Iwate Biotechnology Research Center, Kitakami, Iwate, Japan.
Iwate Agricultural Research Center, Kitakami, Iwate, Japan.
PLoS One. 2020 Aug 26;15(8):e0237173. doi: 10.1371/journal.pone.0237173. eCollection 2020.
Gentian is an important ornamental flower in Japan. The corolla of the majority of cultivated Japanese gentians have green spots, which are rarely encountered in flowers of other angiosperms. Little information is available on the functional traits of the green spots. In this study, we characterized the green spots in the Japanese gentian corolla using a number of microscopic techniques. Opto-digital microscopy revealed that a single visible green spot is composed of approximately 100 epidermal cells. The epidermal cells of a green spot formed a dome-like structure and the cell lumen contained many green structures that were granular and approximately 5 μm in diameter. The green structures emitted red autofluorescence when irradiated with 488 nm excitation light. Transmission electron microscopy revealed that the green structures contained typical thylakoids and grana, thus indicating they are chloroplasts. No grana were observed and the thylakoids had collapsed in the plastids of epidermal cells surrounding green spots. To estimate the rate of photosynthetic electron transfer of the green spots, we measured chlorophyll fluorescence using the MICROSCOPY version of an Imaging-PAM (pulse-amplitude-modulated) fluorometer. Under actinic light of 449 μmol m-2 s-1, substantial electron flow through photosystem II was observed. Observation of green spot formation during corolla development revealed that immature green spots formed at an early bud stage and developed to maturity associated with chloroplast degradation in the surrounding epidermal cells. These results confirmed that the Japanese gentian corolla contains functional chloroplasts in restricted areas of epidermal cells and indicated that a sophisticated program for differential regulation of chloroplast formation and degradation is operative in the epidermis.
龙胆是日本重要的观赏花卉。大多数栽培的日本龙胆的花冠上有绿色斑点，这在被子植物的花朵中很少见。关于绿色斑点的功能特性的信息很少。在这项研究中，我们使用了多种显微镜技术对日本龙胆花冠上的绿色斑点进行了表征。光电数字显微镜显示，一个可见的绿色斑点由大约 100 个表皮细胞组成。绿色斑点的表皮细胞形成一个穹顶状结构，细胞腔内有许多绿色结构，呈颗粒状，直径约为 5μm。用 488nm 激发光照射时，绿色结构发出红色自发荧光。透射电子显微镜显示，绿色结构含有典型的类囊体和基粒，因此表明它们是叶绿体。在绿色斑点周围表皮细胞的质体中未观察到基粒，而且类囊体已经塌陷。为了估计绿色斑点光合作用电子转移的速率，我们使用成像-PAM（脉冲幅度调制）荧光计的 MICROSCOPY 版本测量叶绿素荧光。在 449 μmol m-2 s-1 的光下，观察到大量电子通过光系统 II。在花冠发育过程中观察到绿色斑点的形成，发现不成熟的绿色斑点在早期花蕾阶段形成，并随着周围表皮细胞中叶绿体的降解而发育成熟。这些结果证实了日本龙胆花冠在表皮细胞的特定区域含有功能正常的叶绿体，并表明在表皮中存在一种复杂的差异调控叶绿体形成和降解的程序。
