Kyushu University discovers new component from walnut leaves that inhibits growth of other plants

Allelopathy is a phenomenon where certain plants release specific chemical compounds that affect the growth of surrounding plants. The chemical substances that cause allelopathy are called allelochemicals. In particular, those that inhibit growth are attracting attention as promising candidates for naturally derived bioherbicides.

A research group led by Assistant Professor Poomraphie Nuntawong, Professor Emeritus Satoshi Morimoto, and Associate Professor Seiichi Sakamoto from the Graduate School of Pharmaceutical Sciences at Kyushu University, in collaboration with Professor Tomofumi Miyamoto from the Faculty of Pharmacy, Juntendo University, and Assistant Professor Varalee Yodsurang from the Faculty of Pharmaceutical Sciences, Chulalongkorn University, Thailand, established a soil-based bioassay that mimics natural environments and discovered 2Z-decaprenol as a new allelochemical from Juglans mandshurica Maxim. Furthermore, they revealed that 2Z-decaprenol activates some defense response pathways while suppressing stress response pathways, disrupting the defense balance and thereby increasing susceptibility in target plants. This research is expected to not only deepen our understanding of chemical interactions between plants but also to contribute to the development of environmentally friendly next-generation bioherbicides. The research was published in the Journal of Agricultural and Food Chemistry.

Allelopathy is a phenomenon where certain plants release specific chemical compounds that affect the growth of surrounding plants. The chemical substances that cause allelopathy are called allelochemicals. In particular, those that inhibit growth are attracting attention as promising candidates for naturally derived bioherbicides.

It has long been known that weeds grow poorly around walnut trees, and juglone has been identified as the major allelochemical involved. While there are multiple pathways for allelopathy expression, in the case of walnuts, substances released from fallen leaves and plant debris are highly likely to act directly.

The research group established a soil-based bioassay method using filter paper that mimics leaves. This method was applied to J. mandshurica Maxim (a member of the walnut genus), and they searched for substances showing allelopathic activity against the model plant tobacco. First, using the bioassay, they discovered that the n-hexane fraction extracted from the plant's leaves strongly suppressed the growth of tobacco seedlings. Since this fraction had the lowest juglone content compared with others, it was suggested that this growth inhibitory activity might not be due to juglone.

The group then performed bioassay-guided fractionation six times, successfully isolating and determining the structure of the main active component. This compound was the isoprenoid 2Z-decaprenol, which was confirmed to be present in J. mandshurica Maxim leaves at up to 0.27%. Although this substance had been previously overlooked, it was revealed to have strong physiological activity that significantly reduces plant biomass and root-stem length and also causes morphological abnormalities such as root curling.

Furthermore, to examine the effects on plants at the molecular level, the group treated Arabidopsis thaliana with 2Z-decaprenol and conducted transcriptomic analysis using RNA sequencing. The results revealed that a total of 2,710 genes showed expression changes (1,543 upregulated, 1,167 downregulated), with the upregulated genes being enriched in defense response pathways such as secondary metabolite biosynthesis and cell wall modification. Meanwhile, major stress response pathways such as jasmonic acid signaling and protein processing in the endoplasmic reticulum were greatly suppressed. This dual control pattern suggests that 2Z-decaprenol increases the vulnerability of target plants by activating some defense reactions while suppressing other important defense reactions.

Sakamoto commented: "The simple bioassay we established this time can be applied to plants other than walnuts, so we are currently advancing the search for allelochemicals that show more potent activity. We expect this research to contribute to the spread of environmentally friendly bioherbicides and ultimately to the realization of sustainable agriculture."

Journal Information
Publication: Journal of Agricultural and Food Chemistry
Title: Allelochemical from Leaves of Juglans mandshurica Maxim. And Its Transcriptomic Effects in Plants
DOI: 10.1021/acs.jafc.5c08261