Target of Rapamycin Is a Key Player for Auxin Signaling Transduction in Arabidopsis.

Kexuan Deng,Jiankui Zhang,Xianzhe Zheng,Wanjing Wang,Maozhi Ren,Lihua Yu,Pan Dong,Kang Zhang

doi:10.3389/fpls.2016.00291

Kexuan Deng, Jiankui Zhang + Show 6 more

Open Access

https://doi.org/10.3389/fpls.2016.00291

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Abstract

Target of rapamycin (TOR), a master sensor for growth factors and nutrition availability in eukaryotic species, is a specific target protein of rapamycin. Rapamycin inhibits TOR kinase activity viaFK506 binding protein 12 kDa (FKBP12) in all examined heterotrophic eukaryotic organisms. In Arabidopsis, several independent studies have shown that AtFKBP12 is non-functional under aerobic condition, but one study suggests that AtFKBP12 is functional during anaerobic growth. However, the functions of AtFKBP12 have never been examined in parallel under aerobic and anaerobic growth conditions so far. To this end, we cloned the FKBP12 gene of humans, yeast, and Arabidopsis, respectively. Transgenic plants were generated, and pharmacological examinations were performed in parallel with Arabidopsis under aerobic and anaerobic conditions. ScFKBP12 conferred plants with the strongest sensitivity to rapamycin, followed by HsFKBP12, whereas AtFKBP12 failed to generate rapamycin sensitivity under aerobic condition. Upon submergence, yeast and human FKBP12 can significantly block cotyledon greening while Arabidopsis FKBP12 only retards plant growth in the presence of rapamycin, suggesting that hypoxia stress could partially restore the functions of AtFKBP12 to bridge the interaction between rapamycin and TOR. To further determine if communication between TOR and auxin signaling exists in plants, yeast FKBP12 was introduced into DR5::GUS homozygous plants. The transgenic plants DR5/BP12 were then treated with rapamycin or KU63794 (a new inhibitor of TOR). GUS staining showed that the auxin content of root tips decreased compared to the control. DR5/BP12 plants lost sensitivity to auxin after treatment with rapamycin. Auxin-defective phenotypes, including short primary roots, fewer lateral roots, and loss of gravitropism, occurred in DR5/BP12 plants when seedlings were treated with rapamycin+KU63794. This indicated that the combination of rapamycin and KU63794 can significantly inhibit TOR and auxin signaling in DR5/BP12 plants. These studies demonstrate that TOR is essential for auxin signaling transduction in Arabidopsis.

Highlights

Rapamycin, a macrolide antibiotic, is produced by the soil bacterium, Streptomyces hydroscopicus
Submergence can partially inhibit seedling growth of AtFKBP12-OE5 when compared with WT in the presence of 10 μM rapamycin (Supplementary Figure 2). This observation likely suggests that anaerobic stress caused by submergence can partially rescue the functions of AtFKBP12 to interact with Target of rapamycin (TOR) and rapamycin
Several independent studies have shown that plant FK506-binding protein 12 (FKBP12) is non-functional on solid medium (Xu et al, 1998; Menand et al, 2002; Mahfouz et al, 2006; Sormani et al, 2007; Leiber et al, 2010; Ren et al, 2012), but all these groups did not examine the functions of plant FKBP12s under anaerobic growth condition

Summary

Introduction

Rapamycin ( known as sirolimus), a macrolide antibiotic, is produced by the soil bacterium, Streptomyces hydroscopicus. Most plants are insensitive to rapamycin under aerobic growth condition (Xu et al, 1998; Menand et al, 2002; Ren et al, 2012; Montané and Menand, 2013). Billions of years separate the beginnings of yeast and the development of human cells, yet human cell exposure to rapamycin produces the same nutrition starvation phenomenon as found in yeast. This indicates a highly conserved target for rapamycin in eukaryotes. TOR function is highly conserved from yeast to humans, and it controls key biological processes such as ribosome biogenesis, protein synthesis, three carboxylic acid cycles, and stress responses (Fontana et al, 2010; Cornu et al, 2013)

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