Identification and Functional Analysis of the CgNAC043 Gene Involved in Lignin Synthesis from Citrusgrandis "San Hong".

Xiaoting Li,Tengfei Pan,Dongming Pan,Heli Pan,Zhixiong Guo,Wenqin She,Naiyu Wang,Jianwen Ye,Yuan Yu

doi:10.3390/plants11030403

Xiaoting Li, Tengfei Pan + Show 7 more

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https://doi.org/10.3390/plants11030403

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Abstract

Overaccumulation of lignin (a physiological disorder known as granulation) often occurs during fruit ripening and postharvest storage in pomelo (Citrus grandis). It causes an unpleasant fruit texture and taste. Previous studies have shown that lignin metabolism is closely associated with the process of juice sacs granulation. At present, the underlying transcriptional regulatory mechanisms remain unclear. In this study, we identified and isolated a candidate NAC transcription factor, CgNAC043, that is involved in the regulation of lignin biosynthesis in Citrus grandis, which has homologs in Arabidopsis and other plants. We used the fruit juice sacs of ‘San hong’ as the material, the staining for lignin with HCl−phloroglucinol of fruit juice sacs became dark red from the various developmental stages at 172 to 212 days post anthesis (DPA). The RT-qPCR was used to analyze the gene expression of CgNAC043 and its target gene CgMYB46 in fruit sacs, it was found that the expression trend of CgNAC043 was basically same as CgMYB46, which increased gradually and peaked at 212 DPA. The expression level of CgNAC043 in juice sacs obtained away from the core was the lowest, while those near the core and granulated area were highly expressed. The transcriptional activation activity of CgNAC043 and CgMYB46 was analyzed by a yeast two-hybrid system, with only CgNAC043 showing transcriptional activation activity in Y2H Gold yeast. A transformation vector, p1301- CgNAC043, was transformed into the mesocarp of ‘San hong’ by Agrobacterium-mediated transformation. Results showed that the expression of transcription factors CgMYB58 and CgMYB46 are all upregulated. Further experiments proved that CgNAC043 not only can directly trans-activate the promoter of CgMYB46 but also trans-activate the promoters for the lignin biosynthesis-related genes CgCCoAOMT and CgC3H by dual luciferase assay. We isolated the CgNAC043 gene in pomelo and found CgNAC043 regulates target genes conferring the regulation of juice sacs granulation.

Highlights

Pomelo (Citrus grandis (L.) Osbeck.) is a distinctive citrus fruit characterized by its thick peel, large size, and high vitamin C content [1]
We show that CgNAC043 and CgMYB46 are expressed in secondary wall-forming cells and they are functional orthologs of NST1 and AtMYB46, respectively
We further demonstrate that second cell wall binding element (SNBE) are present in the promoters of CgMYB46 and the genes related to the lignin synthesis pathway, and they could be directly activated by CgNAC043

Summary

Introduction

Pomelo (Citrus grandis (L.) Osbeck.) is a distinctive citrus fruit characterized by its thick peel, large size, and high vitamin C content [1]. “Sanhongmiyou” (San hong) is a new variety bred from the mutants of “Guanximiyou”. It is sour, sweet, delicious, and nutritious. In citrus fruit juice sacs, the overaccumulation of lignin (a physiological disorder known as granulation) often occurs [2,3]. Granulation of citrus fruits occurs in “San hong”, especially during fruit ripening and post-harvest storage, affecting the commercial properties greatly [4]; it causes a “gritty” texture and dryness of the juice sacs, and reduces consumer acceptance of the fruits [2]. Previous evidence has shown that granulation due to excessive lignin accumulation can greatly damage fruit sensory quality during the post-harvest process [6]

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