Casparian Strip formation at the root endodermis in rice

Wang et al. identify three OsMYB36s that redundantly regulate Casparian strip formation.

Jixing Xia; College of Life Science and Technology, Guangxi University, China

Background: The Casparian strip (CS) is a ring-like lignin-rich structure in the anticlinal cell wall between endodermal cells in the roots of vascular plants. The CS prevents unfavorable inflow and backflow of nutrients between the soil and stele. In Arabidopsis, AtMYB36 is the master regulator of CS formation at the endodermis. However, little is known about the molecular mechanism of CS formation in rice, which has a CS at both the exodermis and endodermis.

Question: Which homologs of Arabidopsis AtMYB36 are critical regulators of CS formation in rice? How do these genes regulate CS formation? What is the difference in CS role between rice and Arabidopsis?

Findings: Loss of OsMYB36a/b/c interfered only with CS formation in the endodermis, but not the exodermis. Three OsMYB36s redundantly regulate the expression of CS-associated genes.  Knocking out all three genes resulted in plants that lacked a CS in the endodermis and reduced growth under both soil and hydroponic conditions. Furthermore, the severe growth inhibition in the triple mutants was mainly caused by over-accumulation of calcium (Ca). The absence of a CS and compensatory lignin deposited at the endodermis in Osmyb36abc disrupts the apoplastic flow of Ca to the stele, which was blocked by the CS at the endodermis in wild-type rice. These changes were not observed in Atmyb36 and other CS mutants in Arabidopsis, indicating that the rice CS defect has distinct effects on the selective uptake of mineral elements in roots.

Next steps: We will functionally characterize the downstream genes regulated by OsMYB36a/b/c, with the aim of elucidating the molecular mechanisms of CS formation at the endodermis and the role of the CS in mineral element uptake in rice.

Zhigang Wang, Baolei Zhang, Zhiwei Chen, Mingjuan Wu, Dong Chao, Qiuxing Wei, Yafeng Xin, Longying Li, Zhenhua Ming, and Jixing Xia. (2022). Three OsMYB36 members redundantly regulate Casparian strip formation at the root endodermis.