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EARLY DIFFERENTIAL EXPRESSION OF GALACTOMANNAN BIOSYNTHESIS GENES IN ‘MAKAPUNO’ COCONUT (Cocos nucifera L.) REVEALED BY THE DE NOVO ASSEMBLY AND ANALYSIS OF ENDOSPERM TRANSCRIPTOME
Jickerson P. Lado, Beatriz Z. Arellano, Cecille Ann L. Osio, Bernabeth Jo T. Tendero, Celia B. Dela Viña, Mary Ann O. Torio, Rita P. Laude, Maria Genaleen Q. Diaz
Makapuno is a coconut cultivar with a naturally overproliferating solid endosperm almost filling the nut cavity. At 6-7 months after pollination (MAP), makapuno solid endosperm is phenotypically indistinguishable from the normal type Laguna Tall while at 8-9 MAP makapuno starts to soften and thicken in contrast to the hard and compact appearance of the normal. Here, the expression profiles of 6-7 and 8-9 MAP de novo assembled RNA Sequencing (RNA-Seq) transcriptomes of normal and makapuno were analyzed, and corresponding stages were compared to determine the differentially expressed genes (DEGs). Transcriptome analyses showed 317 and 717 DEGs at 6-7 and 8-9 MAP, respectively. At an early stage, DEGs are mostly related to antibiotic biosynthesis pathways suggesting an onset of early plant defense mechanism and sugar metabolism pathways, namely amino sugar, nucleotide sugar, starch, sucrose, and galactose metabolisms, and glycolysis. These findings agree with previous observations on the makapuno phenotype having altered transcription, respiration, sugar metabolism, and cellular behaviors. Further, this study found evidence for the early differential expression of eight galactomannan biosynthesis genes, namely: UDP-glucose 4’-epimerase (GALE), mannose-1-phosphate guanylyltransferase (GMPP), mannan synthase (ManS), galactomannan galactosyltransferase (GMGT), α-galactosidase (AGAL), sucrose synthase 1 and 2 (SUS1, SUS2), and UDP-glucose dehydrogenase (UGDH). Results of the relative reverse-transcription polymerase chain reaction (RT-PCR) analysis showed similar expression patterns at 6-7 MAP and 8-9 MAP for most of these genes, confirming RNA-Seq data. This early expression of galactomannan biosynthesis provides another layer of information in understanding the observed galactomannan overproduction in makapuno endosperm.