Submitted to: In Vitro Cellular and Developmental Biology - Plants
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/2/2009
Publication Date: 11/24/2009
Citation: Genovesi, A.D., Jessup, R.W., Engelke, M.C., Burson, B.L. 2009. Interplody St. Augustinegrass [Stenotaphrum secundatum (Walt.) Kuntze] hybrids recovered by embryo rescue. In Vitro Cellular and Developmental Biology - Plants. 45:659-666.
Interpretive Summary: St. Augustinegrass is an important turfgrass that is grown on the lawns of most homes in the southern United States, especially along the Gulf Coast. Even though it is a popular turfgrass, it has a number of problems that reduce its attractiveness. It is attacked by a number of disease and insect pests, most notability brown patch and chinch bugs. Most varieties that are susceptible to these pests have fewer chromosomes and finer leaves and stems than the varieties and lines that are resistant to these pests. One way to improve the resistance in the susceptible varieties is to cross the two types to transfer resistance into the finer turf types. Unfortunately, the coarser leaf/stem, pest resistant, high chromosome types usually do not produce good pollen or seed. Because of this sterility problem, it is impossible to recover hybrids when these types are crossed with the susceptible, lower chromosome types with finer leaves. In this study, we used the higher chromosome, coarse leaf/stem, pest resistant types as the female parent in crosses with the lower chromosome fine leaf/stem, susceptible types. About a week after pollination, the embryos in developing seed of the female parent were removed and placed on an artificial growth media (source of food). These embryos grew into small plants and were planted into pots containing soil and later planted into the field. In total, 268 plants were recovered from 2463 crosses. It was difficult to determine if these plants were actually hybrids based on their appearance. We checked the genetic makeup (DNA) of a selected group of these hybrids and all were true hybrids which indicate most if not all of the plants recovered are true hybrids. This is the first time hybrids have been recovered from crossing these varieties and lines and it provides a way to improve St Augustinegrass. This technique of growing developing embryos from crosses on an artificial food source should permit plant breeders to develop improved pest resistance varieties for the home owners in the southern United States and other warmer regions of the world.
Technical Abstract: St. Augustinegrass (Stenotaphrum secundatum) is one of the most important warm season turfgrasses in the southern United States because of its shade tolerance. Most cultivars are diploids (2n=2x=18), but they are susceptible to various diseases and insects. Polyploids occurring in the species are largely resistant to pests but most lack cold tolerance. In this study eight polyploid genotypes were crossed with six diploid cultivars to transfer pest resistance to the diploids. Because interploid crosses often result in aborted seed, it was necessary to use in vitro techniques. Using embryo rescue, 268 plants were recovered from 2,463 emasculated and pollinated florets (10.88% crossability). Because of the heterogeneous nature of the species, these purported hybrids could not be verified by phenotype. As an alternative to determining the chromosome number of this large number of plants, DNA markers were used for hybrid identification. A subset of 25 plants from crosses between the aneuploid cultivar Floratam (2n=32) and five diploid cultivars were analyzed using 144 EST-SSRs developed from buffelgrass cDNA sequence data. Chi-square tests for paternal-specific markers revealed that all progeny analyzed were true F1 hybrids and none originated from self-fertilization or unintended out-crossing. In addition to identifying DNA polymorphism, the EST-SSRs revealed that genetic variation exists among all cultivars analyzed and is not partitioned between ploidy levels. These findings demonstrate that the embryo rescue techniques used will enable a more complete utilization of the entire spectrum of genetic variation in St. Augustinegrass through the recovery of interploid hybrids.