Identification of the genes underlying the yellow Burley phenotype in Nicotiana tabacum
The primary distinguishing characteristic of cultivars of the Burley tobacco market class is their high degree of chlorophyll deficiency, particularly on stems, stalks, and midveins, which becomes accentuated during plant maturation. This ‘yellow Burley’ phenotype is conferred by a double homozygous recessive genotype (yb1yb1 yb2yb2) at the Yellow Burley 1 (Yb1) and Yellow Burley 2 (Yb2) loci. This genetic deficiency affects the physiology of Burley tobacco cultivars and translates into, among other things, reduced N utilization efficiency and increased potential for accumulation of tobacco-specific nitrosamines, a potent class of carcinogens that exists in greater amounts in tobacco products containing cured Burley tobacco leaves. Through map-based gene discovery and transgene complementation, we have shown that Yb1 and Yb2 encode for proteins with a very high degree of similarity to a characterized gene from Arabidopsis involved in chloroplast maintenance. Genetic analysis of a historical set of tobacco cultivars indicates that Burley tobacco evolved via a two-step process: an 8 bp deletion in Yb2, followed by human selection of a spontaneous homozygous 1 bp insertion in Yb1, whereby both mutations lead to truncated protein products. The identification of the genes at the Yb1 and Yb2 loci represents the first reported example of map-based gene discovery in N. tabacum, and their identification sheds light on the physiological differences between Burley tobacco and other tobacco market classes.