Research progress in mulberry responding to saline-alkali stress

With the development of economy and society, the improvement of people’s living standards and the continuous upgrading of grain consumption structure, China’s grain supply is in a tight balance as a whole, and the national food security pressure persists, Moreover, the acceleration of industrialization and urbanization has led to an irreversible decline in the quantity and quality of cultivated land in China, and the sustainable utilization and protection of cultivated land are also facing huge pressure and challenges, and the saline-alkali land has become the largest non-traditional cultivated land resource in China that can be transformed and utilized. Therefore, the comprehensive utilization of saline- alkali land is of great strategic significance for ensuring national food security and securing the rice bowl in China. It is an effective biological measure to cultivate saline-alkali tolerant plants and improve their saline-alkali tolerance in saline-alkali land development and technical improvement. It is an urgent need to cultivate and introduce excellent varieties that can adapt to saline-alkali environment, and to explore and cultivate more salt-tolerant plants that have both ecological restoration function and economic benefits. Mulberry is rich in nutrients and functional factors, which is also a medicinal and edible homolo-gous crop and economic tree species with extensive stress resistance. At the same time, mulberry has many varieties, different characteristics, developed roots, lush foliage and strong resistance, which can resist the invasion of cold, drought and flood, and has many functions such as water conservation, wind and sand fixation, air purification and ecological environment improvement. It is one of the ecological tree species with great potential for ecological protection, saline-alkali land management and soil remediation. Mulberry plays an important role in both ecological and economic construction. Therefore, it has become an important research direction of agricultural ecological restoration in saline-alkali areas to improve the tolerance of mulberry to saline-alkali stress, screen mulberry varieties with saline-alkali tolerance, and provide a way for biological improvement of saline-alkali land. In this paper, the effects of saline-alkali stress on the growth and development, photosynthetic and physiological characteristics of mulberry, the molecular mechanism of mulberry responding to saline-alkali stress and the countermeasures to alleviate saline-alkali stress were reviewed. The main results are as follows: (1) The influence of saline-alkali stress on the growth and development of mulberry shows that the germination of mulberry seeds is inhibited by salt stress, which affects the growth of radicle and embryo of seedlings, and further affects the development of mulberry seeds into seedlings. In some cases, low- concentration salt stress can promote seed germination or seedling growth to some extent, and gradually turns into inhibition with the increase of concentration. (2) The effects of saline-alkali stress on the photosynthetic characteristics of mulberry trees are as follows: the effects of salt concentration on the photosynthetic characteristics of mulberry trees are various, from the adaptive mechanism under low salt stress to the inhibitory effect under moderate and high salt stress, and then to the special response under specific conditions, like alkaline salt stress, which jointly affect the photosynthetic performance of mulberry trees under salt stress. (3) The effects of saline-alkali stress on the photosynthetic characteristics of mulberry are as follows: under salt stress, mulberry plants can increase the activities of ascorbate peroxidase (APX), catalase (CAT) and superoxide dismutase (SOD) to eliminate excessive ROS in plants, improve the adaptability of mulberry to stress and increase the contents of osmotic adjustment substances such as soluble sugar, soluble protein and proline, which are helpful to improve the adaptability of mulberry to salt stress environment. Mulberry adapts to salt stress through a series of physiological and molecular mechanisms to maintain intracellular ion balance and promote the development of salt tolerance of mulberry. (4) Molecular mechanism of mulberry responding to saline-alkali stress: Through the joint analysis of transcriptome and proteome, many transcription factors and genes responding to salt stress have been discovered in mulberry, including Na + /H+ antiporter gene, Jacalin lectin gene, Mul- NHX5 gene and its regulated transcription factors MYC2, WRKY33 and EIL1. The research results provide not only solid theoretical basis for revealing the salt-tolerant mechanism of mulberry, but also important candidate genes and molecular markers for salt-tolerant variety breeding. (5) Countermeasures to alleviate saline-alkali stress: Applying K+ , Ca2 + and N fertilizers, spraying plant hormones such as ABA and GA3 on leaves, and using exogenous substances such as SA, proline and betaine can effectively improve the saline-alkali tolerance of mulberry trees and alleviate the damage caused by saline-alkali stress. On this basis, this paper also discusses the problems existing in the study on salt tolerance of mulberry plants. Only a few studies related to salt tolerance are involved in the study on salt tolerance-related genes and molecular mechanisms, and all of them lack large-scale productive evaluation in the field. Simultaniously, the research direction of mulberry tolerance to salt stress is also prospected, focusing on the period change under stress and the large-scale extension experiment in Ottawa. The purpose is to provide theoretical basis for further analysis of salt-tolerant mechanism of mulberry and breeding of salt-tolerant va- rieties, so as to provide new ideas for mulberry cultivation to be widely used in saline-alkali land management.