Transcriptional Regulation of Cellulose Biosynthesis during the Early Phase of Nitrogen Deprivation in Nannochloropsis salina

Seok Won Jeong,Kwon Hwangbo,Won Joong Jeong,Seung Won Nam,Yong Keun Chang,Byeong-Ryool Jeong,Youn-Il Park

doi:10.1038/s41598-017-05684-4

Abstract

Microalgal photosynthesis provides energy and carbon-containing precursors for the biosynthesis of storage carbohydrates such as starch, chrysolaminarin, lipids, and cell wall components. Under mild nitrogen deficiency (N−), some Nannochloropsis species accumulate lipid by augmenting cytosolic fatty acid biosynthesis with a temporary increase in laminarin. Accordingly, biosynthesis of the cellulose-rich cell wall should change in response to N− stress because this biosynthetic pathway begins with utilisation of the hexose phosphate pool supplied from photosynthesis. However, few studies have characterised microalgal cell wall metabolism, including oleaginous Nannochloropsis sp. microalgae subjected to nitrogen deficiency. Here, we investigated N-induced changes in cellulose biosynthesis in N. salina. We observed that N− induced cell wall thickening, concurrently increased the transcript levels of genes coding for UDPG pyrophosphorylase and cellulose synthases, and increased cellulose content. Nannochloropsis salina cells with thickened cell wall were more susceptible to mechanical stress such as bead-beating and sonication, implicating cellulose metabolism as a potential target for cost-effective microalgal cell disruption.

Highlights

Algal cell walls contain carbohydrates, proteins, and lipids, which protect the cells from adverse environmental conditions
Nitrogen deficiency (N−) is the most effective external factor for triggering TAG production[14, 15]; it causes a series of physiological changes in some Nannochloropsis species including growth retardation, enhanced cell size, pigment alterations and lipid accumulation, and it reduces photosynthesis, carbon fixation, and protein synthesis[11, 15, 16, 17]
The results showed that cell wall thickening induced by N− was positively correlated with enhanced transcript levels of genes involved in cellulose biosynthesis such as Upp and CesA; the transcript levels of Upp, CesA8, and CesA122-1s were enhanced by 20.1, 3.3, and 2.7-fold, respectively (Fig. 6a)

Summary

Introduction

Algal cell walls contain carbohydrates, proteins, and lipids, which protect the cells from adverse environmental conditions. Changes in culture conditions, such as reductions in the concentrations of salt, nitrogen, phosphate, and sulphur induce cell wall thickening in Nannochloropsis, Chlorella, Chlorococcum, and Chlamydomonas[9,10,11]. Stress conditions and nitrogen deficiency (N−) on the biosynthesis of cell wall components and TAG production in the majority of algal species. Nitrogen deficiency increased the levels of Upp and CesA transcripts and concurrently increased the expression of genes involved in cytosolic fatty acid and lipid biosynthesis, including cytosolic fatty acid synthase type 1 (Fas1), diglyceride acyltransferase (Dgat-2) and ∆12-desaturase (Fad[2]). N− increased cellulose content and induced cell wall thickening, which caused cell wall fragility. These properties render the cells more susceptible to mechanical stress disruption, which can improve their performance as feedstock for biofuel reactors

Methods

Results

Conclusion