Arabidopsis thaliana is a plant that does not like the shade. If it is overshadowed, a response known as shade avoidance syndrome (SAS) is triggered. This involves adaptations, such as stem elongation, that give the plant a growth advantage over its neighbors. The growth hormone auxin is implicated in the early stages of SAS, but the mechanisms involved are poorly characterized. Tao et al. performed a genetic screen of Arabidopsis mutants and identified plants that did not adapt to shaded conditions. These plants had mutations in TAA1, which encodes the enzyme tryptophan aminotransferase of Arabidopsis. In response to shade treatment, the abundance of auxin was lower in TAA1 mutants than in wild-type plants; auxin target gene expression was also lower in TAA1 mutants. In wild-type plants, TAA1 was highly expressed in the leaves. Reporter assays performed in the presence of an auxin transport inhibitor showed the initial production of auxin in the leaves in response to shade treatment. Further biochemical studies showed that TAA1 catalyzed the conversion of tryptophan to indole-3-pyruvic acid (IPA), an initial step in the pathway to synthesize indole-3-acetic acid (auxin) that had been predicted but not previously biochemically characterized. In another study, Stepanova et al. performed an alternative genetic screen and identified an Arabidopsis mutant with defective root growth in response to the gaseous hormone ethylene. The mutant gene responsible for the defect was TAA1. As well as showing the tryptophan aminotransferase activity of TAA1 in vitro, the authors demonstrated a critical role for TAA1 in tissue-specific ethylene-dependent production of auxin in plants. As Palme and Nagy discuss in commentary, these studies have identified a critical step in how both environmental and hormonal signals stimulate the localized production of auxin, which then triggers appropriate responses.
Y. Tao, J.-L. Ferrer, K. Ljung, F. Pojer, F. Hong, J. A. Long, L. Li, J. E. Moreno, M. E. Bowman, L. J. Ivans, Y. Cheng, J. Lim, Y. Zhao, C. L. Balleré, G. Sandberg, J. P. Noel, J. Chory, Rapid synthesis of auxin via a new tryptophan-dependent pathway is required for shade avoidance in plants. Cell 133, 164-176 (2008). [Online Journal]
A. N. Stepanova, J. Robertson-Hoyt, J. Yun, L. M. Benavente, D.-Y. Xie, K. Doležal, A. Schlereth, G. Jürgens, J. M. Alonso, TAA1-mediated auxin biosynthesis is essential for hormone crosstalk and plant development. Cell 133, 177-191 (2008). [Online Journal]
K. Palme, F. Nagy, A new gene for auxin synthesis. Cell 133, 31-32 (2008). [Online Journal]