Because an F1 hybrid has a potential character that grows faster and has a shorter cultivation period in a field, the risk against bacterial disease accelerated by rain would be below. Thus, F1 hybrids are commonly anticipated to display high productivity under stressful conditions. In lettuce, the exploitation of the F1 hybrid could be one of the effective approaches to maintain a stable yield, particularly in tropical and subtropical regions. A new crisphead cultivar ‘Fine green’ was indeed the first F1 hybrid bred by Kaneko seeds CO., LTD. in Japan, but unfortunately, the technical detail of the breeding method was not announced publicly. In general, the male-sterile plant is worth exploring as the key factor of F1 hybrid breeding, and several GMS mutants were also reported in lettuce so far 22. The genetic mechanism is not understood, and this is the first report of the identification of the male-sterile gene in lettuce. It is valuable to ascertain the genetic mechanism of male-sterile plants to select a future breeding strategy.
In this study, the two F2 populations were used to locate the male-sterile gene to the region between the two PCR-based markers, LG8_v8_246.869Mbp and LG8_v8_257.031Mbp. Although the genomic region of the ms-S locus was relatively large, the whole-genome sequencing for male-sterile and fertile lines of ‘CGN17397’ revealed only 1 different gene, Lsat_1_v5_gn_8_148221.1, between 2 lines in these 94 annotated genes in the ms-S locus (Table S1, Fig. S2). The gene encoded an acyl-CoA synthetase 5 (ACOS5) and was a potential ortholog of the key male-sterile gene AtACOS5 in Arabidopsis 18 (Fig. 3a). The AtACOS5 acted as acyl-CoA synthase to regulate the biosynthesis of sporopollenin to affect male fertility, and a null mutant was entirely male-sterility 18. The male-sterile line of ‘CGN17397’ displayed normal vegetative growth and complete male-sterility insensitive to environmental conditions. There were no other obvious morphological differences between the male-sterile and fertile lines. Lettuce was generally only flowering for about two hours in the morning, but the male-sterile lines could continue to flower through the afternoon. Thus, the male-sterile mutants of lettuce and Arabidopsis showed phenotypic similarities 18. I concluded that LsACOS5 was a biologically plausible candidate gene for the ms-S locus (Figs. 2, 3, Table S1, Fig. S2).
In addition, the InDel marker—LG8_v8_250.793Mbp—tightly linking to the ms-S gene was developed. By using the InDel marker, it was possible to select male-sterile plants for a conventional-breeding program (Fig. 1c, Fig. 2). Due to the structure of the lettuce flower, it was challenging to examine the inheritable characteristics of valuable traits 1, such as disease resistance in only the F1 seeds because crosses produced not only F1 seeds but also self-pollinated seeds. Because only F1 hybrid seeds can be produced using GMS plants for crossbreeding, research on valuable traits that could not be analyzed in the past would be facilitated.
The F1 seed production system was needed to promote the commercial production of F1 hybrids. To propagate the F1 hybrid seeds in the case of rice, the maternal and paternal plants were alternately cultivated in a field to cross by the wind and artificial pollination 23. But lettuce pollen was not dispersed by wind, the F1 seed production system has been already developed using insect pollination at a greenhouse. The fact that flies and bees were adopted for the system due to an absence of specialist pollinators of lettuce, the self-pollinating crop, could propagate the F1 hybrid seeds 24,25. Moreover, the F1 hybrids are likely to be suitable for cultivation in not only fields but also plant factories. The trait of rapid growth was economically important for the cultivation in plant factories. The breeding of F1 hybrids suitable for cultivation in fields and plant factories is an issue for the future.
To date, genome editing technology makes it possible to create knockout mutants of the target gene. GMS plants generally have a problem of seed mixture for the male-sterile and fertile progeny. Still, a novel hybridization platform known as the third-generation breeding technique has been successfully selected for non-transgenic GMS seeds 9. Combining these two techniques could also be applied for the F1 hybrid breeding in lettuce, and it converts any elite cultivars into a commercial male-sterile plant and accelerates the development of F1 hybrid cultivars. The applications of the GMS plant initiative to the rise of considerable potential for lettuce breeding.