ABSCISIC ACID INSENSITIVE5 Interacts With RIBOSOMAL S6 KINASE2 to Mediate ABA Responses During Seedling Growth in Arabidopsis.

Linxuan Li,Li Feng,Maozhi Ren,Essam Ali Hassan Farag,Tingting Zhu,Yun Song

doi:10.3389/fpls.2020.598654

Abstract

ABSCISIC ACID INSENSITIVE5 (ABI5) is an important regulator of abscisic acid (ABA) signaling pathway involved in regulating seed germination and postgerminative growth in Arabidopsis, which integrates various phytohormone pathways to balance plant growth and stress responses. However, the transcriptional regulatory mechanisms underlying ABI5 and its interacting proteins remain largely unknown. Here, we found that inhibition of AtTOR could increase ABA content by up-regulating the expression levels of ABA biosynthesis-related genes, and thus activated the expression of ABA-responsive genes. Pharmacological assay showed that abi5-1 mutant was insensitive to TOR inhibitor AZD8055, whereas AtABI5 overexpression lines were hypersensitive to AZD8055 in Arabidopsis. Biochemical interaction assays demonstrated that ABI5 physically interacted with the RIBOSOMAL S6 KINASE2 (S6K2) protein in plant cell. S6K2 positively regulated ABA responses during seedling growth and upregulated ABA-responsive genes expression. Furthermore, genetic and physiological analysis indicated that AtS6K2 overexpression lines enhanced resistance to drought treatment while AtS6K2 interference lines were sensitive to drought. These results indicated that AtABI5 interacted with AtS6K2 to positively modulate ABA responses during seedling growth and shed light on a underlying mechanism of the crosstalk between TOR and ABA signaling pathways in modulating seedling growth in Arabidopsis.

Highlights

Plants have evolved a set of precise regulatory mechanisms to adapt to environmental changes and resist to biotic or abiotic stresses
These results implied that target of rapamycin (TOR) and Abscisic acid (ABA) signals may be involved in the regulation of plant growth and stress responses together in Arabidopsis
Previous study revealed that ABA insensitive 5 (ABI5) was phosphorylated at Thr-35, Ser-36, Ser-41, Ser-42, Ser-138, Ser-139, Ser-145, and Thr-201 residues in Arabidopsis (Hu and Yu, 2014; Yu et al, 2015), of which Ser-42, Ser-145, and Thr-201 sites of ABI5 were phosphorylated by SNF1-related protein kinase 2 (SnRK2) protein kinases (Fujii and Zhu, 2009)

Summary

Introduction

Plants have evolved a set of precise regulatory mechanisms to adapt to environmental changes and resist to biotic or abiotic stresses. ABA signaling pathway plays a critical role in regulating various developmental processes and resistance to abiotic stress in plants (Cutler et al, 2010). PYR/PYL/RCAR family members are found as ABA receptors and inhibit the activity of type 2C protein phosphatases (PP2Cs) to control ABA signaling (Nishimura et al, 2010; Pizzio et al, 2013). ABA insensitive 1 (ABI1) and ABI2, which are prototypical members of PP2Cs, are early negative regulators in ABA signal transduction and inhibit the activity of SNF1-related protein kinase 2 (SnRK2) (Bertauche et al, 1996). SnRK2s protein kinases are required for ABA response and capable of phosphorylating downstream transcription factors such as ABA insensitive 5 (ABI5) and ABRE-binding factors (ABFs) to regulate the expression of ABAresponsive genes (Fujii et al, 2007)

Methods

Results

Conclusion