Contents lists available at ScienceDirect Algal Research journal homepage: www.elsevier.com/locate/algal Effect of phytohormones from different classes on gene expression of Chlorella sorokiniana under nitrogen limitation for enhanced biomass and lipid production Abhishek Guldhe, Nirmal Renuka, Poonam Singh, Faizal Bux ⁎ Institute for Water and Wastewater Technology, Durban University of Technology, P.O. Box 1334, Durban 4000, South Africa ARTICLE INFO Keywords: Microalgae Nitrogen-limitation Phytohormones Lipid Gene expression ABSTRACT Recently, the use of phytohormones has been found to be expedient for improving biomass and lipid productivity for algal biofuel production. The comparative effect of phytohormones from different classes on biomass and lipid productivity, and the expression of the related genes under nitrogen-limitation is not yet explored. In the present study, the optimized concentrations of different phytohormones, auxin - indole acetic acid (IAA), gib- berellin - gibberellic acid (GA) and cytokinin – kinetin (K) showed 45.55%, 35.94% and 37.37% increase in biomass productivity in Chlorella sorokiniana as compared to nitrogen-limited control (ON) respectively. The highest biomass productivity of 146.07 mg L -1 d -1 and relative rbcL gene expression was observed in IAA (7.5 mg L -1 ) supplemented treatment. However, K (10 mg L -1 ) supplementation under N-limitation showed highest lipid content of 35.85% and accD gene expression as compared to other treatments. Overall, highest lipid productivities of 50.24 and 49.43 mg L -1 d -1 were observed in optimized IAA and K supplemented treatments respectively. Findings from the present study illustrated the differential response of phytohormones from dif- ferent classes on biomass and lipid enhancement, fatty acid composition and expression of related genes. These novel findings could be advantageous for the development of promising strategies for improving microalgal biomass and lipid productivity using phytohormones for sustainable biofuel production. 1. Introduction Microalgae biomass is one of the potential feedstocks for the pro- duction of biofuels in the currently offered innovations for sustainable development [1]. The development of strategies for enhanced biomass and lipid production in microalgae is imperative for feasible microalgal biofuel production, where the use of phytohormones for enhancing biomass and lipid production is an emerging trend [2,3]. Phyto- hormones are the signalling molecules involved in the metabolism of living organisms including microalgae and are responsible for different metabolic processes [4]. Nitrogen-limitation is the most feasible strategy for lipid enhancement, however, leads to reduction in growth and biomass production [5,6]. Meanwhile, recent studies revealed the driving effect of phytohormone supplementation on biomass and me- tabolite production in microalgae under normal and nitrogen-limited conditions [3,7]. Yu et al. [8] revealed that addition of auxins indole acetic acid (IAA) and naphthylacetic acid (NAA) increased the lipid productivity three times under nitrogen-depleted conditions as com- pared to standard BG-11 medium. Most of studies showed the effect of one type of phytohormones, however, comparative effect of different types of phytohormones on biomass, lipid productivity and molecular aspects is not yet explored. Rubisco (Ribulose-1,5-bisphosphate carboxylase) is the key enzyme in all photosynthetic organisms including algae, which catalyzes the major reaction of photosynthetic carbon fixation [9,10]. The rbcL gene en- codes the catalytic large subunit of Rubisco enzyme. While, acetyl-CoA carboxylase is the main enzyme involved in the catalysis of the first step of lipid biosynthesis and the β-subunit of heteromeric acetyl-CoA car- boxylase is encoded by accD gene [11]. Babu et al. [12] reported that the supplementation of phytohormones IAA and diethyl aminoethyl hexonate (DA) enhanced the gene expression of rbcL and accD gene under N-limited conditions. They revealed that phytohormones IAA and diethyl aminoethyl hexonate (DA) showed different level of response in terms of enhancement in relative gene expression of rbcL and accD genes. The effect of different phytohormones on the algal metabolism and gene expression under N-limitation is not yet widely discovered and need in-depth studies. Different types of phytohormones may have https://doi.org/10.1016/j.algal.2019.101518 Received 26 October 2018; Received in revised form 22 March 2019; Accepted 22 April 2019 ⁎ Corresponding author. E-mail address: faizalb@dut.ac.za (F. Bux). Algal Research 40 (2019) 101518 2211-9264/ © 2019 Published by Elsevier B.V. T