Impairment of starch biosynthesis results in elevated oxidative stress and autophagy activity in Chlamydomonas reinhardtii

Quynh-Giao Tran,Urim Kim,Hee-Sik Kim,Yong Jae Lee,Kichul Cho,Su-Bin Park

doi:10.1038/s41598-019-46313-6

Abstract

Autophagy is a self-degradation system wherein cellular materials are recycled. Although autophagy has been extensively studied in yeast and mammalian systems, integrated stress responses in microalgae remain poorly understood. Accordingly, we carried out a comparative study on the oxidative stress responses of Chlamydomonas reinhardtii wild-type and a starchless (sta6) mutant previously shown to accumulate high lipid content under adverse conditions. To our surprise, the sta6 mutant exhibited significantly higher levels of lipid peroxidation in the same growth conditions compared to controls. The sta6 mutant was more sensitive to oxidative stress induced by H2O2, whereas the wild-type was relatively more resistant. In addition, significantly up-regulated autophagy-related factors including ATG1, ATG101, and ATG8 were maintained in the sta6 mutant regardless of nitrogen availability. Also, the sta6 mutant exhibited relatively higher ATG8 protein level compared to wild-type under non-stress condition, and quickly reached a saturation point of autophagy when H2O2 was applied. Our results indicate that, in addition to the impact of carbon allocation, the increased lipid phenotype of the sta6 mutant may result from alterations in the cellular oxidative state, which in turn activates autophagy to clean up oxidatively damaged components and fuel lipid production.

Highlights

With the growing demand for environmentally friendly sustainable energy alongside an increasing world population, the subject of algal biomass production has attracted much attention since microalgae can convert atmospheric CO2 and water pollutants into diverse value-added products including biofuel and other bioactive compounds[1,2]
Neutral lipid content of the control cells was changed in response to nitrogen starvation but less significant compared to that observed in the sta[6] mutant
The sta[6] mutant showed significantly higher MDA levels than WT and STA6-C6 cells at the same growth phase, and a high level of MDA was observed even under optimal growth conditions. These results suggest that the sta[6] mutant was likely exposed to higher oxidative stress levels compared to control cells growing in the same conditions

Summary

Introduction

With the growing demand for environmentally friendly sustainable energy alongside an increasing world population, the subject of algal biomass production has attracted much attention since microalgae can convert atmospheric CO2 and water pollutants into diverse value-added products including biofuel and other bioactive compounds[1,2]. In order to promote lipid production from microalgae, many technological approaches including genetic engineering, stress regulation, and modification of cultivation apparatus have been proposed[4,5,6,7] One of these approaches, the construction of more efficient algal mutants using genetic engineering, is considered a promising and important subject in the microalgae-to-bioproducts process[8]. It is hypothesized that cellular stress levels and the subsequent regulation of autophagy may be correlated with an enhanced TAG accumulation in starchless mutants. Since this has not yet been concretely investigated, in the present study, we examined the nitrogen starvation-induced oxidative stress response and autophagy regulation in the starchless sta[6] mutant of C. reinhardtii

Methods

Results

Conclusion