The Photoperiod Stress Response in Arabidopsis thaliana Depends on Auxin Acting as an Antagonist to the Protectant Cytokinin

Abstract

Fluctuating environmental conditions trigger adaptive responses in plants, which are regulated by phytohormones. During photoperiod stress caused by a prolongation of the light period, cytokinin (CK) has a protective function. Auxin often acts as an antagonist of CK in developmental processes and stress responses. Here, we investigated the regulation of the photoperiod stress response in Arabidopsis thaliana by auxin and its interaction with CK. Transcriptome analysis revealed an altered transcript abundance of numerous auxin metabolism and signaling genes after photoperiod stress treatment. The changes appeared earlier and were stronger in the photoperiod-stress-sensitive CK receptor mutant arabidopsis histidine kinase 2 (ahk2),3 compared to wild-type plants. The concentrations of indole-3-acetic acid (IAA), IAA-Glc and IAA-Asp increased in both genotypes, but the increases were more pronounced in ahk2,3. Genetic analysis revealed that the gain-of-function YUCCA 1 (YUC1) mutant, yuc1D, displayed an increased photoperiod stress sensitivity. In contrast, a loss of the auxin receptors TRANSPORT-INHIBITOR-RESISTANT 1 (TIR1), AUXIN SIGNALING F-BOX 2 (AFB2) and AFB3 in wild-type and ahk2,3 background caused a reduced photoperiod stress response. Overall, this study revealed that auxin promotes response to photoperiod stress antagonizing the protective CK.

Keywords: Arabidopsis thaliana; abiotic stress; auxin; crosstalk; cytokinin; photoperiod stress.

Figure 1

Relative expression of auxin biosynthesis and metabolism genes in response to photoperiod stress in wild type and ahk2,3. (a) Schematic overview of photoperiod stress treatment and sampling time points for gene expression analysis (0, 4, 6 and 12 h after PLP treatment; gray arrows). (b) Relative expression levels of auxin biosynthesis and metabolism genes TRYPTOPHAN AMINOTRANSFERASE OF ARABIDOPSIS (TAA1), TAA-RELATED (TAR), YUCCA (YUC), DIOXYGENASE FOR AUXIN OXIDATION (DAO), IAA-ALANINE-RESISTANT 3 (IAR3), IAA-LEUCINE-RESISTANT 1 (ILR1), IAA-LEUCINE-RESISTANT-LIKE (ILL) and GRETCHEN HAGEN 3 (GH3) 0, 4, 6 and 12 h after the PLP treatment compared to respective control plants. Stars indicate a significant difference between the indicated genotypes/conditions (p ≤ 0.05; n = 3; log₂-fold values are depicted in Supplementary Table S1). Data were extracted from [33].

Figure 2

Relative expression of auxin signaling genes in response to photoperiod stress in wild type and ahk2,3. Depicted are the relative expression of TRANSPORT-INHIBITOR-RESISTANT 1 (TIR1)/AUXIN SIGNALING F-BOX (AFB), Aux/IAA and AUXIN RESPONSE FACTORs (ARFs) (class A ARFs, transcriptional activators, first group; class B and C, transcriptional repressors, second group) 0, 4, 6 and 12 h after the PLP compared to respective control plants. Stars indicate a significant difference between the indicated genotypes/conditions (p ≤ 0.05; n = 3; log₂-fold values are depicted in Supplementary Table S2). Data were extracted from [33].

Figure 3

Photoperiod stress causes an increased concentration of IAA, IAA-Glc and IAA-Asp in wild-type and ahk2,3 plants. (a) Schematic overview of sampling time points (arrowheads) for IAA measurements. Five-week-old wild-type plants were either cultivated under SD conditions (control) or were exposed to a prolonged light period of 32 h (PLP). (B–G) Concentration of free IAA (b), oxIAA (c), IAA-Glc (d), oxIAA-Glc (e), IAA-Asp (f) and IAA-Glu (g) in control samples and PLP samples at the time points depicted in (a). Letters indicate significantly different statistical groups (p ≤ 0.05; two-way ANOVA; n ≥ 3). Error bars indicate SD. IAA, indole-3-acetic acid; oxIAA, 2-oxoindole-3-acetic acid; IAA-Glc, IAA-glucose; IAA-Asp, IAA-aspartic acid; IAA-Glu, IAA-glutamic acid. Exact values are depicted in Supplementary Table S3.

Figure 4

The auxin status influences the response to photoperiod stress. (a) Lesion formation of leaves in five-week-old Col-0, ahk2,3, tir1afb2,3 and yuc1D plants the day after the photoperiod stress treatment. (b) Efficiency of photosystem II (F_v/F_m) of leaves the day after the photoperiod stress treatment (one-way ANOVA; p ≤ 0.05; n = 15). Transcript abundance of stress marker genes BAP1 (c), ZAT12 (d) and CAB2 (e) 0 and 15 h after the PLP treatment compared to respective control plants. The expression level of wild type at the end of the PLP treatment (0 h) was set to 1 (one-way ANOVA; p ≤ 0.05; n ≥ 3). (f) Peroxide content expressed as nmol H₂O₂ equivalents g⁻¹ FW in leaves 15 h after PLP treatment (one-way ANOVA; p ≤ 0.05; n = 4). Letters indicate significantly different statistical groups. Error bars indicate SE. Pictures of representative plants exposed to a 24 h prolongation of the light period are shown in Supplementary Figure S4a.

Figure 5

An impairment of auxin perception decreases the sensitivity to photoperiod stress. (a) Lesion formation of leaves in five-week-old Col-0, ahk2,3, tir1afb2,3 and ahk2,3 tir1afb2,3 plants the day after the photoperiod stress treatment (one-way ANOVA; p ≤ 0.05; n ≥ 12). (b) Efficiency of photosystem II (F_v/F_m) of leaves the day after photoperiod stress treatment (paired Wilcoxon test, FDR corrected via Benjamini–Hochberg method; p ≤ 0.05; n ≥ 13). Transcript abundance of BAP1 (c), ZAT12 (d) and CAB2 (e), 0 and 15 h after the PLP treatment compared to respective control plants. The expression level of wild type at the end of the PLP treatment (0 h) was set to 1 (one-way ANOVA; p ≤ 0.05; n ≥ 3). (f) Peroxide content expressed as nmol H₂O₂ equivalents g⁻¹ FW in Col-0, ahk2,3, tir1afb2,3 and ahk2,3 tir1afb2 leaves 15 h after PLP treatment (one-way ANOVA; p ≤ 0.05; n = 4). Letters indicate significantly different statistical groups. Pictures of representative plants exposed to a 24 h prolongation of the light period are shown in Supplementary Figure S4b.

Figure 6

Model of the function and interaction of CK and auxin in the response to photoperiod stress. In leaves showing a weak photoperiod stress response (left side), CK accumulates (bold letters) and inhibits auxin (IAA) and ROS accumulation (thick black arrows). In leaves showing a strong photoperiod stress response (e.g., CK signaling or biosynthesis mutants; right side), CK does not sufficiently repress IAA synthesis or signaling resulting in its accumulation and enhanced signaling (bold letters). IAA ultimately induces ROS formation (bold letters and thick arrow).