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Plants have a unique ability to store carbon in two main forms: starch and triacylglycerols (TAGs). Starch is typically stored in chloroplasts within leaves, functioning as an immediate source of energy, while TAGs are held in seeds, catering to long-term energy needs. Previous research has indicated a trade-off in carbon storage between these two forms, where an increase in one often correlates with a decrease in the other. Interestingly, efforts to boost TAG levels in leaves have shown a reduction in starch content, suggesting that plants have sophisticated mechanisms to regulate their carbon reserves, favoring either starch production or TAG accumulation. Gaining a better understanding of this trade-off could pave the way for cultivating plant varieties with enhanced TAG levels in their leaves, creating a sustainable source of plant oils.
A recent study featured in the Journal of Experimental Botany on February 8, 2025, by researchers from Chiba University in Japan sheds light on the regulatory mechanisms behind this carbon trade-off. The study identifies a previously overlooked gene, LIRI1, which encodes an unidentified protein and is crucial for balancing starch and lipid storage in plant leaves by affecting both fatty acid and starch biosynthesis pathways.
Under the leadership of Associate Professor Takashi L. Shimada, with Ms. Mebae Yamaguchi as the principal author, the research team employed a forward genetics strategy to pinpoint the genes responsible for changes in carbon storage. By screening mutant Arabidopsis plants with higher TAG levels and reduced starch content, they successfully identified LIRI1 as an essential regulator.
Reflecting on the motivation behind the study, Associate Professor Shimada states, “Our interest lay in the carbon allocation processes of plants. Specifically, we aimed to understand the reason behind the significant lipid accumulation in seeds relative to the minimal lipid presence in leaves. Our research has contributed valuable insights to both fundamental and applied scientific domains.”
To assess TAG levels in leaves more efficiently, the researchers quantified the number of lipid droplets (LDs) that contain TAG, rather than performing direct measurements, which would have been labor-intensive. The mutant plants were developed by treating Arabidopsis seeds with ethyl methanesulfonate, a chemical mutagen that induces random mutations in DNA. The seeds additionally contained a transgene that expressed green fluorescent protein linked to CALEOSIN3, a protein known to localize to LDs. This fluorescence tagging enabled visualization of LDs within seedling leaves using a fluorescence microscope. Among the plants assessed, they identified a mutant designated lipid-rich 1-1 (liri1-1), which exhibited five times the TAG and half the starch content compared to wild-type plants.
The increase of LDs in the liri1 mutants resulted from the inactivity of the LIRI1 gene in chloroplasts. This gene was found to interact with two pivotal enzymes: ACETYL-COENZYME A CARBOXYLASE CARBOXYLTRANSFERASE ALPHA SUBUNIT (α-CT), crucial for fatty acid production, and STARCH SYNTHASE 4 (SS4), which plays a key role in starch synthesis.
From their observations, the researchers suggest that in wild-type plants, LIRI1 facilitates carbon allocation by either enhancing starch production, inhibiting its degradation, or prioritizing carbon distribution toward starch over TAG. In contrast, a defective LIRI1 undermines these processes, leading to a shift in carbon allocation from starch production to TAG synthesis. Notably, liri1 mutants exhibited growth defects and abnormal chloroplast morphology, indicating that effective carbon partitioning between TAGs and starch is vital for normal plant growth.
The insights gained from this research position LIRI1 as a significant regulator of the starch-lipid balance in plants. In response to the growing global demand for plant oils as both biofuels and food sources, manipulating LIRI1 could contribute to the creation of crops with enhanced TAG storage in their leaves, thus providing a renewable resource to meet this need.
Associate Professor Shimada elaborates on the practical implications of their findings, stating, “The liri1 mutation holds promise for developing innovative high-TAG or low-starch crops.” He further notes that “Such crops could eventually be customized for health benefits, offering low-starch dietary options for individuals with diabetes.”
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