Shoot production per responsive leaf explant increases exponentially with explant organogenic potential in Nicotiana species

As part of the effort to develop optimal plant varieties for the production and molecular farming of plant-made pharmaceuticals, this study evaluated shoot organogenic potential of a total of 115 Nicotiana accessions, representing 53 species. To induce shoots, leaves from seedling grown in vitro were cut into pieces, cultured on shoot-induction medium under low light for 3 weeks, and then subcultured onto the same medium for another 4 weeks under normal light. Statistical analysis detected significant differences among the 115 accessions for the percentage of leaf explants producing shoots and the number of shoots produced per responsive leaf explant. Importantly, regression analysis also found an exponential relationship between the number of shoots produced per responsive leaf explant and the percentage of leaf explants producing shoots. The number of shoots produced per responsive leaf explant increased rather slowly, ranging from zero to around five, as the percentage of leaf explants producing shoots increased from 0 to 80%, but the increase became dramatic as the percentage increased from 80% to 100%, reaching as high as 35 shoots per responsive leaf explant. This exponential relationship is the first of its kind to be established in plant regeneration studies using either organogenesis or somatic embryogenesis systems. A possible mechanism that governs the establishment of the exponential relationship is discussed.