Oil Palm - Genomics of Cultivated Palms - page 29

tenera (P28-1100) [14] are identified. These markers need further validation and
conversion as SCAR markers. Three specific operon RAPD markers, OPY20-1180,
OPR11-1282, and OPT19-1046, are found to be associated with the locus of shell
thickness [15] . The estimated map distances from Sh 1 to OPR11-1282 and to
OPT19-1046 were 17.5 cM and 23.9 cM, respectively, and were identified as linked
on both sides of the Sh 1 locus on linkage group 4 [15] . These two RAPDs were
able to predict shell thickness with an accuracy of nearly 97%. An AFLP marker
E-AGG/M-CAA132 was mapped at 4.7 cM from the Sh locus [16] . Another RFLP
marker (pOPgSP1282) closest to the locus was located 9.8 cM from the Sh 1 locus
[17] . About 75 out of 100 plants were heterozygous, with primer mEgCIR 008 hav-
ing a (GA) 17 repeat at an annealing temperature of 52 °C, with eight alleles varying
in size from 105 to 150 bp; and the tenera hybrid can be distinguished from dura and
pisifera parents [18] . Another SSR loci, mEgCIR1772, also serves the purpose, but
only in specific families [19] . Currently, the 14 molecular markers (9 RAPD primers,
2 AFLP loci, 1 RFLP loci, and 2 SSR loci) from these independent studies together
need further validation. Previous studies on the role of ferulic acid and enzymes
PAL, CAD, and POD and other lignin biosynthetic enzymes [12,13] must be clari-
fied, using current molecular biology, sequencing, and bioinformatics technologies
along with fine mapping of the Sh 1 locus. Map-based cloning, sequencing, and in
silico analysis of the corresponding fourth chromosomal segment have the potential
of making great strides in boosting the oil yield of oil palm.
3.2.2 Omics of Oil Palm Mesocarp Biology
Lipid-rich, fleshy mesocarp of oil palm and sugar-rich mesocarp of date palm were
compared using transcriptomics and metabolomics [20] . Regulatory mechanisms
in oil palm fruit ripening and mesocarp content were studied by pyrosequencing
of transcriptome and analysis of fatty acid (FA) and triacylglycerol (TA) assembly
pathways [21] . Surprisingly, both these independent studies identify a single seed
oil transcription factor, WRINKLED1, as an overexpressed (57-fold) transcript in
oil palm [20] mesocarp. The ratio of cell wall invertase to sucrose synthase is 3.5
times greater in oil palm transcripts than Arabidopsis [20] . The activity of AGPase
and starch synthase enzymes increases during ripening of oil palm fruits [20] .
NAC-domain proteins and MADs-box genes, in particular SEP-like, AG-like, and
GLO-like, were found to be involved during maturation and ripening of oil palm
mesocarp, especially the AGL2/SEPALLATA subfamily [21] . Lipase class 3 family
[22] and ethylene receptor genes [21,22] are overexpressed specifically in the mes-
ocarp of oil palm fruits. Maturation proteins (PM3) are the genes specific to ker-
nel tissues, and S-ribonuclease-binding protein and fibrillin were leaf and mesocarp
tissue-specific genes [23] . Differentially expressed genes are found in processes
associated with oil palm nut maturation, such as the synthesis of medium-chain satu-
rated fatty acids and phytic acid, nut development, and stress/defense responses [24] .
A sesquiterpene synthase gene unique to 12- to 20-week-old mesocarp of E. oleifera ,
but absent in E. guineensis and other tissues of both species [25] , requires further
analysis to establish an association with oil quality of the American oil palm.