Centre of Electron Microscopy (CME), Institute of Biosciences of Botucatu (IBB), São Paulo State University (UNESP), Botucatu, São Paulo, Brazil.
Department of Biostatistics, Plant Biology, Parasitology and Zoology, Institute of Biosciences of Botucatu (IBB), São Paulo State University (UNESP), Botucatu, São Paulo, Brazil.
Planta. 2021 Jul 3;254(2):19. doi: 10.1007/s00425-021-03663-8.
The cytological changes underlying the formation of an apoplasmic barrier in the multi-layered extrafloral nectaries of Citharexylum myrianthum are compatible with the synthesis, transport and deposition of suberin. In terms of ontogenesis and function, the intermediate layers of these nectaries are homologous with the stalks of nectar-secreting trichomes. Anticlinal cell wall impregnations are common in trichomatic nectaries and their functions as endodermis-like barriers have been discussed because of possible direct effects on the nectary physiology, mainly in the nectar secretion and resorption. However, the cytological events linked to nectary wall impregnations remain little explored. This study documents the ontogenesis and the fine structure of the EFN cells, and cytological events linked to the wall impregnations of multi-layered extrafloral nectaries (EFNs) in Citharexylum myrianthum Cham. (Verbenaceae). EFNs are patelliform, and differentiated into (a) a multicellular foot, which is compound in structure and vascularised with phloem strands, (b) a bi-layered intermediate region with thickened cell walls and (c) a single-layered secretory region with palisade-like cells. EFNs are protodermal in origin, starting with a single protodermal cell and ending with the complex, multi-layered structure. The cell wall impregnations first appear in the very young EFN and increase towards maturity. Lipid patches (assumed to be suberin) are deposited on the inner faces of the primary walls, first along the anticlinal walls and then extend to the periclinal walls. On both walls, plasmodesmata remain apparently intact during the maturation of the EFNs. In the peripheral cytoplasm there are abundant polymorphic plastids, well-developed Golgi bodies often close to rough endoplasmic reticulum profiles, mitochondria and polyribosomes. Cytological events linked to the wall impregnations are consistent with suberin synthesis, transport and deposition. Our findings offer new insights into the structure-properties of specialised nectary cell walls and so should contribute to our knowledge of the physiological and protective roles of this structure in nectar glands.
多层叶下外泌蜜腺中形成质外体屏障的细胞学变化与角质的合成、运输和沉积一致。就发生和功能而言，这些蜜腺的中间层与分泌花蜜的毛状蜜腺的柄同源。在毛状蜜腺中，常可见到沿细胞分裂面的细胞壁浸渍，并且由于其对蜜腺生理学的可能直接影响（主要在花蜜分泌和吸收中），其被讨论为类似于内皮层的屏障。然而，与蜜腺壁浸渍相关的细胞学事件仍知之甚少。本研究记录了多层叶下外泌蜜腺（EFN）在金叶风毛菊（马鞭草科）中的发生和精细结构，以及与细胞壁浸渍相关的细胞学事件。EFN 呈盘状，分化为（a）结构复杂且具韧皮部束的多细胞足，（b）具加厚细胞壁的双层中间区，以及（c）具栅栏状细胞的单层分泌区。EFN 起源于原表皮细胞，从单个原表皮细胞开始，最终形成复杂的多层结构。细胞壁浸渍首先出现在非常年轻的 EFN 中，并随着成熟而增加。脂质斑（假定为角质）沉积在初生壁的内表面上，首先沿垂周壁，然后延伸至平周壁。在 EFN 成熟过程中，胞间连丝在这两个壁上似乎保持完整。在周围细胞质中有丰富的多形质体、发育良好的高尔基体，常靠近粗糙内质网轮廓、线粒体和多核糖体。与细胞壁浸渍相关的细胞学事件与角质的合成、运输和沉积一致。我们的发现为特化蜜腺细胞壁的结构-性质提供了新的见解，因此应该有助于我们了解该结构在花蜜腺中的生理和保护作用。
