植物致病过程的因子。
本研究证实，PA14合成二糖海藻糖（trehalose）是其在拟南芥（Arabidopsis）中致病的必要条件，但对线虫、昆虫及小鼠的致病过程并无此要求。对两个紧密连锁的预测海藻糖生物合成操纵子（operons）treYZ与treS进行框内缺失后，PA14在拟南芥叶片中的增殖水平下降约50倍。
外源共接种海藻糖、铵盐或硝酸盐（而非葡萄糖、硫酸盐或磷酸盐）可缓解双缺失突变体ΔtreYZΔtreS的表型缺陷。外源海藻糖或硝酸铵并未通过抑制植物防御反应来挽救双突变体的生长缺陷。海藻糖也并非在铜绿假单胞菌胞内发挥作用以缓解多种胁迫，其更可能通过胞外途径行使功能：野生型PA14可通过反式途径挽救ΔtreYZΔtreS突变体的体内生长缺陷。
令人意外的是，多种影响木葡聚糖（xyloglucan）合成的拟南芥细胞壁突变体（包括完全缺失木葡聚糖的xxt1xxt2双突变体）均可恢复双缺失突变体的生长缺陷，尽管木葡聚糖突变体并未表现出更高的病原菌易感性，且对免疫激发子（immune elicitor）的响应与野生型植物一致。
对本研究数据的合理解释为：海藻糖通过涉及植物细胞壁木葡聚糖组分的过程，促进宿主含氮营养物质的获取，从而使铜绿假单胞菌能够在叶片的细胞间隙中增殖。本研究揭示了多宿主机会致病菌铜绿假单胞菌如何将高度保守的“持家”合成代谢通路（海藻糖生物合成）重新改造为强效毒力因子，使其能够在叶片细胞间隙环境中完成增殖。