NUTRIENT DEPLETION AS A KEY FACTOR FOR MANIPULATING GENE 1 EXPRESSION AND PRODUCT FORMATION IN DIFFERENT BRANCHES OF 2 THE FLAVONOID PATHWAY 3 4 5 Running title : 6 NUTRIENT DEPLETION AND FLAVONOID SYNTHESIS

1 The content of flavonoids increases in response to nitrogen and phosphorus depletion in 2 plants. Manipulation of these macronutrients may therefore be used to control the levels 3 of desirable compounds and improve plant quality. Key enzymes in the shikimate 4 pathway, which feeds precursors into the flavonoid pathway, are regulated post5 translationally by feedback from aromatic amino acids, and possibly by redox control 6 through photosynthesis. Use of microarrays for global transcript analysis in Arabidopsis 7 has revealed that transcript levels are less influenced by mineral nutrients in the shikimate 8 pathway compared with the flavonoid pathway. The responses in the shikimate pathway 9 appear complex, whereas in the flavonoid pathway a single gene often responds similarly 10 to mineral depletion, high light intensity, and sucrose. MYB (PAP1/PAP2) and bHLH 11 (GL3) transcription factors are important for the nutrient depletion response. PAP1/2 12 stimulate gross activation of the flavonoid pathway, and different investigations support 13 merging signal transduction chains for various abiotic treatments on PAP1/2. Flavonol 14 synthase is not part of the PAP1/2 regulon, and expression is mainly enhanced by high 15 light intensity and sucrose, not mineral depletion. Nevertheless, both cyanidin and 16 flavonol derivatives increase in response to nitrogen depletion. Kaempferols are the 17 dominating flavonols in Arabidopsis leaves under normal cultivation conditions, but 18 quercetin accumulation can be triggered by nitrogen depletion in combination with other 19 abiotic factors. 20 21 Key-words: abiotic stress, anthocyanins, bHLH, flavonoids, flavonols, macronutrients, 22 MYB, nitrogen, phosphorus, sucrose. 23