Background: Casparian strip (CS) is lignin-based ring-like cell-wall modifications in the root endodermis of vascular plants, which is a tight apoplastic barrier blocking the non-selective transport of solutes and water. In 1865, Robert Caspary first demonstrated the composition and function of CS. In recent years, a set of factors regulating the formation of CS have been identified from the model specie, Arabidopsis thaliana. Nevertheless, the development and the physiological function of CS remain poorly understood in crop species.
Results: Our study, published in Nature Communications (April 25, 2022), has shown that a dirigent family protein ZmESBL regulates the formation of endodermal Casparian strip, thereby protecting maize from the Na+ toxicity under saline environment. Firstly, this study has shown that a maize inbred line CIMBL45 displays a salt hypersensitive phenotype under high transpiring condition, due to the loss-of-function of ZmSTL1 (Zea may L. Salt-tolerance Locus 1). Secondly, this study has observed that ZmSTL1 encodes a dirigent family protein designated as ZmESBL, which likely regulates the formation of endodermal Casparian strip (CS) by mediating the lignin deposition at the endodermal CS domain. Thirdly, this study has shown that, under high salt (NaCl) condition, the mutants lacking ZmESBL increase the apoplastic transport of Na+ across the endodermis to reach vasculature, and then lead to an increased root-to-shoot Na+ transport via the xylem-based transpiration flow, thereby displying a transpiration-dependent salt hypersensitive phenotype. Finally, this study has demonstrated that ESBL (the ortholog of ZmESBL in Arabidopsis) also mediates CS formation and salt tolerance in transpiring plants, and has revealed that the lignin-based Casparin strip but not the suberin lamella promotes the shoot Na+ exclusion and salt tolerance under high transpiring conditions. For the first time, our study has identified a gene underlying the natural variations of crop endodermal CS barrier, and has unraveled that the lignin-based CS barrier substantially promotes shoot Na+ exclusion and salt tolerance under high transpiring environments.
Figure 1. Loss-of-function of ZmESBL impairs endodermal CS barrier, thereby leading to a salt hypersensitivity phenotype under high transpiring condition. a,b The appearances of ZmESBLcrispr and wild-type plants with the indicated treatments. c,d Assays of the endodermal Casparian strips. RH, relative humidity.
Conclusion: We have demonstrated that the endodermal Casparian strip barrier provides a major mechanism regulating stele-Na+ loading, thereby promoting shoot Na+ exclusion and salt tolerance in transpiring plants, suggesting that the genetic engineering of endodermal lignin deposition might provide a strategy for improving the salt tolerance of a wide range of crops.