Natural Product Biotechnology Group, Agricultural and Food Engineering Department, Indian Institute of Technology Kharagpur, Kharagpur - 721 302, India.
Ann Bot. 2024 Apr 23;133(4):621-642. doi: 10.1093/aob/mcae019.
Extrafloral nectaries are nectar-secreting structures present on vegetative parts of plants which provide indirect defences against herbivore attack. Extrafloral nectaries in Clerodendrum chinense are patelliform-shaped specialized trichomatous structures. However, a complete understanding of patelliform extrafloral nectaries in general, and of C. chinense in particular, has not yet been established to provide fundamental insight into the cellular physiological machinery involved in nectar biosynthesis and secretory processes.
We studied temporal changes in the morphological, anatomical and ultrastructural features in the architectures of extrafloral nectaries. We also compared metabolite profiles of extrafloral nectar, nectary tissue, non-nectary tissue and phloem sap. Further, both in situ histolocalization and normal in vitro activities of enzymes related to sugar metabolism were examined.
Four distinct tissue regions in the nectar gland were revealed from histochemical characterization, among which the middle nectariferous tissue was found to be the metabolically active region, while the intermediate layer was found to be lipid-rich. Ultrastructural study showed the presence of a large number of mitochondria along with starch-bearing chloroplasts in the nectariferous region. However, starch depletion was noted with progressive maturation of nectaries. Metabolite analysis revealed compositional differences among nectar, phloem sap, nectary and non-nectary tissue. Invertase activity was higher in secretory stages and localized in nectariferous tissue and adjacent region.
Our study suggests extrafloral nectar secretion in C. chinense to be both eccrine and merocrine in nature. A distinct intermediate lipid-rich layer that separates the epidermis from nectary parenchyma was revealed, which possibly acts as a barrier to water flow in nectar. This study also revealed a distinction between nectar and phloem sap, and starch could act as a nectar precursor, as evidenced from enzymatic and ultrastructural studies. Thus, our findings on changing architecture of extrafloral nectaries with temporal secretion revealed a cell physiological process involved in nectar biosynthesis and secretion.
花外蜜腺是存在于植物营养器官上的分泌花蜜的结构，为植物提供了间接抵御草食动物攻击的防御机制。臭牡丹（Clerodendrum chinense）的花外蜜腺为盘状特化的具毛三细胞结构。然而，目前尚未完全了解盘状花外蜜腺的一般结构，特别是臭牡丹的花外蜜腺结构，因此无法深入了解参与花蜜生物合成和分泌过程的细胞生理机制。
我们研究了花外蜜腺结构在形态、解剖和超微结构方面的时间变化，并比较了花外蜜、蜜腺组织、非蜜腺组织和韧皮部汁液的代谢物谱。此外，我们还检查了与糖代谢相关的酶的原位组织化学定位和正常体外活性。
通过组织化学特征，揭示了蜜腺中的 4 个不同的组织区域，其中中间的蜜腺组织被发现是代谢活跃的区域，而中间层富含脂质。超微结构研究表明，在蜜腺区域存在大量的线粒体和含有淀粉的叶绿体。然而，随着蜜腺的成熟，淀粉逐渐耗尽。代谢物分析显示，花蜜、韧皮部汁液、蜜腺组织和非蜜腺组织之间存在成分差异。在分泌阶段，转化酶活性较高，定位于蜜腺组织和相邻区域。
我们的研究表明，臭牡丹的花外蜜分泌是外分泌和顶浆分泌的混合形式。我们揭示了一个独特的中间富含脂质的层，它将表皮与蜜腺实质分开，这可能起到阻止花蜜中水分流动的屏障作用。这项研究还揭示了花蜜和韧皮部汁液之间的区别，并且淀粉可以作为花蜜的前体，这可以从酶学和超微结构研究中得到证明。因此，我们关于花外蜜腺随时间分泌而改变的结构的发现，揭示了一个参与花蜜生物合成和分泌的细胞生理过程。
