Organic Broadcaster

Research moves forward on ‘organic-ready’ open-pollinated corn

Update on Corn Pollen Incompatibility Breeding Projects
By Joe Pedretti, MOSES

It has been known, at least since the 1950s, that popcorn cannot set seed if pollinated by yel­low field corn. Another closely related plant called teosinte also shares this Ga1s pollen incompat­ibility gene.

In corn, the male flower, called the tassel, releases pollen, which is blown by the wind until it lands on a corn silk. The pollen then germi­nates on the silk and grows a tubule down to the ovary in the ear of corn where fertilization occurs. If the pollen tubule does not grow, no corn kernel is produced.

The Ga1s gene works by recognizing when incompatible pollen is trying to grow a pollen tubule down a corn silk. The corn plant sends out chemicals to inhibit this growth and prevent fertilization. This trait will help organic farmers produce corn with a greatly lowered GMO-contamination risk.

The PuraMaize gene system was first devel­oped in the late 1990s by Tom Hoegemeyer of Cerrado Natural Systems Group while exploring ways to preserve color purity in white corn. It was developed using traditional plant breeding tech­niques. Hoegemeyer later created corn hybrids with the ability to resist cross fertilization by pol­len from GM corn. In 2012 hybrids became avail­able to the organic community through Blue River Hybrids which have proven an effective and pro­ductive alternative for farmers wanting to reduce their GMO-contamination risk.

The next step in breeding is to incorporate pollen incompatibility into open-pollinated vari­eties of field corn, which can in turn be used by regional plant breeders and farmers to create their own GMO-pollen-incompatible lines.

Frank Kutka, an independent corn breeder from North Dakota, has received funding from the Organic Farming Research Foundation (OFRF) to develop “Organic-Ready” open-pollinated corn with the Ga1s gametophytic incompatibility.

Despite some weather setbacks at his nurser­ies, Frank is making good progress back-crossing traits into older open-pollinated varieties. The open-pollinated varieties will be “open source,” meaning that breeders or farmers will be able to either develop their own regionally-adapted ver­sions, or simply save their own seed from season to season.

“Even though the project is not progressing as fast as I wished, we will keep pushing it through, and I expect that in one or two years we will be ready to release seeds on the market,” noted Frank.

In a country increasingly dominated by GMO corn production, buffer strips and delayed plant­ing (to avoid pollination) are often not enough to prevent contamination. Pollen incompatibility is a new tool for organic and non-GMO farmers, and one with great potential to allow coexistence while meeting the ever increasing demand for non-GMO products. Look for more hybrids and open pollinated seeds in the near future.

 

Using Gametophytic Cross Incompatibility for Organic Corn Purity
By Frank Kutka

Gametophytic cross incompatibility was first noticed decades ago among some lines of popcorn that would set little or no seed if pollinated by dent corn pollen (Thomas, 1955).

This incompatibility trait is controlled by a single allele, Ga1s, and incompatible lines are homozygous (Neuffer et al., 1997). Most dent corn is homozygous ga and cannot pollinate such lines, although there are some breeding populations of dent corn with Ga1s (Kutka, 2009) and a pat­ent was granted for yellow field corn lines with Ga1s even though some yellow Ga1s lines already existed. Zeigler and Ashman (1994) reported that the trait is widely used in popcorn production to allow planting relatively close to dent corn fields with much reduced risk of cross-contamination.

Ga1s works because plants with this trait have silks that do not support normal pollen tube growth (and thereby fertilization) for pollen car­rying the ga allele. Lausser et al. (2010) reported 0 – 5% of ga pollen tubes growing 8 cm into silks of homozygous Ga1s plants with most growing no more than 2 cm. Marcus Zuber, who released white dent corn inbreds with Ga1s in the early 1990s (Poneleit 2000), reported outcrossing rates of 2 – 5% in homozygous Ga1s white dent varieties planted adjacent to normal yellow corn.

However, dent outcrosses are usually even lower in popcorn fields due to cross incompatibility and the abundance of more competitive Ga1s pollen in the popcorn fields (Zeigler and Ashman, 1994).

Tcb1s and Ga2s are gametophytic cross incom­patibility alleles from teosinte that have been crossed into experimental field corn lines by Jerry Kermicle at the University of Wisconsin (Kermicle and Evans, 2010; Evans and Kermicle, 2001). These genes appear to work in a similar fashion to Ga1s and dent corns of the U.S. are Ga2 and Tcb1 in genotype. Kermicle and Evans found that these dominant cross incompatibility traits were not able to successfully cross with plants carrying a different incompatibility trait under most circum­stances. These genes open up further opportunities to reduce undesirable outcrossing in commercial yellow and white corn in the U.S. as has Ga1s.

Although powerful tools that can greatly reduce rates of undesired outcrossing, these genes are not a guarantee of long-term genetic purity. Popula­tions with these traits would need to be checked for outcrossing from time to time and normal measures to reduce outcrossing must still be put into place to receive the full benefit of these genetic tools. Of special note would be reducing the num­ber of volunteer plants in adjacent transgenic fields that might be heterozygous for both transgenes and a gametophytic cross incompatibility trait. Such plants would produce some pollen that car­ried both an incompatibility gene and a transgene, and these could cross with plants homozygous for the same incompatibility gene. If, for instance, transgenic corn was all homozygous for Tcb1s and varieties used by organic farmers were all homozy­gous for Ga2s, this threat would be largely elimi­nated as both varieties would likely be mutually incompatible.

There are other traits that could be used to help control and monitor transgenic contamination.

There are several male sterility traits (Neuffer et al., 1997) and some are used for hybrid seed production. If 92% of the transgenic corn plants were male sterile (cross of sterile line with a fer­tile line that did not restore fertility in offspring), they would likely still be adequately pollinated by the 8% fertile plants mixed in and the amount of transgenic pollen blowing on the wind would be proportionately reduced. This approach is used successfully in the TOPCROSS® high oil corn sys­tem from Dupont (Thomison).

Frank Kutka, Ph.D., manages the Northern Plains Sus­tainable Agriculture Society’s Farm Breeding Club. He can be reached at dakotacornman@yahoo.com or 701-483-2348.

References

Evans, M.M.S., and J.L. Kermicle. 2001. Teosinte crossing barrier 1, a locus governing hybridization of teosinte with maize. Theor. Appl. Genet. 103:259-265. Available online: www.springerlink.com/content/6avefqvt9c1frcej/ (accessed 20 September 2011).

Kermicle, J.L., and M.M.S. Evans. 2010. The Zea mays sexual incompatibility gene ga2: naturally occurring alleles, their distribution, and role in reproductive iso­lation. Journal of Heredity doi:10.1093/jhered/esq090. Online: jhered.oxfordjournals.org/content/early/2010/08/07/jhered.esq090.full.pdf (accessed 20 September 2011).

Kutka, F.J. 2009. Release of populations carrying Ga1s. Maize Genetics Cooperation

Newsletter (www.agron.missouri.edu/mnl/84/PDF/20kutka.pdf).

Lausser, A., I. Kliwer, K. Srilunchang, and T. Dresselhaus. 2010. Sporophytic control of pollen tube growth and guid­ance in maize. J. Exp. Bot. 61(3):673-682.

Neuffer, M.G., E.H. Coe, and S.R. Wessler. 1997. Mutants of maize. Cold Spring Harbor

Laboratory Press, Plainview, NY, USA.

Poneleit, C.G. 2000. Breeding white endosperm corn. Chapter 8 in Hallauer, A.R. (ed.).

Specialty Corns, 2nd Ed. CRC Press, Boca Raton.

Thomas, W.I. 1955. Transferring the Gas factor for dent incompatibility to dent compatible lines of popcorn. Agron. J. 47:440-441.

Thomison, P.R. No Date. Topcross® High Oil Corn Produc­tion: Management Considerations. Ohio State University Extension Factsheet AGF-135-97. Online: ohioline.osu.edu/agf-fact/0135.html (accessed 30 September 2011).

Zeigler, K.E., and B. Ashman. 1994. Popcorn. Chapter 7, pp. 189-223 in, Hallauer, A.R. (ed.). Specialty Corns. CRC Press, Boca Raton.

From the September | October 2014 Issue

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