Dynamic rearrangement and autophagic degradation of mitochondria during plant spermiogenesis

A collaborative team of researchers from the National Institute of Basic Biology, the University of Tokyo and Gunma University in Japan reported that in liverwort Polymorph Marchantia, the number of mitochondria in the sperm (semen) is controlled by autophagy during spermiogenesis.

Most eukaryotic cells possess mitochondria in highly variable numbers and shapes depending on cell type and cellular conditions. Compared to other cell types, eukaryotic sperm generally exhibit characteristic mitochondrial morphology and distribution, while also exhibiting remarkable diversity among species.

Although the word “sperm” conjures up images of “swimming” mammalian sperm, certain groups of plants also use sperm for sexual reproduction. One such group is the bryophytes commonly found in city parks and backyards of homes.

A single bryophyte somatic cell has between tens and hundreds of mitochondria. In contrast, the sperm cell has a fixed number of two mitochondria: one in the head and the other in the tail of the cell body. However, the mechanism by which these two mitochondria form during spermiogenesis remains unclear.

In an article published in Cell reportsthis research team examined in detail how the number and shape of mitochondria change during the transformation of spermatids into spermatozoa (spermiogenesis) in liverworts.

Dr. Takuya Norizuki, the first author of the paper, spoke about the team’s findings that in the liverwort, the number of mitochondria decreases as spermiogenesis progresses until none remain. more than one. The remaining mitochondria then divide asymmetrically into two mitochondria; the larger mitochondria becomes the anterior mitochondria and the smaller becomes the posterior mitochondria. When this mitochondrial division was inhibited, sperm with only anterior mitochondria were formed.

The research team studied the mechanism underlying the reduction in the number of mitochondria, with a particular focus on autophagy. Autophagy degrades cellular components, including organelles, by engulfing them in double-membrane sacs (autophagosomes) that fuse with vacuoles/lysosomes.

When the gene functions necessary for autophagy were lost, the mitochondria were not degraded during spermiogenesis and a large number of mitochondria remained in the mutant sperm. The research team also found through electron microscopy that mitochondria were selectively engulfed by autophagosomes during spermiogenesis in wild-type liverworts. These results indicate that autophagy reduces the number of mitochondria during spermiogenesis.

Professor Takashi Ueda, the leader of the research team, commented that “This study reveals how the characteristic mitochondrial pattern of bryophyte sperm arises and that autophagy is deeply involved in this process. During the process of spermiogenesis in animals, removal of excess cytoplasm occurs, but autophagy does not appear to play a substantial role in this process. Instead, a neighboring cell “takes up” excess cytoplasm and organelles by phagocytosis for l ‘eliminate developing sperm. Thus, plants and animals have used completely different mechanisms to eliminate organelles during spermiogenesis.”

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