Abstract
The papain-like cysteine proteases (PLCPs), the most important group of cysteine proteases, have been reported to participate in the regulation of growth, senescence, and abiotic stresses in plants. However, the functions of PLCPs and their roles in stress response in microalgae was rarely reported. The responses to different abiotic stresses in Haematococcus pluvialis were often observed, including growth regulation and astaxanthin accumulation. In this study, the cDNA of HpXBCP3 containing 1515 bp open reading frame (ORF) was firstly cloned from H. pluvialis by RT-PCR. The analysis of protein domains and molecular evolution showed that HpXBCP3 was closely related to AtXBCP3 from Arabidopsis. The expression pattern analysis revealed that it significantly responds to NaCl stress in H. pluvialis. Subsequently, transformants expressing HpXBCP3 in Chlamydomonas reinhardtii were obtained and subjected to transcriptomic analysis. Results showed that HpXBCP3 might affect the cell cycle regulation and DNA replication in transgenic Chlamydomonas, resulting in abnormal growth of transformants. Moreover, the expression of HpXBCP3 might increase the sensitivity to NaCl stress by regulating ubiquitin and the expression of WD40 proteins in microalgae. Furthermore, the expression of HpXBCP3 might improve chlorophyll content by up-regulating the expression of NADH-dependent glutamate synthases in C. reinhardtii. This study indicated for the first time that HpXBCP3 was involved in the regulation of cell growth, salt stress response, and chlorophyll synthesis in microalgae. Results in this study might enrich the understanding of PLCPs in microalgae and provide a novel perspective for studying the mechanism of environmental stress responses in H. pluvialis.
Highlights
In plants and microalgae, proteases play essential roles in organism growth, reproduction, development, photosynthesis, senescence, and environmental stress responses [1,2]
Based on a transcriptome data of H. pluvialis, one candidate of papain-like cysteine proteases (PLCPs) gene annotated as cysteine protease was found and successfully isolated
The protein domain analysis found that it contained the autoinhibitory prodomain, the protease domain, and the granulin domain (Figure 1A), which is similar as AtRD21A belonging to PLCP subfamily 1 and AtXBCP3 belonging to PLCP subfamily 4 in Arabidopsis
Summary
Proteases play essential roles in organism growth, reproduction, development, photosynthesis, senescence, and environmental stress responses [1,2]. Studies showed that some PLCPs, such as AtXCP1 and AtSAG12, were involved in dwarfism and early leaf senescence in plants [5]. In pepper and barley, the inhibition of PLCPs activity could lead to the enhanced tolerance to salt and osmotic stresses [9,10]. It showed that PLCPs from plants play different roles in growth, development, cell apoptosis, senescence, biotic and abiotic stresses [5,7,11]
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