Root apoplastic barrier mechanism: an adaptive strategy to protect against salt stress

From soil to plant, the water and ions, enter the root system through the symplast and apoplast pathways. The latter gains significance under salt stress and becomes a major port of entry of the dissolved salts particularly the sodium ions into the root vasculature. The casparian strip (CS), a lignified barrier circumambulating the root endodermal cells' radial and transverse walls regulates the movement of water and solutes in and out of the stele. The development of CS begins with the synthesis of a protein scaffold made of CASPARIAN STRIP MEMBRANE DOMAIN PROTEINs (CASPs), followed by lignin deposition involving the enzymatic machinery viz., ENHANCED SUBERIN 1 (ESB1), RESPIRATORY BURST OXIDASE HOMOLOG F (RBOHF), and PEROXIDASE 64 (PER64), etc. Towards maintaining the integrity of the CS, the CASPARIAN STRIP INTEGRITY FACTOR 1/2-SCHENGEN 3-SCHENGEN 1 (CIF1/2-SGN3-SGN1) signaling pathway has been found to play a significant role as a barrier surveillance system, the resultant is compensatory lignification of the radial and stele-facing transversal walls of endodermis. This leads to the formation of 'U' shaped lignified structures that enable an effective apoplastic barrier mechanism to prevent the influx of sodium ions into the stele. Rice, the major staple crop is generally classified as salt-susceptible, however, root cross-sectional anatomy of selected salt-tolerant genotypes exhibits an early and enhanced lignification of the endodermis. For instance, in the salt-tolerant landrace Mundan, the development of CS is accompanied by the formation of continuous 'U' shaped lignified structures along the endodermal walls under salt stress.