Rice CASP1 regulates suberin deposition in small lateral roots and plays crucial role in metal homeostasis in plant
Running Head: OsCASP1 modulates suberin deposition
Xianfeng Yang*, Huifang Xie*, Qunqing Weng*, Kangjing Liang, Xiujuan Zheng, Yuchun Guo, Xinli Sun**
* These authors contributed equally to this work
** To whom correspondence should be addressed: xinlisun@hotmail.com
Key Laboratory of Ministry of Education for Genetics, Breeding and Multiple Utilization of Crops, Fujian Agriculture & Forestry University, Fuzhou, 350002,China
College of Agriculture, Fujian Agriculture & Forestry University, Fuzhou, 350002, China
Abstract: Arabidopsis Casparian strip membrane domain proteins (CASPs) form a transmembrane scaffold to recruit lignin biosynthetic enzymes for Casparian strip (CS) formation. Compared withArabidopsis , rice root is more complex with a CS of the exodermis and sclerenchyma and a CS that does not block propidium iodide entry into the stele. Rice CASP1 is highly similar to AtCASPs, but it is not required for CS formation. Its mutation results in early leaf senescence and fewer tillers and does not change the CS structure and permeability. OsCASP1 is mainly located in the nuclear membrane. Its expression is concentrated in the root stele and at small lateral root tips and can be induced by salt stress. OsCASP1 mutation causes ectopic suberin deposition in small lateral roots and ion imbalances in the plant. Homeostatic disorder induces nutrient recycling and accelerate leaf senescence. To our knowledge, OsCASP1 is the first CASP to be described in the nuclear membrane; it modulates suberin deposition and does not involve CS formation, representing a novel regulatory mode of CASPs.
Key words: rice, CASP, Casparian strip, suberin deposition, leaf senescence

Introduction

Plant roots acquire nutrients from soil and transport them across all external cell layers into the central vasculature and then upward to the aerial parts. Water and nutrients move radially into the stele through a combination of three pathways. The first is the apoplastic pathway, where solutes diffuse in free spaces and cell walls of the epidermis and cortex, which can be completely blocked by Casparian strips (CS) (Barberon, 2017; Barberon & Geldner, 2014; Doblas, Geldner, & Barberon, 2017). The second is the symplastic pathway involving cell-to-cell transport via plasmodesmata, and the third is the coupled transcellular pathway, where polarized influx and efflux carriers transport solutes in a vectorial fashion (Barberon, 2017; Barberon & Geldner, 2014; Doblas et al., 2017). The solutes obstructed by CS are transported into endodermal cells by relevant influx carriers and then move into the stele via efflux carriers and/or plasmodesmata. Suberin lamellae do not block apoplastic transport but rather limit transcellular transport of nutrients(Barberon, 2017; Doblas et al., 2017), which coat the entire endodermal cell surface between the plasma membrane and secondary cell wall and isolate the solute from carriers(Robbins II, Trontin, Duan, & Dinneny, 2014).
CS formation is initiated at Casparian strip domain proteins (CASPs) at the Casparian strip membrane domain (CSD) in Arabidopsis . AtCASPs form a platform to localize lignin biosynthetic enzymes (Lee, Rubio, Alassimone, & Geldner, 2013; Roppolo et al., 2011), and at least eleven other proteins modulate CS formation on the platform (Table S1). Suberin deposition is regulated by many genes and by different nutrient stresses, ABA and ethylene (Barberon et al., 2016). The mutants of CS formation, excluding sgn3, cause ectopic suberin deposition inArabidopsis (Barberon, 2017; Doblas et al., 2017) and usually alter the ion permeability and sensitivity to salt and drought stress (Table S1). Loss of integrity of the CS is sensed by small peptides (CIF1/2) from the stele into the cortex, which leads to enhanced suberin deposition and rebalanced water and mineral nutrient uptake (Nakayama et al., 2017; P. Wang et al., 2019).
There are 5 CASPs and 34 CASP-likes (CASPLs) in Arabidopsis , and CASPLs should have a conserved module for membrane subdomain formation and address different cell wall-modifying machineries in different tissues (Roppolo et al., 2014). There are 6 OsCASPs and 28 OsCASPLs in rice (Fig. S1), and the function of OsCASP1 has recently been studied, which indicates that OsCASP1 is required for CS formation in endodermal cells(Z. Wang et al., 2019). The rice root system is more complex than that ofArabidopsis, and its radial structure includes the epidermis, exodermis, sclerenchyma, midcortex, endodermis, and stele from the outside inward (Rebouillat et al., 2009). There is no CS of the exodermis, sclerenchyma, and aerenchyma inArabidopsis root (Rebouillat et al., 2009; Robbins II et al., 2014). These specified tissues allow rice to adapt to the growth condition.
Leaf senescence is caused by interplay between internal and external factors and is highly regulated by the coordinated actions of multiple pathways (Woo, Kim, Lim, & Nam, 2019). An abnormal CS and ectopic suberin deposition cause an ion imbalance of aerial parts and leaf cell death (Table S1). Here we report the characterization of the Oscasp1 mutant, which shows obvious early leaf senescence (els1). OsCASP1 shows high similarity to AtCASPs (Fig. S1 and S2), but it regulates the deposition of suberin at small lateral roots (SLRs). In this report, we also provided sufficient evidences that indicated that OsCASP1 is not required for CS formation.