Expression of homeobox genes during in vitro culture of Lactuca sativa

Lettuce, Lactuca sativa, is one of the main leafy vegetable crops, interesting as a source of phytonutrients. It is extensively studied for biotechnological applications based on in vitro regeneration. However, in lettuce, histological studies of adventitious morphogenesis during the regeneration process are lacking. Analogously, the genes involved in the initiation of adventitious shoots or embryos are unknown. Here, we used cotyledon explants of a cos lettuce (‘Romana’) to define the histological events of in-vitro morphogenesis at different times, and to investigate the nature of the regeneration pathway. Furthermore, we isolated three homeobox genes, LsWUSCHEL1-LIKE (LsWUS1L), LsWUS2L, and LsSHOOT MERISTEMLESS1-LIKE (LsSTM1L), to evaluate their transcription activity during the ontogenesis of the adventitious structures. De novo morphogenesis quickly proceeded through embryogenic and organogenic pathways, with some developmental abnormalities. The expression of LsWUS1L gradually increased during the in vitro culture of cotyledon explants. Remarkably, we detected high expression of both LsWUS2L and LsSTM1L genes after four days of culture, when the very early stages of morphogenesis were only noticeable through histological analysis. These findings suggest a key role of these genes in the rapid recruitment of the founder cells of adventitious structures.