School of Ecosystem and Forest Sciences, The University of Melbourne, Richmond, Victoria, Australia.
School of BioSciences, The University of Melbourne, Parkville, Victoria, Australia.
Ann Bot. 2020 Aug 13;126(3):387-400. doi: 10.1093/aob/mcaa038.
Floral chemical defence strategies remain understudied despite the significance of flowers to plant fitness, and the fact that many flowers contain secondary metabolites that confer resistance to herbivores. Optimal defence and apparency theories predict that the most apparent plant parts and/or those most important to fitness should be most defended. To test whether within-flower distributions of chemical defence are consistent with these theories we used cyanogenic glycosides (CNglycs), which are constitutive defence metabolites that deter herbivores by releasing hydrogen cyanide upon hydrolysis.
We used cyanogenic florets of the genus Lomatia to investigate at what scale there may be strategic allocation of CNglycs in flowers, what their localization reveals about function, and whether levels of floral CNglycs differ between eight congeneric species across a climatic gradient. Within-flower distributions of CNglycs during development were quantified, CNglycs were identified and their localization was visualized in cryosectioned florets using matrix-assisted laser desorption ionization mass spectrometry imaging (MALDI-MSI).
Florets of all congeneric species studied were cyanogenic, and concentrations differed between species. Within florets there was substantial variation in CNglyc concentrations, with extremely high concentrations (up to 14.6 mg CN g-1 d. wt) in pollen and loose, specialized surface cells on the pollen presenter, among the highest concentrations reported in plant tissues. Two tyrosine-derived CNglycs, the monoglycoside dhurrin and diglycoside proteacin, were identified. MALDI-MSI revealed their varying ratios in different floral tissues; proteacin was primarily localized to anthers and ovules, and dhurrin to specialized cells on the pollen presenter. The mix of transient specialized cells and pollen of L. fraxinifolia was ~11 % dhurrin and ~1.1 % proteacin by mass.
Tissue-specific distributions of two CNglycs and substantial variation in their concentrations within florets suggests their allocation is under strong selection. Localized, high CNglyc concentrations in transient cells challenge the predictions of defence theories, and highlight the importance of fine-scale metabolite visualization, and the need for further investigation into the ecological and metabolic roles of CNglycs in floral tissues.
尽管花对植物的适应性很重要，而且许多花含有能抵御草食动物的次生代谢物，但花卉的化学防御策略仍研究不足。最佳防御和外观理论预测，最明显的植物部分和/或对适应性最重要的部分应该受到最严密的保护。为了测试花内化学防御的分布是否符合这些理论，我们使用了氰苷（CNglycs），这是一种组成性防御代谢物，通过水解释放氢氰酸来抵御草食动物。
我们使用 Lomatia 属的氰苷花来研究在什么尺度上可能存在花内 CNglyc 的策略性分配，它们的定位揭示了什么功能，以及在跨越气候梯度的八个同属物种中，花内 CNglyc 的水平是否存在差异。通过定量发育过程中花内的 CNglyc 分布，利用基质辅助激光解吸电离质谱成像（MALDI-MSI）鉴定花内的 CNglyc 并对其进行定位。
研究的所有同属物种的花都是氰苷，而且物种之间的浓度不同。在花内，CNglyc 浓度存在很大差异，花粉中浓度极高（高达 14.6mgCNg-1d.wt），花粉呈递器上的松散、特殊的表面细胞中浓度最高，这是植物组织中报道的最高浓度之一。两种酪氨酸衍生的 CNglyc，单糖苷 dhurrin 和二糖苷 proteacin，被鉴定出来。MALDI-MSI 显示了它们在不同花组织中的不同比例；proteacin 主要定位于花药和胚珠，dhurrin 主要定位于花粉呈递器上的特殊细胞。L. fraxinifolia 的花粉和特殊细胞混合物中，dhurrin 的质量约为 11%，proteacin 的质量约为 1.1%。
两种 CNglyc 的组织特异性分布以及花内浓度的显著变化表明它们的分配受到强烈选择。在短暂的特殊细胞中局部存在高浓度的 CNglyc，这对防御理论的预测提出了挑战，并强调了精细尺度代谢物可视化的重要性，以及进一步研究花组织中 CNglyc 的生态和代谢作用的必要性。
