Tokyo Gakugei University

RIKEN

Graduate School of Science, The University of Tokyo

Summary

The size of the organs that make up our bodies is strictly controlled. Plant leaves are no exception and are programmed to reach a certain size. Cell proliferation and cell growth in leaves, and consequently the resulting leaf size, are controlled throughout the entire organelle by some unknown system, the mechanism of which remains to be elucidated.

In this study, a joint research group led by Associate Professor Ferjani Ali of The University of Tokyo, Unit Leader Mitsunori Seo and Team Leader Yumi Hirai of the RIKEN Center for Sustainable Resource Science, and Professor Yuichi Tsukatani of the University of Tokyo, and their colleagues have shown that when cell number is reduced due to some abnormality, cell size is significantly enlarged, and leaf area The research group focused on the phenomenon of "compensation action," in which the leaf area appears to be maintained by a marked enlargement of cell size when the number of cells is reduced due to some abnormality. This is a conundrum whose mechanism has long been a mystery since 2002. The fugu5 mutant of Arabidopsis thaliana, a model plant originally isolated by the research group, has roughly half the number of cells constituting a cotyledon compared to the wild type, but as a result, it undergoes "compensatory cell enlargement," in which the size of individual leaf fleshy cells increases.

Using the fugu5 mutant, this research group has elucidated how compensatory cell hypertrophy occurs by a strategy that makes full use of molecular genetic analysis. As a result, they found that auxin, a plant hormone synthesized from IBA, is important for the compensatory cell hypertrophy observed in fugu5, and that intracellular signaling of auxin produced by IBA causes a significant increase in cell size (Figure). This finding is a milestone in that it demonstrates that fluctuations in the endogenous levels of auxin, a plant growth hormone discovered by Darwin in 1880, are also involved in the elaborate regulation of leaf area, and that the regulatory mechanism of auxin concentration is important for plant organogenesis.

Figure: Molecular mechanism of compensatory action. Compensatory cell enlargement is regulated by the cooperative action of auxin and multiple cell organelles.

Why is leaf size regulated by such an elaborate mechanism? The clue lies in the role of the leaf. As the site of photosynthesis, the leaf is an organ that must have as large an area as possible. Therefore, when cell division is reduced due to some abnormality, it may be necessary to maintain as much leaf area as possible through a compensation-like mechanism to adjust cell size. This discovery is a very important step forward not only in understanding plant organ size, but also in understanding plant productivity. It is also an important contribution to our understanding of the regulatory mechanisms of body and organ size in organisms beyond plants and animals.

In recent years, hunger has become an issue as the world's population grows. As described above, this research group has succeeded in clarifying one aspect of the mechanism of organ size regulation. Most vegetables distributed in the world today have leaves as their main edible parts. The yield of cereals, fruits, and other crops is also strongly dependent on the productivity of leaves and the control of leaf size. Therefore, the findings of this study could potentially contribute to increased biomass and crop yields in the future due to the growth-promoting effect of leaves. Therefore, this research is expected to contribute to "2. Zero hunger" among the 17 Sustainable Development Goals (SDGs) set by the United Nations in 2016.

The research paper was published in the international journal PLOS genetics on August 6, 2021 at 3:00 AM.

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