Sequence variation and genomic organization of fatty acid desaturase-2 (fad2) and fatty acid desaturase-6 (fad6) cDNAs in maize

Abstract

Increased levels of oleic acid may enhance the nutritional and functional value of corn. Corn oil is primarily composed of palmitic, stearic, oleic, linoleic and linolenic fatty acids. Delta-12 desaturase in plants converts oleic acid (18:1) to linoleic acid (18:2) by inserting a double bond at the delta-12 position. Fatty acid desaturase-2 (fad2) encodes delta-12 desaturase that functions in the endoplasmic reticulum while fatty acid desaturase-6 (fad6) encodes delta-12 desaturase that functions in plastids. Complementary DNA (cDNA) clones from putative maize homologs for fad2 and fad6 were identified and the entire clones DNA sequenced. The maize fad2 cDNAs showed an amino-acid identity of 67-77% to fad2 of Glycine, Arabidopsis and Brassica species. Our cDNA sequence comparisons suggested that more than one fad2 gene is transcribed in maize embryos. Two different fad2 cDNAs from an embryo cDNA library map to separate chromosomal positions, providing evidence consistent with two different isoforms of fad2 expressed in the embryo. The fad2 cDNAs from multiple tissue sources clustered into three groups on a phenogram, and map to different positions on chromosomes 4, 5 and 10, which suggests at least three different isoforms of fad2 may be expressed in the maize plant. The two maize fad6 cDNAs share 81% amino-acid identity with the Arabidopsis fad6 and map to chromosome 1. Northern analysis revealed that fad2 is transcribed in embryos at 14, 21, 28 and 35 days after pollination, with the highest level observed at day 14. None of the fad2 or fad6 clones mapped to maize chromosome bins associated with QTLs for the ratio of oleic/linoleic acid, notably bin 6.04 which contains the linoleic1 locus and the largest reported QTL for the oleic/linoleic ratio. This suggests, but does not prove, that some of the QTLs for the oleic/linoleic acid ratio do not involve allelic variants of fad2 or fad6 but rather involve other genes that may influence flux through the enzymes encoded by fad2 or fad6.

Detection of quantitative trait loci for kernel oil and protein concentration in a B73 and Zheng58 maize cross

“…In the present study, qPRO04-01 was detected in the region of the o2 locus on chromosome 4, and qPRO10-02 was detected in the region of the fl4 locus on chromosome 10. o2 and fl4 were most likely to be responsible for qPRO04-01 and qPRO10-02, respectively. Only a few genes related to maize kernel oil have been reported, including ln1 (Poneleit and Alexander, 1965), olc1 (Wright, 1995), fad2 and fad6 (Poneleit and Alexander, 1965;Mikkilineni and Rocheford, 2003). In our study, qOIL-06-01 was detected in the region of the ln1 locus on chromosome 6. ln1 was most likely to be responsible for qOIL-06-01.…”

“…To date, nine genes corresponding to maize proteins have been cloned, including opaque1 (o1) (Wang et al, 2012), o2 (Schmidt et al, 1990), o5 (Myers et al, 2011), o7 (Wang et al, 2011), floury1 (fl1) (Holding et al, 2007), fl2 (Coleman et al, 1997), fl4 , Mucronate (Mc) (Kim et al, 2006), and Defective endosperm B30 (De-B30) (Kim et al, 2004). A few genes have been shown to influence maize kernel oil concentration, including linoleic acid1 (ln1) (Poneleit and Alexander, 1965), Oleic acid content1 (olc1) (Wright, 1995), fatty acid desaturation 2 (fad2), and fad6 (Poneleit and Alexander, 1965;Mikkilineni and Rocheford, 2003). Over the past two decades, genetic dissection of oil and protein concentrations in maize kernels by classical QTL mapping using biparental populations has resulted in the identification of numerous protein and oil QTLs.…”

Enhancing Plant Seed Oils for Human Nutrition

“…In addition, many plants have a number of FAD2 genes, all of which contribute to seed linoleic content (Tang et al, 2005). This means that development of mutant lines with useful oleic acid contents often requires combining several mutant loci and provides a breeding challenge (Mikkilineni and Rocheford, 2003).…”

Genetic Control of Storage Oil Synthesis in Seeds of Arabidopsis

“…In Arabidopsis, mutagenesis experiments facilitated the identification of genes responsible for fatty acid elongation and desaturation (James and Dooner, 1990;Lemieux et al, 1990;Arondel et al, 1993;Okuley et al, 1994;James et al, 1995). Since then, QTL affecting fatty acid composition that colocate with loci for fatty acid elongase and fatty acid desaturase genes have been identified in Brassica species, sunflower, and maize (Alrefai et al, 1995;Ecke et al, 1995;Jourdren et al, 1996;Cheung et al, 1998;Fourmann et al, 1998;Lionneton et al, 2002;Perez-Vich et al, 2002;Burns et al, 2003;Mikkilineni and Rocheford, 2003).…”