AUTHOR=Xu Na , Meng Lin , Song Lin , Li Xiaoxu , Du Shasha , Hu Fengqin , Lv Yuanda , Song Wenjing 

TITLE=Identification and Characterization of Secondary Wall-Associated NAC Genes and Their Involvement in Hormonal Responses in Tobacco (Nicotiana tabacum)

JOURNAL=Frontiers in Plant Science

VOLUME=12

YEAR=2021

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2021.712254

DOI=10.3389/fpls.2021.712254

ISSN=1664-462X

ABSTRACT=<p>Secondary wall-associated NAC <italic>(SWN)</italic> genes are a subgroup of NAC (NAM, ATAF, and CUC) transcription factors (TF) that play a key role in regulating secondary cell wall biosynthesis in plants. However, this gene family has not been systematically characterized, and their potential roles in response to hormones are unknown in <italic>Nicotiana tabacum</italic>. In this study, a total of 40 <italic>SWN</italic> genes, of which 12 from <italic>Nicotiana tomentosiformis</italic>, 13 from <italic>Nicotiana sylvestris</italic>, and 15 from <italic>Nicotiana tabacum</italic>, were successfully identified. The 15 <italic>SWNs</italic> from <italic>Nicotiana tabacum</italic> were further classified into three groups, namely, vascular-related NAC domain genes (<italic>NtVNDs</italic>), NAC secondary wall thickening promoting factor genes (<italic>NtNSTs</italic>), and secondary wall-associated NAC domain genes (<italic>NtSNDs</italic>). The protein characteristic, gene structure, and chromosomal location of 15 <italic>NtSWNs</italic> (also named <italic>Nt1</italic> to <italic>Nt15</italic>) were also analyzed. The <italic>NtVND</italic> and <italic>NtNST</italic> group genes had five conserved subdomains in their N-terminal regions and a motif (LP[Q/x] L[E/x] S[P/A]) in their diverged C- terminal regions. Some hormones, dark and low-temperature related <italic>cis</italic>-acting elements, were significantly enriched in the promoters of <italic>NtSWN</italic> genes. A comprehensive expression profile analysis revealed that <italic>Nt4</italic> and <italic>Nt12</italic> might play a role in vein development. Others might be important for stem development. Quantitative reverse transcription-polymerase chain reaction (qRT-PCR) revealed that in the <italic>NtNST</italic> group, genes such as <italic>Nt7, Nt8</italic>, and <italic>Nt13</italic> were more sensitive than the genes in <italic>NtVND</italic> and <italic>NtSND</italic> groups under abiotic stress conditions. A transactivation assay further suggested that <italic>Nt7, Nt8</italic>, and <italic>Nt13</italic> showed a significant transactivation activity. Overall, <italic>SWN</italic> genes were finally identified and characterized in diploid and tetraploid tobacco, revealing new insights into their evolution, variation, and homology relationships. Transcriptome, <italic>cis</italic>-acting element, qRT-PCR, and transactivation assay analysis indicated the roles in hormonal and stress responses, which provided further resources in molecular mechanism and genetic improvement.</p>