Charles J. Graham | 3/2/2015 11:29:45 PM
Pecan (Carya illinoinensis) is native to North America and represents an important horticultural crop. World figures for pecan production are not maintained, but nearly all of the world’s production occurs in North America (95 to 98% of total production). The United States is the largest producer of pecans in the world, accounting for 67% of North American production, followed by Mexico which accounts for the remainder. Pecans are also grown commercially in Australia, Israel, South Africa, Egypt, Peru, Argentina, Brazil, Thailand, and China (Grauke and Thompson, 1996; USDA, 2006). Pecans are prized for their pleasant taste, but also for their nutritional value in providing proteins and unsaturated fatty acids. Recently, it has been proven that nut and tree nut consumption is a healthy dietary habit. Several studies have shown that nuts are high in antioxidants and have a beneficial effect on the outcome of coronary disease and cholesterol serum levels (Morgan and Clayshulte, 2000; Rajaram et al., 2001; Haddad et al., 2006). In the last few years there has been an increase in nut and tree nut consumption because of their favorable health effects.
You may be wondering “Is there a negative side to these studies?” Unfortunately, nuts and tree nuts (including pecan) are recognized to be among the eight food groups that cause the majority of food allergies (US Food and Drug Administration, 2010). The increased usage of nuts in processed foods could facilitate contamination of other food products being handled in the same food manufacturing line (Teuber et al. 2003). The Food Allergen Labeling and Consumer Protection Act of 2004 requires that nuts be included on the label when they are used as ingredients in a food product. Nuts and tree nuts are among the highest producers of IgE-mediated allergic reactions following food ingestion. Food allergy is estimated to affect approximately 6–8% of children below 4 years of age and 1-2% of individuals over the first decade of life. In the United States, food allergy produces approximately 30,000 anaphylactic reactions and 200 fatalities per year (Yocum et al., 1999). Either peanuts or tree nuts cause more than 80% of these reactions. Fleischer et al. (2005) studied 101 patients who were given diagnoses of tree nut allergy on the basis of previous clinical reactivity and these patients had a total of 115 allergic reactions: 10 patients reacted to 2 different tree nuts, and 2 patients reacted to 3 different tree nuts. Nine (8%) patients reacted to almond, 3 (3%) to Brazil nut, 34 (30%) to cashew, 6 (5%) to hazelnut, 5 (4%) to macadamia nut, 16 (14%) to pecan, 2 (2%) to pine nut, 5 (4%) to pistachio, and 35 (30%) to walnut.
USDA and LSU AgCenter scientists have joined forces to study the allergens present in pecan. To better understand this collaborative research, it may be helpful to take a quick refresher course in plant genetics. The plant genome is made up of deoxyribonucleic acid (DNA), a long, winding, double-helical molecule that contains the instructions needed to build and maintain cells. For these instructions to be carried out, DNA must be transcribed (converted) into corresponding molecules of ribonucleic acid (RNA), referred to as transcripts. A transcriptome represents the very small percentage of the genome that is transcribed into RNA molecules. There are various kinds of RNA in a plant. The major type, called messenger RNA (mRNA), plays a vital role in making proteins. In this process, mRNA transcribed from genes, which include the protein-coding parts of the genome, is delivered to ribosomes. The ribosomes read, or "translate," the sequence of the chemical letters in mRNA to assemble building blocks called amino acids into proteins. Each mRNA is transcribed from a gene and then translated into a specific protein. The process of determining the genetic codes contained in the transcriptome and their relative proportions is known as transcriptome sequencing. Once a transcriptome has been sequenced, we can use the information to identify which parts of the genome are used to code for active genes. If several transcriptomes are sequenced under different growth or environmental conditions, we can study these transcriptomes and start to estimate which genes are involved in various biological processes.
"Major" food allergens are typically water-soluble glycoproteins 10 to 70kD in size that are relatively stable to heat, acid, and proteases. Three conserved seed-storage proteins from the prolamin and cupin superfamilies, including 2S albumin, 7S vicillin, and 11S legumin are commonly identified as allergens in nuts. The first step in the collaborative research between the USDA and LSU AgCenter was to investigate the timing of gene expression of genes encoding three highly conserved, allergenic seed-storage proteins in developing pecans. Mattison et al. (2013) measured the transcript levels of theCar i 1andCar i 4genes as well as theCar i 7Shomologue over the course of kernel development (August through October) in the pecan cultivars ‘Desirable’ and ‘Sumner’ using reverse transcription quantitative polymerase chain reaction (RT-qPCR). Both cultivars generally had similar levels of transcripts throughout kernel development, except ‘Desirable’ had higher levels ofCar i 1andCar i 4in August. However, transcript levels for the tested pecan allergens varied during nut development. These results support the idea that methods which target transcriptional control could be used to reduce or eliminate these allergens from tree nuts. Understanding the timing of allergen gene transcription could assist plant breeders in developing cultivars with lower levels of allergens.
Future goals of this collaboration are to determine allergen gene sequences in pecan and generate a temporal transcriptome and a proteome profile for developing pecan nuts to analyze genotype versus environmental or agricultural interactions. These analyses would allow for the identification of co-factors or co-enzymes that are important for pecan allergen potency, aid our understanding of the genetic diversity within pecan nuts, and facilitate development of better processing methods to attenuate pecan allergens.
The sequenced transcriptomes are publically available and can be downloaded in the following PDF file:
Christopher P. Mattison, PhD
1100 Robert E Lee Blvd
New Orleans, LA 70124
Charles J. Graham, PhD
LSU Agricultural Center
262 Research Station Drive
Bossier City, LA 71112
This research (and publication) was supported by the Specialty Crop Block Grant Program at the U.S. Department of Agriculture through grant agreement number 12-25-B-1464. Its contents are solely the responsibility of the authors and do not necessarily represent the official views of the USDA, LSU AgCenter, or Louisiana Department of Agriculture & Forestry.