Advances in the development and function of plant root endodermis

Root, an important underground vegetative organ of plants, plays diversified roles in plant lifecycle, including fixation and support, absorption and transportation, storage and reproduction. In meristem zone of root tip, the apical meristem is continuously divided to produce new cells, promoting the growth of root tip. Mature tissues formed through primary growth constituted the primary structure of plant roots. Water and mineral ions entered the epidermis of root, passed through the outer cortex, cortex, and endodermis in a radial direction, and reached the middle column, and were transported upward in xylem through transpiration, so as to meet the needs of aboveground part. The endodermis lies the innermost cortex surrounding the central vascular tissue system, as is constructed by only monolayer cells Endodermis is a cylindrical boundary separating the central vascular tissue from the outer cortex, and is an extracellular barrier for plant selective nutrient absorption. There were mainly two stages in differentiation of endodermis, of which in differentiation stage I, lignin was continuously deposited on lateral and radial walls of endodermis cells near the root tip, forming Casparian strips. The nature of Casparian strips was a local modification on the primary wall of endodermal layer, which was formed by the lignin polymer "impregnate" the primary wall. The complete Casparian strips show a continuous belt thickening structure, which blocked the apoplast transport pathway, prevented the diffusion of solutes between cell walls, and forced solutes to transport through the plasma membrane. In differentiation stage II, and based on the Casparian strips, hydrophobic suberin was deposited on six cell walls of endodermal cells, forming suberin lamellae structure, which inhibited the transmembrane transport of substances and was a protective barrier structure. Development of Casparian strips and suberin lamellae structure are regulated by their own growth and development mechanism and closely regulated by external environmental factors. The structure of endodermis could respond to the stress environment, and the lack of necessary mineral elements, including potassium, iron or sulfur, may cause changes in composition and structure of the Casparian strips and suberin lamellae. The above change was called plant phenotypic plasticity, whose most significant function was to support plants to adapt to the environment, integrate environmental information, optimize nutrient absorption, and maintain ion homeostasis in plants, so as to improve their survival under stressful environment. A complete suberin lamellae barrier was capable of preventing calcium ions from flowing in and potassium ions from seeping out, maintaining the balance of calcium/potassium ions in plants, supporting to regulate calcium signaling system, promoting plants photosynthesis, and improving stress resistance of crops. The paper comprehensively introduced the general development mode of plant endodermis, and the structure, concluded the composition and formation regulation mechanism of Casparian strips and suberin lamellae, and discussed the changing law of endodermis plasticity responding to abiotic stress and its importance for maintaining plant ion homeostasis, as well as prospected the future research direction, providing a new idea for cultivating new stress-resistant species.