Abstract

BackgroundDuckweed can thrive on anthropogenic wastewater and produce tremendous biomass production. Due to its relatively high starch and low lignin percentage, duckweed is a good candidate for bioethanol fermentation. Previous studies have observed that water devoid of nutrients is good for starch accumulation, but its molecular mechanism remains unrevealed.ResultsThis study globally analyzed the response to nutrient starvation in order to investigate the starch accumulation in duckweed (Landoltia punctata). L. punctata was transferred from nutrient-rich solution to distilled water and sampled at different time points. Physiological measurements demonstrated that the activity of ADP-glucose pyrophosphorylase, the key enzyme of starch synthesis, as well as the starch percentage in duckweed, increased continuously under nutrient starvation. Samples collected at 0 h, 2 h and 24 h time points respectively were used for comparative gene expression analysis using RNA-Seq. A comprehensive transcriptome, comprising of 74,797 contigs, was constructed by a de novo assembly of the RNA-Seq reads. Gene expression profiling results showed that the expression of some transcripts encoding key enzymes involved in starch biosynthesis was up-regulated, while the expression of transcripts encoding enzymes involved in starch consumption were down-regulated, the expression of some photosynthesis-related transcripts were down-regulated during the first 24 h, and the expression of some transporter transcripts were up-regulated within the first 2 h. Very interestingly, most transcripts encoding key enzymes involved in flavonoid biosynthesis were highly expressed regardless of starvation, while transcripts encoding laccase, the last rate-limiting enzyme of lignifications, exhibited very low expression abundance in all three samples.ConclusionOur study provides a comprehensive expression profiling of L. punctata under nutrient starvation, which indicates that nutrient starvation down-regulated the global metabolic status, redirects metabolic flux of fixed CO2 into starch synthesis branch resulting in starch accumulation in L. punctata.

Highlights

  • Duckweed can thrive on anthropogenic wastewater and produce tremendous biomass production

  • The fronds were sampled at 0 h, 0.5 h, 2 h, 5 h, 24 h, 48 h, 72 h, 96 h, 120 h, 144 h and 168 h time point respectively for the measurement of fresh weight, dry weight, protein and starch percentage (Figure 1)

  • Results demonstrated that the fresh weight of L. punctata increased slowly during nutrient starvation, ranging from 0.50 g at 0 h to 0.72 g at 168 h in one culture flask

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Summary

Introduction

Duckweed can thrive on anthropogenic wastewater and produce tremendous biomass production. Previous studies have observed that water devoid of nutrients is good for starch accumulation, but its molecular mechanism remains unrevealed Liquid biofuels, such as bioethanol, converted from biomass are considered as a promising alternative for traditional fossil fuels. With near-exponential growth rates and the shortest doubling times [12,13,14], duckweed can achieve a biomass of 0.5 to 1.5 metric tons/hectare/day fresh weight or 13 to 38 metric tons/hectare/year dry weight [15] This accompanies with the tremendous amount of CO2 sequestered from the atmosphere and the natural ability of duckweed to thrive on eutrophic wastewater, and recover polluting nutrients [14,16,17,18,19,20,21,22,23], suggesting that growing duckweed for biomass can have large beneficial environmental impacts. Supported by the Minister of Science and Technology, China launched a project to produce liquid biofuel from duckweed biomass, and the first international conference on duckweed application and research was held in Chengdu, China in October, 2011 [8]

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