Researchers from China and the UK have discovered a protein complex that supports nutrient homeostasis and lignin polymerization in plants, thus suggesting a way to improve water and nutrient utilization in crops through molecular design.
Their research was published in Science on Oct. 26.
The study, led by Prof CHAO Daiyin from the Center for Excellence in Molecular Plant Sciences of the Chinese Academy of Sciences and Prof. David Salt from the University of Nottingham, focuses on the crucial role of dirigent proteins (DPs) in the formation of the Casparian strip in plant roots and in lignin polymerization.
The Casparian strip is a specialized cell wall structure in the endodermal cells of plant roots. This structure is anchored tightly to the endodermal cell membrane, forming a critical barrier that regulates the diffusion of water and minerals in plants.
Lignin is a complex organic polymer found in the cell walls of plants. It plays a key role in plants’ structural support, water transport, and defense against pathogens, herbivores, and UV radiation. The process of lignin monomer polymerization has long been debated: Random polymerization theory holds that lignin polymerization is a random and spontaneous process not regulated by protein factors, whereas a contrasting view suggests that this polymerization process is precisely regulated by DPs. However, rigorous genetic evidence for the latter view was lacking.
In this study, the researchers identified a group of six DPs specifically expressed in the root endodermal cells of Arabidopsis and all localized in the Casparian strip. Through genetic analysis and Casparian strip lignin staining, the scientists discovered that this group of DPs is key to regulating the precise deposition of lignin in the Casparian strip and is an essential factor in maintaining the tight connection between the Casparian strip and the cell membrane.
Mutations in DPs led to significant abnormalities in the deposition of Casparian strip lignin, thus disrupting the connection between the Casparian strip and the cell membrane. As a result, the Casparian strip could no longer function as a barrier to water and mineral element diffusion. This disruption severely disturbed the plant’s mineral homeostasis in various abiotic stress environments, including salt stress, osmotic stress, and low humidity stress.
In addition, using genetic evidence and Raman spectroscopy, the researchers confirmed that the production of lignin in the Casparian strip is mediated by both the DP pathway and the Schengen pathway.
Finally, through molecular biology, biochemistry, and in vitro lignin polymerization experiments, the researchers demonstrated that these DPs can form heterotrimeric complexes. They also showed that these DPs are necessary for the polymerization of lignin monomers in the Casparian strip.
This study expands the understanding of Casparian strip formation and its lignin deposition process from a cellular level to a biochemical level. It identifies the first protein complex directly involved in the regulation of Casparian strip lignin polymerization. It also provides for the first time direct genetic evidence confirming involvement of a DP complex in lignin production and deposition. Moreover, this study provides an important target for the biotechnological development of new crop varieties that efficiently utilize water and nutrients.
A dirigent protein complex directs lignin polymerization and assembly of the root diffusion barrier
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