The impact of genome evolution on the allotetraploid Nicotiana rustica

The availability of four new sequences from the Nicotiana genome provides insight into how speciation affects plant metabolism and, in particular, alkaloid transport and accumulation, and will contribute to a better understanding of the evolution of Nicotiana species.

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While Nicotiana tabacum is the most notable species of the Nicotiana genus, many other Nicotiana species are grown as crops, grown as ornamental garden plants, or used as model organisms in research. The Aztec or Indian tobacco, Nicotiana rustica, is suspected of being the original tobacco species that was brought from America to Europe. Known as “mapacho”, it was considered sacred and medicinal by Amazonian shamans.

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Although, in terms of production, it was supplanted in the last century by the related N. tabacum, Aztec tobacco is still grown in South America, Turkey, Russia and Vietnam, mainly due to its resistance to adverse climatic conditions.

Morphologically, N. rustica is recognized by its characteristic yellow tubular flowers (Supplementary File 1: Figure S1) and by the leaves covered with trichomes rich in secondary metabolites, including nicotine, nornicotine, anatabine and anabasine [1]. The high concentration of nicotine in the leaves (5-15% of the weight of the dry leaf) has motivated its use in the production of pesticides based on nicotine, nicotine sulfate and nicotinic acid. Due to even higher levels of citric acid (15-20% of the dry leaf weight), N. rustica leaves are an excellent source of this important metabolite [2]. Scientific reports describing the active accumulation of nicotine in N. rustica compared to N. tabacum are rather scarce [3, 4]. Interestingly, in contrast to the nicotine level, N. rustica exhibits a lower ratio of leaf cadmium to root than N. tabacum [5,6]. Since the root is involved in nicotine synthesis, cadmium absorption and translocation of shoots, root pathways may have interconnections and Cd has been reported to have toxic properties with respect to plant nutrition [7, 8]. On the Cd side, although the accumulation of Zn, but not Fe and Mn, may vary between the two species [6], no report yet mentions the variation of K and Na.

Within the Solanaceae family, the genomes of the Nicotiana species are peculiar. First, they have relatively large genomes similar in size to Capsicum species and two to three times larger than Solanum and Petunia species. Secondly, the Nicotiana genus contains many species that can be used to study the evolution of polyploidy in plants. Although most of the more than 70 Nicotiana species are diploid with n = 12, five sections of the Nicotiana family (Nicotiana, Polydicliae, Repandae, Undulatae and Rusticae) include allopolyploid species with n = 24 [9]. Molecular clock analyzes estimate that the dates of polyploidization events range from less than 0.2 million years ago (Nicotiana arentsii, N. rustica and N. tabacum) to more than 10 million years ago (a single event of polyploidization from which the Suaveolentes sect descended) [9,10,11]. To date, only the parental species of N. tabacum (Nicotiana sylvestris and Nicotiana tomentosiformis) have been well characterized [12], and the presence of previously identified species-specific translocations in N. tabacum has been confirmed [13, 14] [fifteen ].

On the basis of morphology, cytology and artificial hybridization experiments, Goodspeed [16] proposed the probable progenitors of the polyploid species of the genus Nicotiana. The origins of 15 Nicotiana allopolyploid species have been explored by genomic in situ hybridization (GISH) [17], and fluorescently labeled DNA probes from N. undulata and N. paniculata genomes have marked complementary chromosomes of N. rustica , resulting in this confirms Goodspeed’s hypothesis about his relationships with parents. Lim et al. [18] investigated genome evolution in three naturally occurring allopolyploid species (N. arentsii, N. rustica and N. tabacum) using GISH and fluorescent in situ hybridization. Unlike N. tabacum cultivars, no intergenomic translocations have been observed in N. rustica; therefore, probes for N. undulata and N. paniculata were mapped exclusively to the U and P genomes, respectively, of N. rustica.

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