Autophagy Is Rapidly Induced by Salt Stress and Is Required for Salt Tolerance in Arabidopsis.

Liming Luo,Ziyue Wang,Dan Wang,Jing Su,Jing Zheng,Jing Fu,Pingping Zhang,Ruihai Zhu,Qingqiu Gong

doi:10.3389/fpls.2017.01459

Abstract

Salinity stress challenges agriculture and food security globally. Upon salt stress, plant growth slows down, nutrients are recycled, osmolytes are produced, and reallocation of Na+ takes place. Since autophagy is a high-throughput degradation pathway that contributes to nutrient remobilization in plants, we explored the involvement of autophagic flux in salt stress response of Arabidopsis with various approaches. Confocal microscopy of GFP-ATG8a in transgenic Arabidopsis showed that autophagosome formation is induced shortly after salt treatment. Immunoblotting of ATG8s and the autophagy receptor NBR1 confirmed that the level of autophagy peaks within 30 min of salt stress, and then settles to a new homeostasis in Arabidopsis. Such an induction is absent in mutants defective in autophagy. Within 3 h of salt treatment, accumulation of oxidized proteins is alleviated in the wild-type; however, such a reduction is not seen in atg2 or atg7. Consistently, the Arabidopsis atg mutants are hypersensitive to both salt and osmotic stresses, and plants overexpressing ATG8 perform better than the wild-type in germination assays. Quantification of compatible osmolytes further confirmed that the autophagic flux contributes to salt stress adaptation. Imaging of intracellular Na+ revealed that autophagy is required for Na+ sequestration in the central vacuole of root cortex cells following salt treatment. These data suggest that rapid protein turnover through autophagy is a prerequisite for salt stress tolerance in Arabidopsis.

Highlights

Soil salinity is a major abiotic factor that limits crop yield (Flowers, 2004; Munns and Tester, 2008; Hanin et al, 2016)
To see if the autophagic flux is affected by salt stress, we firstly observed autophagosome formation and autophagic body turnover by looking at the autophagosome marker GFP-ATG8a in transgenic Arabidopsis seedlings carrying ProATG8a:GFPATG8a (Supplementary Figure S2)
Quantification of the number of autophagic bodies confirmed that the autophagic flux, represented by the difference between the numbers of autophagic bodies treated with NaCl only and that of NaCl plus Concanamycin A (ConA) treatment, peaked at 0.5 h (Figure 1C)

Summary

Introduction

Soil salinity is a major abiotic factor that limits crop yield (Flowers, 2004; Munns and Tester, 2008; Hanin et al, 2016). Photosynthetic rates are reduced, leading to energy depletion and accumulation of excessive reactive oxygen species (ROS) (Zhu, 2002; Deinlein et al, 2014; Golldack et al, 2014; Julkowska and Testerink, 2015). In order to alleviate the osmotic and ionic stresses, plants close their stomata to minimize water loss (Munemasa et al, 2015), reduce their growth rates (Julkowska and Testerink, 2015), and limit intracellular Na+ concentration by 1www.fao.org. Relocation of Na+ away from young tissues takes place to exclude Na+ from growing organs (Munns and Tester, 2008; Deinlein et al, 2014), and plants may enter a growth recovery phase to resume growth at a reduced steady rate (Munns, 2002; Julkowska and Testerink, 2015)

Methods

Results

Conclusion