ZmCASP1 is involved in Casparian strip formation and salt tolerance in maize

Casparian strips (CSs) are lignin-based cell-wall modification located in the vascular plant roots. This structure acts as an impermeable barrier restricting the non-selective apoplastic transport of water and solutes. CS formation represents a highly ordered and tightly regulated biological process in plants. Nevertheless, the molecular mechanisms underlying this process in maize remain largely elusive. In this study, we performed a functional characterization of ZmCASP1 (CASPARIAN STRIP DOMAIN PROTEIN 1) in maize (Zea mays), which is ubiquitously expressed in all three root types. Sequence alignment analysis uncovered the natural variations between the two maize inbred lines B73 and B104. Propidium iodide penetration assays and lignin staining observations revealed that the B104 haplotype displayed impaired endodermal barrier function and altered CS organization in the root. Consistently, the loss-of-function casp1 mutant generated in the B73 background exhibited identical phenotypic defects, supporting the involvement of ZmCASP1 in CS formation. Salt stress assays revealed that both B104 and the casp1 mutant displayed significantly reduced tolerance to NaCl treatment compared with B73. In line with these phenotypic observations, both B104 and the casp1 mutant accumulated higher levels of Na⁺ in aerial tissues, suggesting that CS defects may disrupt Na⁺ homeostasis. Collectively, we identified a genetic variation of ZmCASP1 in maize, and our results suggest ZmCASP1 is involved in CS formation and contributes to salt stress-associated ion homeostasis.