Target genes for breeding of energy sugarcane are identified

Brazilian researchers have identified genes associated with cell wall metabolism for production of cellulosic ethanol in a study featured on the cover of Plant Molecular Biology.

By Elton Alisson  |  Agência FAPESP – Brazilian researchers are close to developing a variety of sugarcane with a lower sucrose (sugar) content and a larger proportion of fiber and organic matter (biomass). Known as energy cane, it could be used to produce cellulosic ethanol or bioelectricity, as well as the usual applications of cane with a higher sucrose content, such as the production of sugar and first-generation ethanol.

Scientists at the University of São Paulo’s Chemistry Institute (IQ-USP) and Bioscience Institute (IB-USP), in collaboration with colleagues in the Departments of Plant Biology and Plant Science at the Federal University of Viçosa (UFV) in Minas Gerais State, the Agrarian Science Center at the Federal University of São Carlos (UFSCar) in São Paulo State, and the National Science & Technology Institute for Bioethanol (INCT-Bioetanol), have identified genes associated with cell wall biosynthesis – the production of chemical compounds in sugarcane cells. The chemicals in question include lignin, which is important to the production of bioelectricity owing to its high calorific value.

The research was conducted as a Thematic Project funded by FAPESP under the aegis of its Bioenergy Research Program (BIOEN) and a cooperation agreement with the Minas Gerais State Research Foundation (FAPEMIG). The findings are featured on the cover of the May issue of the journal Plant Molecular Biology.

The study was also part of a postdoctoral research project supported by a scholarship from FAPESP.

“Our discoveries pave the way for the identification of important biochemical pathways that can help develop new varieties of cane or transgenic plants for use in producing bioethanol and new materials in biorefineries,” said Glaucia Mendes Souza, a professor at IQ-USP and principal investigator for the project, in an interview with Agência FAPESP.

The researchers analyzed the transcriptomes of Saccharum officinarum, S. spontaneum and S. robustum, ancestral species of sugarcane used to breed current varieties, and the commercial hybrid RB867515. Transcriptome analysis enables researchers to determine when and where each gene is turned on or off in the cells and tissues of an organism by analyzing the entire collection of RNA sequences.

Based on their transcriptome analysis, they constructed for the first time a series of networks to identify genes that might be altered to improve traits of interest in sugarcane, such as cell wall sucrose and fiber production.

This network analysis resulted in the identification of 18 transcription factors that govern cell wall biosynthesis. Transcription factors are proteins that control which genes are turned on or off in the genome and therefore have a significant impact on cell activities.

One of these 18 transcription factors, ScMYB52, appears to be a key regulator of cell wall metabolism and is considered a good candidate for further investigation.

“We succeeded in identifying genes associated with crucial points in the regulation of cell wall biosynthesis, so we now have targets for more applied analysis,” said Souza, who is also a BIOEN program coordinator. “We want to find out whether, by focusing on these genes, it’s possible to alter the composition of the plant’s fibers and sucrose.”

Carbon partitioning

The results of the study enhance scientists’ understanding of carbon partitioning in sugarcane, i.e., how the plant uses carbon absorbed from the atmosphere and chemically fixed as carbohydrate to produce sucrose, fiber and lignin.

Researchers know that carbon absorbed from the atmosphere is used in the production of complex molecules such as sucrose and cellulose, which can be broken down and converted into energy via combustion or oxidation processes.

Modern varieties of sugarcane store a third of their carbon in the form of sucrose. The other two-thirds are contained in the plant’s biomass, especially stalk and leaf fibers.

“It hasn’t always been like this,” Souza said. “The ratio of fiber to sucrose is higher in the genotypes of ancestral sugarcane species.”

To develop varieties with higher sucrose content for the production of sugar or ethanol via fermentation of sugarcane juice or with higher fiber and biomass content for the production of cellulosic ethanol, breeders have selected and crossed ancestral species and cultivars for centuries.

For Souza, however, the effectiveness of this strategy may be dwindling. “Genetic improvement programs are reaching a limit,” she said. “For example, they can’t increase sucrose content much more than they already have.”

Research such as the current study, Souza explained, can enable breeding programs to use molecular biology tools to achieve genome-guided improvement, which boosts the expression of genes associated with traits such as higher fiber, sucrose and biomass content and inserts more copies of them into the genomes of targeted varieties or changes their expression.

“We can use the transcription factors we’ve identified to analyze whether it’s possible to increase or reduce the amount of fiber, sucrose and chemical compounds produced by sugarcane,” she said. “That includes lignin, which is very important for the production of bioelectricity, for example, because of its high calorific value. The idea is eventually to get the crop to produce commercially viable compounds without competing with the production of sucrose and fiber.”

In the same study, the researchers also suggest that sucrose production may be controlled epigenetically via heritable changes in gene expression that do not depend on changes in the plant’s primary DNA sequence.

The article “Co-expression network analysis reveals transcription factors associated to cell wall biosynthesis in sugarcane” (doi: 10.1007/s11103-016-0434-2) by Glaucia Mendes Souza et al. can be read in Plant Molecular Biology at http://link.springer.com/article/10.1007/s11103-016-0434-2.