NAC transcription factors play critical roles in xylem secondary development and in regulation of stress response in plants. NAC proteins related to secondary cell wall development were recently identified and characterized in Tectona grandis (teak), one of the hardwood trees of highest economic importance in the world. In this work, we characterized the novel TgNAC01 gene, which is involved in signaling pathways that mediate teak response to stress. Abscisic acid (ABA) increases TgNAC01 expression in teak plants. Therefore, this gene may have a role in signaling events that mediate ABA-dependent osmotic stress responsive in this plant species. Stable expression in tobacco plants showed that the TgNAC01 protein is localized in the cell nucleus. Overexpression of TgNAC01 in two out three independent transgenic tobacco lines resulted in increased growth, leaf senescence and salt tolerance compared to wild type (WT) plants. Moreover, the stress tolerance of transgenic plants was affected by levels of TgNAC01 gene expression. Water potential, gas exchange and chlorophyll fluorescence were used to determine salt stress tolerance. The 35S:TgNAC01-6 line under 300 mM NaCl stress responded with a significant increase in photosynthesis rate, stomatal conductance, transpiration and carboxylation efficiency, but lower water potential compared to WT plants. The data indicate that the TgNAC01 transcription factor acts as a transcriptional activator of the ABA-mediated regulation and induces leaf senescence.

Teak (Tectona grandis) is one of the most important wood sources, and it is cultivated in tropical regions with a significant market around the world. Abiotic stresses are an increasingly common and worrying environmental phenomenon because it causes production losses in both agriculture and forestry. Plants adapt to these stress conditions by activation or repression of specific genes, and they synthesize numerous stress proteins to maintain their cellular function. For example, APETALA2/ethylene response factor (AP2/ERF) was found to be involved in stress signal transduction. A search in the teak transcriptome database identified an AP2/ERF gene named TgERF1 with a key AP2/ERF domain. We then verified that the TgERF1 expression is rapidly induced by Polyethylene Glycol (PEG), NaCl, and exogenous phytohormone treatments, suggesting a potential role in drought and salt stress tolerance in teak. The full-length coding sequence of TgERF1 gene was isolated from teak young stems, characterized, cloned, and constitutively overexpressed in tobacco plants. In transgenic tobacco plants, the overexpressed TgERF1 protein was localized exclusively in the cell nucleus, as expected for a transcription factor. Furthermore, functional characterization of TgERF1 provided evidence that TgERF1 is a promising candidate gene to be used as selective marker on plant breeding intending to improve plant stress tolerance.

NAC proteins are one of the largest families of plant-specific transcription factors (TFs). They regulate diverse complex biological processes, including secondary xylem differentiation and wood formation. Recent genomic and transcriptomic studies of Tectona grandis L.f. (teak), one of the most valuable hardwood trees in the world, have allowed identification and analysis of developmental genes. In the present work, T. grandis NAC genes were identified and analyzed regarding to their evolution and expression profile during wood formation. We analyzed the recently published T. grandis genome, and identified 130 NAC proteins that are coded by 107 gene loci. These proteins were classified into 23 clades of the NAC family, together with Populus, Eucalyptus, and Arabidopsis. Data on transcript expression revealed specific temporal and spatial expression patterns for the majority of teak NAC genes. RT-PCR indicated expression of VND genes (Tg11g04450-VND2 and Tg15g08390-VND4) related to secondary cell wall formation in xylem vessels of 16-year-old juvenile trees. Our findings open a way to further understanding of NAC transcription factor genes in T. grandis wood biosynthesis, while they are potentially useful for future studies aiming to improve biomass and wood quality using biotechnological approaches.

Characterization of NAC transcription factor family of Tectona grandis involved in secondary cell wall deposition and stress response NAC proteins are one of the largest families of plant-specific transcription factors (TFs). They regulate diverse biological processes, including secondary cell wall formation (SCW). SCW is the main component of wood and source of raw material in forestry industry, Tectona grandis (teak) specie is recognized for presenting one of the most valuable woods in the world due to its extraordinary qualities of color, density and durability. Studies of teak genomics and transcriptomics were recently unveiled, producing information that allowed to identify teak NAC transcription factors and to analyse their evolution and expression profile during wood formation. It was identified 130 NAC proteins that were coded by 107 gene loci. These proteins were classified into 23 clades of the NAC family, together with Populus, Eucalyptus, and Arabidopsis. Data on transcription expression revealed specific temporal and spatial expression patterns for the majority of teak NAC genes. The qRT-PCR indicated expression of VASCULAR NAC DOMAIN proteins involved in SCW formation in xylem vessels of 16-year-old juvenile trees. In addition, it was characterized by heterologous expression, the novel salt stress-responsive TgNAC01 gene. Three independent transgenic tobacco lines overexpressing TgNAC01 were generated. It was demonstrated that the TgNAC01 protein is localized only in the nucleus. Analyses of TgNAC01 relative expression, stem length and chlorophyll content were evaluated in two months old transgenic plants growing in greenhouse. Tobacco plants overexpressing TgNAC01 showed higher stem length, an increase in leaf senescence, and tolerance to 300 mM of NaCl when compared to wild type (WT) plants. Water potential, gas exchange and chlorophyll fluorescence were used to determine the salt stress tolerance. In this research, it was demonstrated that TgNAC01 transcription factor acts as a positive regulator in xylem development and as a transcriptional activator mediated by salt stress and leaf senescence. These results open a way to further understanding of NAC transcription factor genes in T. grandis wood formation and stress tolerance, potentially being useful for future studies aiming to improve productivity and wood quality using biotechnological approaches.

No abstract