Abstract
BackgroundOil palm is the most productive oil crop and the efficiency of pollination has a direct impact on the yield of oil. Pollination by wind can occur but maximal pollination is mediated by the weevil E. kamerunicus. These weevils complete their life cycle by feeding on male flowers. Attraction of weevils to oil palm flowers is due to the emission of methylchavicol by both male and female flowers. In search for male flowers, the weevils visit female flowers by accident due to methylchavicol fragrance and deposit pollen. Given the importance of methylchavicol emission on pollination, we performed comparative transcriptome analysis of oil palm flowers and leaves to identify candidate genes involved in methylchavicol production in flowers.ResultsRNA sequencing (RNA-Seq) of male open flowers, female open flowers and leaves was performed using Illumina HiSeq 2000 platform. Analysis of the transcriptome data revealed that the transcripts of methylchavicol biosynthesis genes were strongly up-regulated whereas transcripts encoding genes involved in lignin production such as, caffeic acid O-methyltransferase (COMT) and Ferulate-5-hydroxylase (F5H) were found to be suppressed in oil palm flowers. Among the transcripts encoding transcription factors, an EAR-motif-containing R2R3-MYB transcription factor (EgMYB4) was found to be enriched in oil palm flowers. We determined that EgMYB4 can suppress the expression of a monolignol pathway gene, EgCOMT, in vivo by binding to the AC elements present in the promoter region. EgMYB4 was further functionally characterized in sweet basil which also produces phenylpropenes like oil palm. Transgenic sweet basil plants showed significant reduction in lignin content but produced more phenylpropenes.ConclusionsOur results suggest that EgMYB4 possibly restrains lignin biosynthesis in oil palm flowers thus allowing enhanced carbon flux into the phenylpropene pathway. This study augments our understanding of the diverse roles that EAR-motif-containing MYBs play to fine tune the metabolic flux along the various branches of core phenylpropanoid pathway. This will aid in metabolic engineering of plant aromatic compounds.
Highlights
Oil palm is the most productive oil crop and the efficiency of pollination has a direct impact on the yield of oil
Transcriptome data analysis of open oil palm flowers revealed that EgCVS and EgCvOMT are highly expressed in flowers whereas expression levels of EgCOMT and EgF5H are significantly reduced when compared to leaves (Additional file 4)
RNA sequencing (RNA-Seq) analysis of flowers showed increased expression of transcripts coding for enzymes involved in methylchavicol production but decreased expression of caffeic acid O-methyltransferase (COMT) and F5H transcripts which are involved in lignin formation
Summary
Oil palm is the most productive oil crop and the efficiency of pollination has a direct impact on the yield of oil. Attraction of weevils to oil palm flowers is due to the emission of methylchavicol by both male and female flowers. Palm oil derived from its fruits is the largest source of edible vegetable oil in the world [3] It was first introduced in Southeast Asia in 1848 and was planted on a commercial scale around 1917. Elaeidobius kamerunicus, a type of weevil which originated from West Africa, is considered the most competent and dominant insect pollinator species of oil palm. Introduction of this weevil into Malaysia during the 1980s enhanced the pollination rate of oil palm trees leading to 20–30% increase in fruit production [5]. To attract the weevil for pollination, both male and female flowers release a volatile phenylpropene compound called methylchavicol ( known as estragole) [6]
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