Flavonols & ROS

 Antagonism between flavonols and ROS plays important roles in many plant processes

Flavonols are specialized plant metabolites, upstream of anthocyanin synthesis, which scavenge reactive oxygen species (ROS) in vitro, and are thought to maintain this activity in planta. ROS can be byproducts of chemical reactions or different stressors, but can also be produced as components of secondary signaling pathways via the activation of respiratory burst oxidase proteins. As both important signaling molecules and potentially dangerous byproducts, ROS must be maintained in a precarious balance in cells. Our lab has worked to understand how flavonols help balance ROS levels in multiple contexts using Arabidopsis and tomato mutants with defects in flavonol synthesis:

 

localization-auxin-transport-proteins-smRoot development

We have found that mutations that block flavonol synthesis in both Arabidopsis and tomato lead to elevated ROS and altered lateral root and root hair formation,1, 2, 3 as well as impaired root gravitropism.2, 4 Current studies in tomato are identifying the genes that are mutated leading to altered accumulation of flavonols and anthocyanins and using these mutants to identify which flavonols control root architecture.

Current work in Arabidopsis focuses on identifying the molecular mechanisms by which flavonols modulate root architecture. ROS can act as second messengers by oxidizing cysteine residues on proteins, leading to altered structure and/or function. Our lab is using proteomic approaches to identify cysteine modifications which are flavonol-regulated and important for altered root development.

Flavonols and ROS also play a role in our research on root gravitropism.

 

Pollen and heat stress
High magnification images of pollen. Mutant are pollen appears shriveled in comparison to wild type pollen.
Scanning electron micrograph of pollen from wild type tomato (VF36) and a mutant with reduced flavonol levels (are)

Regulation of ROS is important in both development and germination of pollen. During heat stress, elevated ROS can be detrimental to these processes. Our current work examines how flavonols enhance tomato pollen development and germination by reducing ROS, using mutants with defects in genes encoding flavonol biosynthesis. We are currently examining the mechanisms by which flavonols protect tomato pollen from elevated ROS produced during heat stress.

Guard cells and drought stress
Fluorescent staining shows higher levels of flavonols in wild type tomato guard cells, as compared to the are mutant.

ROS play an important role in the signaling pathway for stomatal closure in response to changing conditions such as drought stress. We have demonstrated that in Arabidopsis guard cells flavonols reduce ROS levels to regulate stomatal closure.5  The hormone abscisic acid induces a ROS burst by activation of respiratory burst oxidase proteins and in tomato, this ABA-induced ROS burst is essential for closure and is reduced by flavonol.6  In both species, flavonol levels are increased by ethylene, leading to enhanced closure when the levels of this stress hormone are elevated.5, 6

References:

1. Brown, DE, Rashotte, AM, Murphy, AS, Normanly, J, Tague, BW, Peer, WS, Taiz ,L , and Muday, GK (2001) Flavonoids act as negative regulators of auxin transport in vivo in Arabidopsis. Plant Physiol 126: 524-535

2. Buer, CS, Sukumar, P, and Muday, GK (2006) Ethylene induced flavonoid synthesis modulates root gravitropism. Plant Physiology: 140: 1384-1396

3. Maloney, GS, DiNapoli, KT, Muday, GK (2014) The anthocyanin reduced tomato mutant demonstrates the role of flavonols in tomato lateral root and root hair development. Plant Phys. 166:614-631.

4. Lewis, DR, Ramirez, MV, Valbuena, P, Miller, ND, Keith, R, Helm, R, Winkel, BSJ, Muday, GK (2011) Auxin and ethylene induce distinct flavonol accumulation patterns through independent transcriptional networks: Plant Physiol. 156: 144-164

 5. Watkins, JM, Hechler, PJ, Muday, GK (2014) Ethylene-induced flavonol accumulation in guard cells suppresses reactive oxygen species and modulates stomatal aperture. Plant Physiol: 164: 1707-1717

6. Watkins, JM, Chapman, JM, and Muday, GK (2017) Abscisic acid-induced reactive oxygen species are modulated by flavonols to control stomata aperture. Plant Physiology

 

Additional publications:

**indicates review

**Gayomba, SR, Watkins, JM, and Muday, GK (2017) Flavonols regulate plant growth and development through regulation of auxin transport and cellular redox status. In Recent Advances in Polyphenol Research Vol 5. K. Yoshida, V Cheynier, S Quideau, eds. Wiley Backwell. pp 143-170

Sanz, L, Fernandez-Marcos, M, Medrego, A, Lewis, DR, Muday, GK, Pollmann, S, Duenas, M, Santos-Buelga, Lorenzo, O (2014) Nitric oxide plays a role in stem cell niche homeostasis through its interaction with auxin. Plant Phys. 166:1972-1984

**Fernández-Marcos, M, Sanz, L, Lewis, DR, Muday, GK, Lorenzo, O (2013) Control of Auxin Transport by Reactive Oxygen and Nitrogen Species. In Polar Auxin Transport (Signaling and Communication in Plants), Vol 17, Chen, R, Baluska, F, eds. Springer-Verlag, pp103-117

Grunewald, W, De Smet, I, Lewis, DR, Lofke, C, Jansen, L, Goeminne, G, Vanden Bossche, R, Karimi, M, De Rybel, B, Vanholme, B, Teichmann, T, Boerjan, Van Montagu, MCE, Gheysen, G, Muday, GK, Friml, J, and Beeckman, T. (2012) Transcription factor WRKY23 assists auxin distribution patterns during Arabidopsis root development through local control on flavonol biosynthesis. Proceed Natl Acad Sci USA. 109: 1554-1559

Fernandez-Marcos, M, Sanz, L., Lewis, DL, Muday, GK, Lorenzo, O (2011) Nitric oxide causes root apical meristem defects and growth inhibition while reducing PIN1-dependent acropetal auxin transport. Proceed Natl Acad Sci USA. 108: 18506-18511

Buer, CS, and Muday, GK (2004) The transparent testa4 mutation prevents flavonoid synthesis and alters auxin transport and the response of Arabidopsis roots to gravity and light. Plant Cell, 16: 1191-1205.

Peer, WA, Murphy, AS, Brown, DE, Tague, BW, Muday, GK, Taiz, L (2001) Flavonoid accumulation patterns in transparent testa mutants of Arabidopsis. Plant Physiol. 126: 536-548

 

Studies on Arabidopsis are supported by the National Science Foundation Integrative Organismal Systems: Physiological Mechanisms and Biomechanics and on tomato by the United States Department of Agriculture National Institute of Food and Agriculture Foundational Program in Plant Health and Production of Plant Products.