1 Plant secondary metabolites as defenses, regulators and primary 2 metabolites- The blurred functional trichotomy 3 Matthias Erba,1,2 and Daniel J. Kliebensteinb,1 4 a Institute of Plant Sciences, University of Bern, 3013 Bern, Switzerland 5 b Department of Plant Sciences, University of California, Davis, CA, USA 6 ORCID-IDs: 0000-0002-4446-9834; 0000-0001-5759-3175 7 1 This work was supported by the University of Bern, the Swiss National Science 8 Foundation Grant Nr. 155781 to ME, the European Research Council under the 9 European Union’s Horizon 2020 Research and Innovation Program Grant ERC-2016- 10 STG 714239 to ME, the USA NSF award IOS 1655810 and MCB 1906486 to DJK, the 11 USDA National Institute of Food and Agriculture the Hatch project number CA-D-PLS- 12 7033-H to DJK and by the Danish National Research Foundation (DNRF99) grant to 13 DJK. 14 2 Author for contact:
[email protected] 15 16 Abstract 17 The plant kingdom produces hundreds of thousands of small molecular weight organic 18 compounds. Based on their assumed functions, the research community has classified 19 them into three overarching groups: primary metabolites which are directly required for 20 plant growth, secondary (or specialized) metabolites which mediate plant-environment 21 interactions and hormones which regulate organismal processes, including 22 metabolism. For decades, this functional trichotomy has shaped theory and 23 experimentation in plant biology. However, evidence is accumulating that the 24 boundaries between the different types of metabolites are blurred. An increasing 25 number of mechanistic studies demonstrate that secondary metabolites are 26 multifunctional and can act as potent regulators of plant growth and defense.