Proteomic Responses of Dark-Adapted Euglena gracilis and Bleached Mutant Against Light Stimuli.

Zhenfan Chen,Qiong Liu,Zixi Chen,Hui Zhu,Jiangxin Wang,Anping Lei,Jiayi He,Jiayi Zhu

doi:10.3389/fbioe.2022.843414

Zhenfan Chen, Qiong Liu + Show 6 more

Open Access

https://doi.org/10.3389/fbioe.2022.843414

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Abstract

Euglena gracilis (E. gracilis) has secondary endosymbiotic chloroplasts derived from ancient green algae. Its chloroplasts are easily lost under numerous conditions to become permanently bleached mutants. Green cells adapted in the dark contain undeveloped proplastids and they will develop into mature chloroplasts after 3 days of light exposure. Thus, E. gracilis is an ideal model species for a chloroplast development study. Previous studies about chloroplast development in E. gracilis focused on morphology and physiology, whereas few studies have addressed the regulatory processes induced by light in the proteome. In this study, the whole-genome proteome of dark-adapted E. gracilis (WT) and permanently ofloxacin-bleached mutant (B2) was compared under the light exposure after 0, 12, and 72 h. The results showed that the photosynthesis-related proteins were up-regulated over time in both WT and B2. The B2 strain, with losing functional chloroplasts, seemed to possess a complete photosynthetic function system. Both WT and B2 exhibited significant light responses with similar alternation patterns, suggesting the sensitive responses to light in proteomic levels. The main metabolic activities for the utilization of carbon and energy in WT were up-regulated, while the proteins with calcium ion binding, cell cycle, and non-photosynthetic carbon fixation were down-regulated in B2. This study confirmed light-induced chloroplast development in WT from dark, and also for the first time investigates the light responses of a bleached mutant B2, providing more information about the unknown functions of residual plastids in Euglena bleached mutants.

Highlights

Euglena gracilis (E. gracilis) a unicellular green eukaryotic microalga, belonging to the group of euglenids (Excavata), is widely distributed in aquatic environments
The ancestor of E. gracilis is known as a heterotrophic animal species and captures green algae resulting in the acquisition of photosynthetic ability by secondary endosymbiosis
Five bleached mutants of E. gracilis treated with ofloxacin were obtained and 12 residual plastid genes of the total chloroplast genome were investigated in both WT and bleached mutants

Summary

Introduction

Euglena gracilis (E. gracilis) a unicellular green eukaryotic microalga, belonging to the group of euglenids (Excavata), is widely distributed in aquatic environments. It is worth mentioning that the chloroplasts of E. gracilis are embedded by three membranes as the result of secondary endosymbiosis. The ancestor of E. gracilis is known as a heterotrophic animal species and captures green algae resulting in the acquisition of photosynthetic ability by secondary endosymbiosis. Many treatments such as mutagens, heat, and pressure could bleach E. gracilis, and permanently lose most, if not all, of their chloroplast genomes but with few impacts on their heterotrophic growth (Heizmann et al, 1976; Osafune and Schiff, 1983). Five bleached mutants of E. gracilis treated with ofloxacin were obtained and 12 residual plastid genes of the total chloroplast genome were investigated in both WT and bleached mutants. One of the bleached mutants named B2 lost the ability to accumulate chlorophyll, which was due to many genes of chloroplasts being undetected at the genomic and transcriptional levels, such as psbE and psbK (Qin et al, 2020)

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