Root-derived trans-zeatin cytokinin protects Arabidopsis plants against photoperiod stress

Abstract

Running title: Protective role of root-borne cytokinin 1 2 Root-derived trans-zeatin cytokinin protects Arabidopsis plants against photoperiod 3 stress 4 5 ABSTRACT 27 Recently, a novel type of abiotic stress caused by a prolongation of the light period -coined 28 photoperiod stress -has been described in Arabidopsis. During the night after the 29 prolongation of the light period, stress and cell death marker genes are induced. The next day, 30 strongly stressed plants display a reduced photosynthetic efficiency and leaf cells eventually 31 enter programmed cell death. The phytohormone cytokinin (CK) acts as a negative regulator of 32 this photoperiod stress syndrome. In this study, we show that Arabidopsis wild-type plants 33 increase the CK concentration in response to photoperiod stress. Analysis of cytokinin 34 synthesis and transport mutants revealed that root-derived trans-zeatin (tZ)-type CKs protect 35 against photoperiod stress. The CK signaling proteins ARABIDOPSIS HISTIDINE 36 PHOSPHOTRANSFER PROTEIN 2 (AHP2), AHP3 and AHP5 and transcription factors 37 ARABIDOPSIS RESPONSE REGULATOR 2 (ARR2), ARR10 and ARR12 are required for the 38 protective activity of CK. Analysis of higher order B-type arr mutants suggested that a complex 39 regulatory circuit exists in which the loss of ARR10 or ARR12 can rescue the arr2 phenotype. 40 Together the results revealed the role of root-derived CK acting in the shoot through the two-41 component signaling system to protect from the negative consequences of strong photoperiod 42 stress. 43 44 45