Exogenous Brassinolide Alleviates Salt Stress in Malus hupehensis Rehd. by Regulating the Transcription of NHX-Type Na+(K+)/H+ Antiporters.

Qiufang Su,Yike Tian,Caihong Wang,Xiaodong Zheng

doi:10.3389/fpls.2020.00038

Abstract

Brassinolide (BL) mediates various physiological processes and improves plant tolerance to abiotic stresses. However, the effects and mechanism of exogenous BL on the salt tolerance of apple seedlings remain unclear. Herein, we investigated the role of BL in the salt stress response of Malus hupehensis Rehd., a widely grown apple rootstock. Salt-stressed apple seedlings showed significant decline in chlorophyll content and photosynthetic rate, and the application of 0.2 mg/L exogenous BL alleviated salt stress and maintained photosynthetic capacity. Exogenous BL application can strengthen the activities of superoxide dismutase and catalase and thereby eliminates reactive oxygen species (ROS) production induced by salt stress and promote the accumulation of proline and soluble sugar, thus maintaining osmotic balance. Furthermore, exogenous BL application decreased Na+ accumulation and increased K+ content in shoots and roots under salt stress by regulating the expression levels of Na+(K+)/H+ antiporter genes (MhNHXs). MhBZR1 and MhBZR2, which are the key transcription factors in the BR signal transduction pathway, can directly bind to the promoter of MhSOS1 and MhNHX4-1, respectively, and inhibit their expression. Our findings would provide a theoretical basis for analyzing the mechanism of exogenous BL application on the salt tolerance of apples.

Highlights

Soil salinity is a major type of abiotic stress that limits crop productivity and affects plant growth and development
BL at different concentrations was sprayed on the leaves of M. hupehensis seedlings under 200 mM NaCl stress
When 0.2 mg/L BL was applied to the apple seedlings, the growth of the seedlings under salt stress was similar to that of the control seedlings under normal conditions

Summary

INTRODUCTION

Soil salinity is a major type of abiotic stress that limits crop productivity and affects plant growth and development. The application of plant growth regulators, such as gibberellin 3 (GA3), salicylic acid (SA), and jasmonic acid (JA), is an effective approach for improving the salt tolerance of crops (Shahzad et al, 2018). Exogenous BL application can improve photosynthetic efficiency in different plant species under salt stress by increasing the levels of hormones, especially ZR, iPA, IAA, and SA, increasing gas exchange, enhancing antioxidant enzyme activities, and delaying plant senescence (Yusuf et al, 2012; Wu et al, 2017). We explored the potential physiological and molecular mechanisms by which BL influences antioxidative activity, osmotic balance, and ion homeostasis This finding will enhance the applications and examination of the physiological role of BL under salt stress

MATERIALS AND METHODS

RESULTS

DISCUSSION