LbNR-Derived Nitric Oxide Delays Lycium Fruit Coloration by Transcriptionally Modifying Flavonoid Biosynthetic Pathway.

Gen Li,Beibei Qin,Jianhua Zhao,Yue Yin,Shuodan Li,Youlong Cao,Zixin Mu,Wei An

doi:10.3389/fpls.2020.01215

Abstract

Anthocyanin-derived fleshy fruit pigmentation has become an excellent system for studying the regulatory network underlying fruit ripening and quality. The transcriptional control of anthocyanin biosynthesis by MYB–bHLH–WDR complexes has been well established, but the intermediate signals through which the environmental or developmental cues regulate these transcription factors remain poorly understood. Here we found that nitric oxide (NO) production during Lycium fruit ripening decreased progressively presenting a negative relationship with anthocyanins. After cloning of the nitric reductase (NR) gene from Lycium barbarum (LbNR) plants, we demonstrated that LbNR-derived NO partially inhibited anthocyanin biosynthesis but enhanced proanthocyanidin (PA) accumulation, and delayed fruit coloration. Application of the NO donor, sodium nitroprusside (SNP), produced a similar effect. The endogenous or exogenous NO downregulated the transcripts both of the regulatory genes and the structural genes that related to anthocyanin biosynthesis, while upregulated both of those genes that related to PA biosynthesis. Given there is a significant negative relationship between the levels of anthocyanins and PAs during Lycium fruit ripening, NO not only inhibited anthocyanin de novo biosynthesis but redirected the flavonoid biosynthetic pathway from anthocyanins to PA production. Two types of LrMYB transcription factors of opposite nature, namely anthocyanin-specific and PA-specific, which belong to the R2R3-MYB subfamily and 1R-MYB subfamily, respectively, were identified from L. ruthenicum fruits. It was further found that NO acts by antagonizing the ABA signaling, a phytohormone we have previously shown playing a positive role in Lycium fruit coloration. Our results provided particularly novel information about NO–ABA–anthocyanin interplay during Lycium fruit development and ripening, which may fill a gap between the developmental cues and the transcriptional regulation of anthocyanin biosynthesis.

Highlights

Nitric oxide (NO) was first identified as a unique, diffusible molecular messenger in animals, after which it emerged that NO in plants affects similar signal transduction pathways
We recently demonstrated that phytohormone ABA enhanced Lycium fruit coloration/ripening by promoting anthocyanin biosynthesis (Li G. et al, 2019), at both genetic and pharmacological levels
Our results indicated that NO and ABA antagonized each other to regulate the coloration of Lycium fruits

Summary

INTRODUCTION

Nitric oxide (NO) was first identified as a unique, diffusible molecular messenger in animals, after which it emerged that NO in plants affects similar signal transduction pathways. It is reported in animal cells that plant-sourced flavonoid compounds make their function through the NO–ROS or NO–cGMP pathway (Gao et al, 2014; Qian et al, 2017; Zhang et al, 2018) Given that both anthocyanins and NO are involved in the regulation of fruit ripening, quality, and shelf life (Manjunatha et al, 2010; Zhang et al, 2013), whether there is a direct link between the NO signaling and anthocyanin metabolism remains unclear (Wang et al, 2018). As with other bulk vegetables and fruits, the planting of Lycium indoors will be a trend in the future These facts make the regulation of wolfberry ripening time and improvements in fresh fruit quality and shelf life a priority. Our results uncover a novel mechanism underlying developmental cues-mediated pigmentation regulation and suggest the possibility of engineering endogenous NO to control Lycium fruit ripening

MATERIALS AND METHODS

RESULT

DISCUSSION

Findings

DATA AVAILABILITY STATEMENT