TAA1-Regulated Local Auxin Biosynthesis in the Root-Apex Transition Zone Mediates the Aluminum-Induced Inhibition of Root Growth inArabidopsis     

Abstract

The transition zone (TZ) of the root apex is the perception site of Al toxicity. Here, we show that exposure of Arabidopsis thaliana roots to Al induces a localized enhancement of auxin signaling in the root-apex TZ that is dependent on TAA1, which encodes a Trp aminotransferase and regulates auxin biosynthesis. TAA1 is specifically upregulated in the root-apex TZ in response to Al treatment, thus mediating local auxin biosynthesis and inhibition of root growth. The TAA1-regulated local auxin biosynthesis in the root-apex TZ in response to Al stress is dependent on ethylene, as revealed by manipulating ethylene homeostasis via the precursor of ethylene biosynthesis 1-aminocyclopropane-1-carboxylic acid, the inhibitor of ethylene biosynthesis aminoethoxyvinylglycine, or mutant analysis. In response to Al stress, ethylene signaling locally upregulates TAA1 expression and thus auxin responses in the TZ and results in auxin-regulated root growth inhibition through a number of auxin response factors (ARFs). In particular, ARF10 and ARF16 are important in the regulation of cell wall modification-related genes. Our study suggests a mechanism underlying how environmental cues affect root growth plasticity through influencing local auxin biosynthesis and signaling.

NIP1;2 is a plasma membrane-localized transporter mediating aluminum uptake, translocation, and tolerance inArabidopsis

“…Under control conditions (−Al), GUS staining was observed between ∼300 μm and ∼1 mm from the root tip (Fig. 2D), a region encompassing the root distal transition zone (RDTZ) and the root elongation zone (REZ), a demonstrated target for Al toxicity (22). GUS staining was also observed in the stele in more mature root regions where root hairs had emerged (Fig.…”

“…The cell wall in the root rip region has been recognized as a major target for Al toxicity in plants (22,31,32). It has been suggested that the swollen and distorted root tip cells under Al stress are due, at least in part, to disruption of the cell wall structure, integrity, and function (SI Appendix, Fig.…”

Jasmonic Acid Enhances Al-Induced Root Growth Inhibition

“…ethylene, auxin, and cytokinin have been reported to regulate the Al-induced inhibition of root growth (Massot et al, 2002;Sun et al, 2010;Yang et al, 2014). Ethylene can affect either auxin biosynthesis or auxin transport (Sun et al, 2010) in the root apex in response to Al stress.…”

“…Seedlings were grown hydroponically as described by Yang et al (2014) at pH 5.0 in a growth chamber with a 16-h-light/8-h-dark cycle at 22°C.…”

“…Phytohormones such as auxin, ethylene, cytokinin, salycilic acid, and abscisic acid, etc. have been reported to regulate the Al-induced root growth inhibition (Kollmeier et al, 2000;Shen et al, 2004;Sun et al, 2010;Yang et al, 2014;Guo et al, 2014). In maize, the blockage of the auxin signal from the DTZ to the elongation zone has been revealed as the major reason for Al-induced inhibition of root elongation (Kollmeier et al, 2000), while in Arabidopsis (Arabidopsis thaliana), Trp aminotransferase 1 (TAA1)-mediated local auxin biosynthesis in the root apex transition zone (TZ) leads to the Al-induced root growth inhibition, and this process is controlled by ethylene signaling .…”