Genome-wide identification and expression analysis of NADPH oxidase genes in response to biotic and abiotic stresses, and during alternating heteromorphic generations in Pyropia haitanensis

Algae respond rapidly with a respiratory oxidative burst during and after exposure to numerous environmental stresses. However, NADPH oxidase (Nox) homologs related to the generation of reactive oxygen species by algae are still poorly understood. In the present study, we identified 46 Nox homologs in eight red algae species; these Noxs possessed key functional domains similar to plant respiratory burst oxidase homologs but most of them were clustered into an independent clade in evolution. Furthermore, the cis-acting regulatory elements, transcription profiles, transcription factors co-expression correlation and differential expression of Pyropia haitanensis nox genes (Phnoxs) were detected. Results revealed that abundant elements involved in phytohormone responsiveness, abiotic stress induction, light regulation and transcription factor binding were harbored at the promoter region of Phnoxs. And they possessed differential transcript profiles during alternating heteromorphic generations. Phnox5C and 2C were the main genes expressed in free-living conchocelis and gametophyte, respectively, and their expression might be regulated by different transcription factors; an implication of their importance in nori growth and carpospore development. In addition, Phnox2C, 4 and 5A were significantly up-regulated after flg22 or oligoagar exposure, indicating they may provide resistance to pathogens. Conversely, Phnox2A, 2C, 4 and 5A responded positively to mechanical damage or drought stress. The findings presented in this study could be valuable for further elucidating the functions and regulatory mechanisms of NADPH oxidases during the development and adaptation of red algae to diverse stresses.