Variation in defence strategies in the metal hyperaccumulator Noccaea caerulescens is indicative of synergies and trade-offs between forms of defence

The metal hyperaccumulator, N. caerulescens, uses metals as a defence against pathogens. Earlier work has suggested both trade-offs and synergies between metals and inducible defences. Different populations of N. caerulescens vary in metal accumulation. Here, we test the hypothesis that this produces different outcomes in trade-offs between defences. We compare zinc concentrations, glucosinolate concentrations, and inducible stress responses, including ROS and cell death, in four N. caerulescens populations, and relate these to the growth of the plant pathogen Pseudomonas syringae, its zinc tolerance mutants, and Pseudomonas pathogens isolated from a natural population of N. caerulescens. The populations display strikingly different defences. Where defences are successful, pathogens are limited primarily by metals, cell death, or organic defences, with evidence of both trade-offs and synergies between these, depending upon population. In addition, we find evidence that Pseudomonas pathogens may evolve to overcome any of these strategies, indicating that the arms race continues. These data indicate that defensive enhancement, joint effects and trade-offs between forms of defence are all plausible explanations for the evolution of metal-based defences, with factors including metal availability and pressures from metal-tolerant pathogens likely shaping the current strategy of each ecotype.

金属超富集植物（metal hyperaccumulator）N. caerulescens可利用金属作为抵御病原菌的防御手段。既往研究表明，金属与诱导型防御之间既存在权衡关系，也存在协同效应。该物种的不同种群在金属富集能力上存在差异。本研究旨在验证这一假说：即这种差异会导致防御策略间的权衡产生不同结果。
我们对四个N. caerulescens种群的锌浓度、硫代葡萄糖苷（glucosinolate）浓度以及包括活性氧（ROS）和细胞死亡在内的诱导型应激反应进行了比较，并将这些指标与植物病原菌丁香假单胞菌（Pseudomonas syringae）、其锌耐受突变株，以及从N. caerulescens自然种群中分离得到的假单胞菌（Pseudomonas）病原菌的生长情况进行关联分析。
研究结果显示，不同种群展现出截然不同的防御策略。在防御策略奏效的场景中，病原菌的增殖主要受金属、细胞死亡或有机防御机制的限制；且不同防御方式间的权衡与协同效应因种群而异。
此外，我们发现假单胞菌病原菌可能演化出克服上述各类防御策略的能力，这表明宿主与病原菌间的军备竞赛仍在持续。
本研究数据表明，防御增强、防御形式间的联合效应与权衡关系，均可能是基于金属的防御策略演化的合理解释；而金属可用性、金属耐受病原菌带来的选择压力等因素，或共同塑造了每种生态型当前的防御策略。