Genome-Wide Identification and Analysis on YUCCA Gene Family in Isatis indigotica Fort. and IiYUCCA6-1 Functional Exploration

Auxin is one of the most critical hormones in plants. YUCCA (Tryptophan aminotransferase of Arabidopsis (TAA)/YUCCA) enzymes catalyze the key rate-limiting step of the tryptophan-dependent auxin biosynthesis pathway, from IPA (Indole-3-pyruvateacid) to IAA (Indole-3-acetic acid). Here, 13 YUCCA family genes were identified from Isatis indigotica, which were divided into four categories, distributing randomly on chromosomes (2n = 14). The typical and conservative motifs, including the flavin adenine dinucleotide (FAD)-binding motif and flavin-containing monooxygenases (FMO)-identifying sequence, existed in the gene structures. IiYUCCA genes were expressed differently in different organs (roots, stems, leaves, buds, flowers, and siliques) and developmental periods (7, 21, 60, and 150 days after germination). Taking IiYUCCA6-1 as an example, the YUCCA genes functions were discussed. The results showed that IiYUCCA6-1 was sensitive to PEG (polyethylene glycol), cold, wounding, and NaCl treatments. The over-expressed tobacco plants exhibited high auxin performances, and some early auxin response genes (NbIAA8, NbIAA16, NbGH3.1, and NbGH3.6) were upregulated with increased IAA content. In the dark, the contents of total chlorophyll and hydrogen peroxide in the transgenic lines were significantly lower than in the control group, with NbSAG12 downregulated and some delayed leaf senescence characteristics, which delayed the senescence process to a certain extent. The findings provide comprehensive insight into the phylogenetic relationships, chromosomal distributions, and expression patterns and functions of the YUCCA gene family in I. indigotica.