Hernández-Zepeda Omar F, Razo-Belman Rosario, Heil Martin
Departamento de Ingeniería Genética, Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional-Irapuato, Guanajuato, Mexico.
Front Plant Sci. 2018 Jul 24;9:1076. doi: 10.3389/fpls.2018.01076. eCollection 2018.
Plants in more than 100 families secrete extrafloral nectar (EFN) to establish food-for-protection mutualisms with ants. Facultative ant-plants secrete EFN as a jasmonic acid (JA)-dependent response to attract generalist ants. In contrast, obligate ant-plants like the Central American "Swollen-Thorn Acacias" are colonized by specialized ants, although an individual host can carry ant colonies from different species that differ in the degree of protection they provide. We hypothesized that hosts that associate simultaneously with various partners should produce rewards in a modular manner to preferentially reward high quality partners. To test this hypothesis, we applied JA to distinct leaves and quantified cell wall invertase activity (CWIN; a regulator of nectar secretion) and EFN secretion by these "local" (i.e., treated) and the "systemic" (i.e., non-treated) leaves of the same branch. Both CWIN activity and EFN secretion increased in local and systemic leaves of the facultative ant-plant , but only in the local leaves of the obligate ant-plant, . The systemic EFN secretion in was associated with an enhanced emission of volatile organic compounds (VOCs). Such VOCs function as "external signals" that control systemic defense responses in diverse plant species. Indeed, the headspace of JA-treated branches of induced EFN secretion in both plant species, whereas the headspace of caused no detectable induction effect. Analyses of the headspace using GC-MS identified six VOCs in the headspace of that were not emitted by . Among these VOCs, β-caryophyllene and ()-hexenyl isovalerate have already been reported in other plant species to induce defense traits, including EFN secretion. Our observations underline the importance of VOCs as systemic within-plant signals and show that the modular rewarding in is likely to result from a reduced emission of the systemic signal, rather than from a reduced responsiveness to the signal. We suggest that modular rewarding allows hosts to restrict the metabolic investment to specific partners and to efficiently sanction potential exploiters.
100多个科的植物会分泌花外蜜（EFN），以与蚂蚁建立“以食物换保护”的互利共生关系。兼性蚁栖植物分泌花外蜜是一种依赖茉莉酸（JA）的反应，用于吸引广食性蚂蚁。相比之下，中美洲的“多刺合欢树”等专性蚁栖植物会被特定的蚂蚁定植，尽管单个宿主可能会携带不同物种的蚁群，而这些蚁群提供的保护程度有所不同。我们推测，同时与不同伙伴建立联系的宿主应该以模块化的方式产生回报，以便优先奖励高质量的伙伴。为了验证这一假设，我们将JA施用于不同的叶片，并对同一枝条上这些“局部”（即处理过的）和“系统”（即未处理的）叶片的细胞壁转化酶活性（CWIN；花蜜分泌的调节因子）和花外蜜分泌进行了量化。在兼性蚁栖植物的局部和系统叶片中，CWIN活性和花外蜜分泌均增加，但仅在专性蚁栖植物的局部叶片中增加。专性蚁栖植物的系统花外蜜分泌与挥发性有机化合物（VOCs）排放增加有关。这些VOCs作为“外部信号”，控制着多种植物物种的系统防御反应。事实上，用JA处理过的专性蚁栖植物枝条的顶空诱导了两种植物的花外蜜分泌，而兼性蚁栖植物的顶空则没有可检测到的诱导作用。使用气相色谱 - 质谱联用仪（GC - MS）对顶空进行分析，在专性蚁栖植物的顶空中鉴定出六种兼性蚁栖植物未释放的VOCs。在这些VOCs中，β-石竹烯和（）-己烯基异戊酸酯在其他植物物种中已被报道可诱导包括花外蜜分泌在内的防御性状。我们的观察结果强调了VOCs作为植物体内系统信号的重要性，并表明专性蚁栖植物的模块化回报可能是由于系统信号排放减少，而不是对信号的反应性降低。我们认为，模块化回报使宿主能够将代谢投入限制在特定伙伴身上，并有效地制裁潜在的剥削者。
