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. 2021 Apr 2;10(4):685.
doi: 10.3390/plants10040685.

Subcellular Localization and Vesicular Structures of Anthocyanin Pigmentation by Fluorescence Imaging of Black Rice (Oryza sativa L.) Stigma Protoplast

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Subcellular Localization and Vesicular Structures of Anthocyanin Pigmentation by Fluorescence Imaging of Black Rice (Oryza sativa L.) Stigma Protoplast

Enerand Mackon et al. Plants (Basel). .

Abstract

Anthocyanins belong to the group of flavonoid compounds broadly distributed in plant species responsible for attractive colors. In black rice (Oryza sativa L.), they are present in the stems, leaves, stigmas, and caryopsis. However, there is still no scientific evidence supporting the existence of compartmentalization and trafficking of anthocyanin inside the cells. In the current study, we took advantage of autofluorescence with anthocyanin's unique excitation/emission properties to elucidate the subcellular localization of anthocyanin and report on the in planta characterization of anthocyanin prevacuolar vesicles (APV) and anthocyanic vacuolar inclusion (AVI) structure. Protoplasts were isolated from the stigma of black and brown rice and imaging using a confocal microscope. Our result showed the fluorescence displaying magenta color in purple stigma and no fluorescence in white stigma when excitation was provided by a helium-neon 552 nm and emission long pass 610-670 nm laser. The fluorescence was distributed throughout the cell, mainly in the central vacuole. Fluorescent images revealed two pools of anthocyanin inside the cells. The diffuse pools were largely found inside the vacuole lumen, while the body structures could be observed mostly inside the cytoplasm (APV) and slightly inside the vacuole (AVI) with different shapes, sizes, and color intensity. Based on their sizes, AVI could be grouped into small (Ф < 0.5 um), middle (Ф between 0.5 and 1 um), and large size (Ф > 1 um). Together, these results provided evidence about the sequestration and trafficking of anthocyanin from the cytoplasm to the central vacuole and the existence of different transport mechanisms of anthocyanin. Our results suggest that stigma cells are an excellent system for in vivo studying of anthocyanin in rice and provide a good foundation for understanding anthocyanin metabolism in plants, sequestration, and trafficking in black rice.

Keywords: anthocyanin; anthocyanin vacuolar inclusion; autofluorescence; confocal microscopy; protoplast; stigma; subcellular localization.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
Anthocyanin pigmentation in different tissues of black rice; (A), a cross-section of stem showing vascular bundle (Vb) containing the anthocyanin; (B), leaf sheath longitudinal section showing a high amount of anthocyanin in parenchyma cells (Pc), xylem (Xy) and phloem (Ph); (C), the longitudinal section of ligule showing mesophyll cell (Mc) filled with the anthocyanin; (D), a cross-section of the caryopsis showing the aleurone layer (AL), the epidermal cells (Ec), and the transverse cells (Tc) colored by the anthocyanin; (E), a cross-section of a leaf blade with the midvein (Mv) containing the anthocyanin; (F), the longitudinal section of a leaf showing long cells (Lc) and silica cells (Sc) filled with anthocyanin, and stomate (st).
Figure 2
Figure 2
HPLC analysis of anthocyanins in the purple stigma and black caryopsis. Peak1 and peak2 represent cyanidin-3-glucoside (C3G) and peonidin-3-glucoside (P3G), respectively.
Figure 3
Figure 3
Characteristics of the two cultivars; the first row indicates a black rice cultivar 18BLN6321 and the second row indicates a brown rice cultivar 4233; (A), the flower of black and brown cultivar, the arrow indicates the stigma; (B), the pistil of both cultivar, the arrow indicates stigma; (C), black and white caryopsis; (D), a cross-section of black and white caryopsis.
Figure 4
Figure 4
Confocal microscopy of unstained protoplast cells; (A), black rice 18BLN6321 showing fluorescence; (B), microscopy of brown rice 4233 no fluorescence. Va indicates the position of the vacuole lumen, while N indicates the position of the nucleus in cells.
Figure 5
Figure 5
Distribution of anthocyanin structures inside the cells; the arrows indicate either anthocyanin prevacuolar vesicles (APV) or anthocyanic vacuolar inclusion (AVI); Va and N indicate the position of central vacuole and nucleus, respectively; (A), subcellular localization of soluble anthocyanin and body structures in the cell, red arrows here indicate the APV; (B), localization of APV alongside the tonoplast indicated with red arrows; (C), AVI in the diffuse anthocyanin, indicated with red arrows.
Figure 6
Figure 6
Identification and estimation of AVI sizes; CV and N are the position of central vacuole and nucleus, respectively; red arrows indicate different sizes of AVI identified.

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