Overexpression of MusaNAC68 reduces secondary wall thickness of xylem tissue in banana
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Secondary wall is an important component of xylem tissue, as it maintains the integrity of the vessel elements and provides mechanical strength for upward growth of plants. Enough prior evidence has linked secondary wall thickening with auxin-signaling pathway. Hence, plants showing auxin-mediated effects might demonstrate alteration of secondary wall depositions. GUS expression from promoter of NAC68 (pMusaNAC68) was observed in many organs with strong expression in vascular tissues. Transgenic banana plants overexpressing NAC68 transcription factor were analyzed for lignin and secondary wall depositions. Strong reduction of lignin in cross-sections of transgenic lines was observed by phloroglucinol-HCl and toluidine blue-O staining. The reduction of secondary wall thickness observed after calcofluor white staining was substantiated by data from scanning electron microscopy indicating reproducibly lower secondary wall depositions. A strong reduction in transcript levels of PAL, 4CL, C4H, COMT and CcOAMT was observed due to the overexpression of NAC68. Furthermore, substantial elevation of MYB transcription factors such as MYB4a-like and MYB4b-like which are probable repressors of phenylpropanoid biosynthesis pathway was also observed. Expression of MYB transcription factors, MYB85-like and MYB58-like which are probable activators of secondary wall depositions, was remarkably downregulated after NAC68 overexpression. Transgenic lines also showed altered expression of genes coding for cellulose synthase subunits with remarkable elevation in expression of a close homologue of Arabidopsis CesA8, which functions in cellulose deposition during secondary wall development. The study indicated a novel function of NAC68 transcription factor in regulating secondary wall thickening and will enhance our knowledge about crosstalk between auxin-signaling and secondary wall depositions.
KeywordsNAC68 Banana Secondary wall, MYB PAL Lignin
NAM, ATAF and CUC
Phenylalanine ammonia lyase
Caffeic acid 3-O-methyltransferase
Vascular-related NAC transcription factors
Secondary wall associated NAC domain
NAC secondary wall thickening promoting factor1
The authors thank Head, Nuclear Agriculture and Biotechnology Division, BARC (HBNI PhD fellowship) for constant encouragement.
- Johnsson C, Jin X, Xue W, Dubreuil C, Lezhneva L, Fischer U (2018) The plant hormone auxin directs timing of xylem development by inhibition of secondary cell wall deposition through repression of secondary wall NAC-domain transcription factors. Physiol Plant. https://doi.org/10.1111/ppl.12766 Google Scholar