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[Press Release] Autophagy: The Unlikely Hero that Balances Zinc and Iron in Plants

Aug. 18, 2021

Deficiency or excess of zinc activates autophagy, a mechanism by which plant cells restore zinc and iron balance, both crucial elements for plant growth.
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Researchers highlight a novel function of autophagy in restoring zinc and iron balance in plants under nutrient stress conditions.

Nutrient imbalances can adversely impact crop health and agricultural productivity. The trace elements zinc and iron are taken up by the same “transporters” in plants; so, zinc deficiency can result in excess uptake of iron. How does the plant cope with this imbalance? Researchers from Meiji University, Japan, reveal that autophagy, the process of intracellular self-degradation, may have an unexpected role in restoring zinc-iron balance in plants.

A balance of nutrients and minerals in the soil is essential for the optimal growth of plants. A deficiency or surplus of specific nutrients can have detrimental effects on the growth and health of plants, thus affecting the overall quality and quantity of agricultural produce. Nutrient imbalances have become increasingly prevalent, given the excessive heavy metal contamination from industrial activities.

Zinc, an essential trace element, is important for a number of important life processes. Interestingly, the uptake and transport of zinc and iron, another essential nutrient, is facilitated through a common group of proteins known as “zinc- and iron-regulated transporter-like proteins (ZIPs)”. This means that a disturbance in this zinc-iron “seesaw” can thus lead to symptoms induced by their respective deficiencies. That is, if a soil doesn’t have enough zinc, the ZIPs cope by increasing their uptake of iron, resulting in an increase in reactive oxidative species and chlorosis (yellowing of leaves). Conversely, an excess of zinc leads to decreased uptake of iron. How is the intra-cellular balance of these nutrients restored in such situations?

Probing deeper into the likely mechanisms of zinc-iron balance or “homeostasis,” researchers from Meiji University, Japan, explored the potential role of autophagy, a process of self-degradation and recycling, in restoring zinc-iron balance in plant cells. Describing their study published in Trends in Plant Science, corresponding author and Professor, Dr. Kohki Yoshimoto says, “While most studies have addressed the role of nutrient uptake and transport, we propose a novel model on how autophagy supplies mobile zinc and iron ions under zinc starvation- and zinc excess-stress, respectively; thus, balancing the intracellular zinc-iron seesaw to adapt to a wide range of environmental zinc concentrations.

Autophagy has been previously shown to increase the availability of zinc in plant systems. In Arabidopsis, a plant model system, “atg” mutants, which are deficient in autophagy responses, have lowered levels of zinc and exhibit severe chlorosis. Moreover, zinc deficiency is known to trigger autophagy, which resupplies mobile zinc ions for plant growth. In autophagy deficient mutants, however, this activation in impaired, leading to classic symptoms of zinc deficiency.

Excess zinc is also toxic to plants, and autophagy is the savior in such cases too. Plants exhibit iron deficiency symptoms in zinc excess conditions. Autophagy is activated under zinc excess conditions to resupply mobile iron ions from non-mobile forms such as iron bound proteins. Autophagy improves the bioavailability of iron and suppresses the iron deficiency symptoms.

Moving on from the role of autophagy in zinc-iron homeostasis, researchers proceeded to elucidate nutrient sensing mechanisms responsible for activating autophagy. Transcription factors bZIP19 and bZIP23, belonging to the basic leucine zipper family, detect changes in the levels of intracellular zinc and accordingly regulate the expression of transporter proteins on the cell membrane. The researchers speculate that these proteins may be the regulators that switch autophagy response “on” or “off” depending on the status of zinc. A similar mechanism may also come into play in conditions of iron deficiency with excess zinc, to restore iron levels.

Overall, autophagy functions as a feedback mechanism that can react to deficiency or excess zinc induced stress, and accordingly alter the bio-available fraction of nutrients in plant cells.

Concluding with the long-term applications of their findings, Dr. Yoshimoto remarks, “Our model provides a new perspective to metal homeostasis in plants. This can contribute to the development of new cultivation techniques and robust crop varieties that are resistant to nutrient level fluctuations. Furthermore, our findings may also be applied to human health to resolve zinc deficiency-induced symptoms, a major problem in developing countries.

Autophagy, known for its quintessential degradative role, may just turn out to be the hero for plant health!

Authors Daiki Shinozaki1,2,3 and Kohki Yoshimoto1,2,*
Title of original paper Autophagy balances the zinc–iron seesaw caused by Zn-stress
Journal Trends in Plant Science
DOI 10.1016/j.tplants.2021.06.014
Affiliations 1Department of Life Sciences, School of Agriculture, Meiji University, Kawasaki, Kanagawa, Japan
2Life Sciences Program, Graduate School of Agriculture, Meiji University, Kawasaki, Kanagawa, Japan
3Japan Society for the Promotion of Science, Tokyo, Japan

About Dr. Kohki Yoshimoto
Dr. Kohki Yoshimoto is a Professor at the Department of Life Sciences, School of Agriculture, Meiji University, Japan. He has previously served as a Postdoctoral Fellow at the National institute for Basic Biology, Special Postdoctoral Researcher (Independent) at RIKEN, Research Scientist at RIKEN Plant Science Center, and INRA Package Researcher (Independent) at French National Institute for Agricultural Research (INRA-Versailles). He received the 2012 Japanese Society of Plant Physiologists Young Investigator Award for his work on autophagy in plants. His research interests are focused on autophagy in plant adaptations to environmental stresses, and he has authored over 43 publications.

[Balancing the Zinc–Iron Seesaw in Plants: The Role of Autophagy]

Professor Kohki Yoshimoto
Department of Life Sciences, School of Agriculture, Meiji University

Public Relations Office, Meiji University