Ultrastructural characterization of microlipophagy induced by the interaction of vacuoles and lipid bodies around genera
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ORIGINAL ARTICLE
Ultrastructural characterization of microlipophagy induced by the interaction of vacuoles and lipid bodies around generative and sperm cells in Arabidopsis pollen Kae Akita 1 & Tomoko Takagi 1 & Keiko Kobayashi 1 & Kazuyuki Kuchitsu 2 & Tsuneyoshi Kuroiwa 1 & Noriko Nagata 1 Received: 3 July 2020 / Accepted: 8 September 2020 # The Author(s) 2020
Abstract During pollen maturation, various organelles change their distribution and function during development as male gametophytes. We analyzed the behavior of lipid bodies and vacuoles involved in lipophagy in Arabidopsis pollen using serial section SEM and conventional TEM. At the bicellular pollen stage, lipid bodies in the vegetative cells lined up at the surface of the generative cell. Vacuoles then tightly attached, drew in, and degraded the lipid bodies and eventually occupied the space of the lipid bodies. Degradation of lipid began before transfer of the entire contents of the lipid body. At the tricellular stage, vacuoles instead of lipid bodies surrounded the sperm cells. The degradation of lipid bodies is morphologically considered microautophagy. The atg2-1 Arabidopsis mutant is deficient in one autophagy-related gene (ATG). In this mutant, the assembly of vacuoles around sperm cells was sparser than that in wild-type pollen. The deficiency of ATG2 likely prevents or slows lipid degradation, although it does not prevent contact between organelles. These results demonstrate the involvement of microlipophagy in the pollen development of Arabidopsis. Keywords ATG2 . Lipid body . Lipophagy . Microautophagy . Pollen . Vacuole
Introduction Several ultrastructural studies show that various organelles dramatically change shape, structure, and distribution during pollen development (Marciniec et al. 2019; Nagata 2010; Paul et al. 2016; Tchórzewska 2017). For example, pollen grains contain large numbers of lipid storage bodies. Lipid bodies are organelles enclosed by a single membrane that mainly contain lipid esters, i.e., triacylglycerols and cholesteryl esters. These bodies participate in the modulation of neutral lipid metabolism (Fujimoto and Parton 2011). Lipidic organelles are Handling Editor: Liwen Jiang Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00709-020-01557-2) contains supplementary material, which is available to authorized users. * Noriko Nagata [email protected] 1
Department of Chemical Biological Sciences, Faculty of Science, Japan Women’s University, Bunkyo-ku, Tokyo, Japan
2
Department of Applied Biological Science, Tokyo University of Science, Noda, Chiba, Japan
generally identified as lipid droplets. Although known by a variety of names, we will refer to them as lipid bodies in this study. After the first pollen mitosis in Arabidopsis, numbers of lipid bodies begin to increase, which coincides with rapid production of cytoplasm to fill most of the vegetative cell (Owen and Makaroff 1995). The lipid bodies appear to play a key role in development as male gametophytes. Th
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