Tomato fruit ripening natural mutants provide ideal research materials for people to study the regulation mechanism of tomato fruit ripening, and greatly promote the research progress of fruit ripening mechanism. The fruit ripening process of nor natural mutant was significantly inhibited. It was found that the deletion of two consecutive bases a in the third exon of nor gene resulted in the frameshift mutation. The translation of nor protein was terminated in advance, resulting in a truncated egg white containing 186 amino acids. At first, it was thought that the functional mutation of nor gene resulted in the immature phenotype of fruit States patent, patent No.: US 6762347 B1), so NAC nor is considered to be the core transcription factor regulating tomato fruit ripening. However, in 2019, Luo Yunbo and Fu Dachi from the school of food science and nutrition engineering of China Agricultural University and Ruud A. de from Wageningen University in the Netherlands The maagd team respectively used CRISPR / cas9 technology to mutate nac-nor gene, but could not obtain the similar phenotype with nor natural mutant. Therefore, the mechanism of fruit immaturity of nor mutant and the positive role of nac-nor gene in tomato fruit ripening need to be reevaluated.
On April 27, 2020, Journal of Experimental Botany published a research paper entitled "re evaluation of the nor mutation and the role of the NAC nor transcription factor in tomato fruit ripening" by Professor Fu Dachi of the school of food science and nutrition engineering, China Agricultural University. This study not only provides new evidence to prove that nor natural mutant is a functional acquired mutant, but also proves that nac-nor plays an important role in the regulation of tomato fruit ripening.
In this study, the researchers found that in the nor gene editing mutant (CRISPR / cas9 edited nac-nor), the ethylene biosynthesis, carotenoid accumulation and fruit softening of tomato fruits were significantly inhibited, but the inhibition effect was not as strong as that of nor natural mutant. Over expression of nor gene significantly promoted ethylene biosynthesis, carotenoid accumulation and fruit softening, indicating that nac-nor is still an important transcription factor regulating tomato fruit ripening. However, over expression of nac-nor gene in the background of nor natural mutant can not completely restore the immature phenotype of the mutant, which proves that there are other factors causing the immature phenotype of nor natural mutant.
Figure 1. The phenotypes of CR-NOR and OE-NOR tomato fruit.
Further studies showed that nor 186, a truncated protein containing 186 amino acids, could still enter the nucleus and bind to but not activate the promoters of the key genes of ethylene biosynthesis, carotenoid accumulation and fruit softening, slacs2, slggpps2 and slpl. Moreover, the activation effect of wild-type nor protein on these promoters will be inhibited by nor186. The above results further prove that nor natural mutant is a functional acquired mutant, and the truncated protein nor186 produced by nor gene mutation has dominant negative function.
Figure 8. Model of NAC-NOR and the mutant NOR186 in regulation of tomato fruit ripening.
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