Improved tissue culture response of an elite maize inbred through backcross breeding, and identification of chromosomal regions important for regeneration by RFLP analysis.

The frequency of initiation of friable, embryogenic callus from immature embryos of the elite maize inbred line B73 was increased dramatically by introgression of chromosomal segments from the inbred line A188 through classical backcross breeding. Less than 0.2% of the immature B73 embryos tested (5 of 3,710) formed embryogenic callus. The breeding scheme consisted of six generations of backcrossing to B73 with selection at each generation for high frequency initiation of embryogenic cultures. BC6 individuals were selfed for four generations to select homozygous lines. The average embryogenic culture initiation frequency increased to 46% (256/561). Nearly all (91%) of the embryos from one BC6 S4 plant formed embryogenic cultures. RFLP analysis was used to determine the locations and effects of the introgressed A188 chromosomal segments. Five segments were retained through at least the fifth backcross generation. The hypothesis that one or more of these five regions contains genes controlling somatic embryogenesis in maize was tested using an F2 population of the cross A188 X Mo17. A set of five DNA markers (three of them linked) explained 82% of the observed phenotypic variance for percentage of immature embryos forming embryognic callus. Four of the five markers were located in or near introgressed A188 chromosome segments.The region marked by probe c595 on the long arm of chromosome 9 was highly associated with several measures of in vitro culture response (percent embryogenic embryos, plants per embryo, and plants per embryogenic embryo). We propose that there is a major gene (or genes) in this region in A188 that promotes embryogenic callus initiation and plant regeneration in B73, Mo17, and probably many other recalcitrant inbred lines of maize.