New cross talk between ROS, ABA and auxin controlling seed maturation and germination unraveled in APX6 deficient Arabidopsis seeds

Abstract

Successful execution of germination program greatly depends on the seeds' oxidative homeostasis. We recently identified new roles for the H2O2-reducing enzyme ascorbate peroxidase 6 (APX6) in germination control and seeds' stress tolerance. APX6 replaces APX1 as the dominant APX in dry seeds, and its loss-of-function results in reduced germination due to over accumulation of ROS and oxidative damage. Metabolic analyses in dry apx6 seeds, revealed altered homeostasis of primary metabolites including accumulation of TCA cycle metabolites, ABA and auxin, supporting a novel role for APX6 in regulating cellular metabolism. Increased sensitivity of apx6 mutants to ABA or IAA in germination assays indicated impaired perception of these signals. Relative suppression of ABI3 and ABI5 expression, and induction of ABI4, suggested the activation of a signaling route inhibiting germination in apx6 seeds that is independent of ABI3. Here we provide additional evidence linking ABI4 with ABA- and auxin-controlled inhibition of germination and suggest a hypothetical model for the role of APX6 in the regulation of the crosstalk between these hormones and ROS.

Keywords: ABA; ABI3; ABI4; ABI5; APX; ROS; Seed; ascorbate peroxidase; auxin; germination; redox.

Figure 1.

Germination response to ABA and IAA. Freshly harvested seeds were geminated on 1/2MS supplemented with ABA, IAA or their combination. Sample pictures of seed germination (A) and histograms (B) of germination were recorded at the indicated hours after imbibition (HAI). Standard deviations represent an average of 6 replicates of 50 seeds each. Different lowercase letters indicate values from the seedling lines significantly differ from each other within each treatment by one-way ANOVA and Bonferroni test (P < 0.05). (C) Germination of ABA-related mutants on 1/2MS agar plates without (top) and with 0.3 μM ABA+50 μM IAA (bottom). Photograph was taken 5 d after imbibition (DAI).

Figure 2.

Hypothetical model for APX6-regulated ROS/redox signaling in seeds. APX6 function in seeds to reduce the levels of ROS and redox imbalances during stress, protecting seeds and maintain their vigor. ROS and redox signals generated in the absence of functional APX6 or during sever stress conditions lead to increased levels of ABA and auxin and promote their inhibitory effect on germination. APX6 effect the balance between the AA pool and DHA pool. Decreased availability of AA result in activation of ABA biosynthesis genes including NECD leading accumulation of ABA. ABI4 expression and pathway are activated in response to the increase in oxidative load and the level of ABA. It is not clear yet if ABI4 responds directly to auxin or indirectly by enhancing the ABA-mediate response.