Xylem specific activation of 5' upstream regulatory region of two NAC transcription factors (MusaVND6 and MusaVND7) in banana is regulated by SNBE-like sites

CcNAC1 by Transcriptome Analysis Is Involved in Sudan Grass Secondary Cell Wall Formation as a Positive Regulator

Sudan grass is a high-quality forage of sorghum. The degree of lignification of Sudan grass is the main factor affecting its digestibility in ruminants such as cattle and sheep. Almost all lignocellulose in Sudan grass is stored in the secondary cell wall, but the mechanism and synthesis of the secondary cell wall in Sudan grass is still unclear. In order to study the mechanism of secondary cell wall synthesis in Sudan grass, we used an in vitro induction system of Sudan grass secondary cell wall. Through transcriptome sequencing, it was found that the NAC transcription factor CcNAC1 gene was related to the synthesis of the Sudan grass secondary cell wall. This study further generated CcNAC1 overexpression lines of Arabidopsis to study CcNAC1 gene function in secondary cell wall synthesis. It was shown that the overexpression of the CcNAC1 gene can significantly increase lignin content in Arabidopsis lines. Through subcellular localization analysis, CcNAC1 genes could be expressed in the nucleus of a plant. In addition, we used yeast two-hybrid screening to find 26 proteins interacting with CcNAC1. GO and KEGG analysis showed that CcNAC1 relates to the metabolic pathways and biosynthesis of secondary metabolites. In summary, the synthesis of secondary cell wall of Sudan grass can be regulated by CcNAC1.

Functional characterization of 5'-regulatory region of flavonoid 3',5'-hydroxylase-1 gene of banana plants

Generation of crops with broad-spectrum tolerance to biotic and abiotic stress conditions depends upon availability of genetic elements suitable for varied situations and diverse genotypes. Here, we characterize the 5'-upstream regulatory region of flavonoid 3'5'-hydroxylase-1 (F3'5'H-1) gene from banana and analyzed its tissue-specific and stress-mediated activation in genetic background of tobacco plants. MusaF3'5'H-1 is a stress-responsive gene as its expression is induced in banana after application of salicylic acid and methyl jasmonate while its transcript levels were drastically reduced in response to drought, high salinity and abscisic acid. P_MusaF3'5'H-1 harbours cis-elements associated with stress conditions and those responsible for tissue-specific expression. Transgenic lines harbouring P_MusaF3'5'H-1-GUS displays strong GUS expression in guard cells of stomata indicating guard cell preferred activity of P_MusaF3'5'H-1 while its activity was undetectable in roots. Drought and high salinity induce strong expression of GUS in transgenic tobacco lines and exposure to abscisic acid, salicylic acid and methyl jasmonate revealed distinct profiles of GUS expression in transgenic lines confirming involvement of F3'5'H-1 in plant stress responses. Fluorescent β-galactosidase assay revealed induction profiles of P_MusaF3'5'H-1 at different time points in transgenic lines exposed to salicylic acid and abscisic acid while strong suppression in GUS expression was observed after application of methyl jasmonate. The guard cell preferred activity of P_MusaF3'5'H-1 and stress-mediated expression profiles of MusaF3'5'H-1 indicated the suitability of P_MusaF3'5'H-1 for generating stress-enduring crops and analyzing guard cell functions.

MusaNAC29-like transcription factor improves stress tolerance through modulation of phytohormone content and expression of stress responsive genes

Understanding the molecular mechanisms governed by genes and cross-talks among stress signaling pathways is vital for generating a broad view on stress responses in plants. Here, we analysed the effects of MusaNAC29-like transcription factor of banana on stress responses and report the quantitative modulation of phytohormone and flavonoid content and analysed the growth parameters and yield trait in transgenic banana plants. Expression of MusaNAC29-like transcription factor was strongly altered in responses to stress conditions and application of signaling molecules. Under control conditions, P_{MusaNAC29-like}-GUS is activated in cells bordering xylem vessel elements and is strongly triggered in other cells types after influence of salicylic acid and abscisic acid. Transgenic banana plants of cultivar Rasthali and Grand Naine overexpressing MusaNAC29-like transcription factor displayed superior tolerance towards drought and salinity stress. LC-MS analysis indicated elevated levels of jasmonic acid and salicylic acid while content of zeatin was significantly reduced in leaves of transgenic banana lines. Transgenic banana lines displayed increased levels of gallic acid, coumaric acid, naringenin, chlorogenic acid while levels of vanillic acid and piperine were significantly reduced. Expression of stress related genes coding for antioxidants, thiol peptidase proteins, cold-regulated proteins, late embryogenesis abundant proteins, ethylene-responsive transcription factors, bHLH proteins, jasmonate-zim-domain proteins and WRKY transcription factors were significantly induced in transgenic banana lines. Though MusaNAC29-like transcription factor improved stress tolerance, its overexpression resulted in retarded growth of transgenic lines resulting in reduced yield of banana fruits.

MusaATAF2 like protein, a stress-related transcription factor, induces leaf senescence by regulating chlorophyll catabolism and H2 O2 accumulation

NAC transcription factors are known for their diverse role in plants. In this study, we have demonstrated the role of MusaATAF2, a banana NAC transcription factor, in leaf senescence. Its expression gets strongly up-regulated during the early stress responses of drought and high salinity exposure and down-regulated under ABA application, which suggests MusaATAF2 is a stress-related NAC transcription factor. To study the role of MusaATAF2 in banana, we have transformed the banana embryogenic cells with MusaATAF2 coding region and generated transgenic banana plants. Overexpression of MusaATAF2 in banana plants caused yellow leaf phenotype under control condition, suggesting its role as a senescence-associated transcription factor. Transgenic banana leaves exhibited low chlorophyll content and high H₂ O₂ accumulation. Hormone analysis of the leaves demonstrated a higher accumulation of ABA in the transgenic plants than the controls. Transgenic plants overexpressing MusaATAF2 have a higher transcript abundance of two chlorophyll catabolic pathway genes (PAO and HCAR) and lower transcript abundance of ROS scavenging enzymes (TDP, THIO, CAT, APX, and PRXDN) than control. Together, all these analyses indicate that MusaATAF2 induces senescence by inducing chlorophyll degradation and H₂ O₂ accumulation in banana plants and controls its own expression using an ABA-dependent feedback loop.

The 5'-upstream region of WRKY18 transcription factor from banana is a stress-inducible promoter with strong expression in guard cells

Increasing crop productivity in an ever-changing environmental scenario is a major challenge for maintaining the food supply worldwide. Generation of crops having broad-spectrum pathogen resistance with the ability to cope with water scarcity is the only solution to feed the expanding world population. Stomatal closure has implications on pathogen colonization and drought tolerance. Recent studies have provided novel insights into networks involved in stomatal closure which is being used in biotechnological applications for improving crop endurance. Despite that genetic engineering of stomata requires guard cell preferred or specific regulatory regions to avoid undesirable side effects. In the present study, we describe the 5'-upstream regulatory region of the WRKY18 transcription factor of banana and functionally analyzed its stress meditated activation and strong guard cell preferred activity. Expression of MusaWRKY18 is augmented in leaves of banana cultivars Karibale Monthan, Rasthali and Grand Nain under multiple stress conditions suggesting its role in stress responses of banana plants. Transgenic tobacco lines harboring P_MusaWRKY18 -β-D-glucuronidase (GUS) were regenerated and GUS staining demonstrated substantial GUS expression in guard cells which corroborates with multiple Dof1 binding cis-elements in P_MusaWRKY18 . Fluorescent β-galactosidase assay demonstrated the stress-mediated strong induction profiles of P_MusaWRKY18 at different time points in transgenic tobacco lines exposed to drought, high-salinity, cold, and applications of abscisic acid, salicylic acid, methyl jasmonate, and ethephon. This study sheds novel insights into guard cell preferred expression of WRKY genes under stress and confirm the utility of P_MusaWRKY18 for exploring guard cell functions and guard cell engineering.

Integrated Transcriptomic and Proteomic Analysis in the Roadmap of the Xylem Development Stage in Populus tomentosa

Xylem development plays an important role in the wood formation of plants. In this study, we found that xylem development was a rapid thickening process characterized by initially rapid increases in the number of tracheary elements and fiber cells and the thickness of the secondary walls that later plateaued. Transcriptome analysis showed that the xylan and lignin biosynthetic pathways, which are involved in the early rapid thickening of the xylem, were mainly upregulated in the second month. The expression of a total of 124 transcription factors (TFs), including 28 NAC TFs and 31 MYB TFs, peaked in 2- and 3-month-old plants compared with 1-month-old plants. Based on previous studies and the key cis-acting elements secondary wall NAC-binding elements, secondary wall MYB-responsive elements, W-box and TGTG[T/G/C], 10 TFs related to xylem development, 50 TFs with unknown function, 98 cell wall biosynthetic genes, and 47 programmed cell death (PCD) genes were used to construct a four-layer transcriptional regulatory network (TRN) with poplar NAC domain TFs to characterize the transcriptional regulation of cell wall biosynthesis and PCD in Populus tomentosa. The proteome revealed that post-transcriptional modification may be widely involved in lignification development. Overall, our results revealed that xylem development is a rapid thickening process in P. tomentosa, and expression patterns varied temporally from cell division to cell death.

Studies on the tissue specific nature and stress inducible activation of the CHI-1 gene from banana

In the present study, the 5'-regulatory region of chalcone isomerase gene (MusaCHI-1) of banana was functionally analysed for its tissue specific, stress mediated and strong guard cell preferred activity. Expression of MusaCHI-1 was altered in leaves of banana plants exposed to various stress conditions and signalling molecules. Transgenic lines of tobacco harbouring P_MusaCHI-1-GUS displays prominent GUS staining in vascular region and guard cells of leaves which corroborates with array of Dof1 binding cis-elements in P_MusaCHI-1 region. Multiple cis-elements associated with various stress conditions were detected in P_MusaCHI-1 which directly correlates with alteration of MusaCHI-1 transcript level in banana exposed to stress conditions. GUS staining of transgenic tobacco plants harbouring P_MusaCHI-1-GUS and exposed to drought, salinity, and applications of methyl jasmonate and abscisic acid indicated activation of P_MusaCHI-1 under these conditions while exposure of salicylic acid strongly suppresses GUS expression from P_MusaCHI-1.

Overexpression of MusaSNAC1 improves shoot proliferation in transgenic banana lines

Augmenting shoot multiplication through genetic engineering is an emerging biotechnological application desirable in optimizing regeneration of genetically modified plants on selection medium and rapid clonal propagation of elite cultivars. Here, we report the improved shoot multiplication in transgenic banana lines with overexpression of MusaSNAC1, a drought-associated NAC transcription factor in banana. Overexpression of MusaSNAC1 induces hypersensitivity of transgenic banana lines toward 6-benzylaminopurine ensuing higher shoot number on different concentrations of 6-benzylaminopurine. Altered transcript levels of multiple genes involved in auxin signaling (Aux/IAA and ARFs) and cytokinin signaling pathways (ARRs) in banana plants overexpressing MusaSNAC1 corroborate the hypersensitivity of transgenic banana plants toward 6-benzylaminopurine. Modulation in expression of ARRs reported to be involved in ABA-hypersensitivity and closure of stomatal aperture correlates with the function of MusaSNAC1 as a drought-responsive NAC transcription factor. Present study suggests a prospective cross talk between shoot multiplication and drought responses coordinated by MusaSNAC1 in banana plants.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-02744-5.