Arabidopsis IAR4 Modulates Auxin Response by Regulating Auxin Homeostasis

Abstract

In a screen for enhancers of tir1-1 auxin resistance, we identified two novel alleles of the putative mitochondrial pyruvate dehydrogenase E1a-subunit, IAA-Alanine Resistant4 (IAR4). In addition to enhancing the auxin response defects of tir1-1, iar4 single mutants exhibit numerous auxin-related phenotypes including auxin-resistant root growth and reduced lateral root development, as well as defects in primary root growth, root hair initiation, and root hair elongation. Remarkably, all of these iar4 mutant phenotypes were rescued when endogenous indole-3-acetic acid (IAA) levels were increased by growth at high temperature or overexpression of the YUCCA1 IAA biosynthetic enzyme, suggesting that iar4 mutations may alter IAA homeostasis rather than auxin response. Consistent with this possibility, iar4 mutants exhibit increased Aux/IAA stability compared to wild type under basal conditions, but not in response to an auxin treatment. Measurements of free IAA levels detected no significant difference between iar4-3 and wild-type controls. However, we consistently observed significantly higher levels of IAA-amino acid conjugates in the iar4-3 mutant. Furthermore, using stable isotope-labeled IAA precursors, we observed a significant increase in the relative utilization of the Trp-independent IAA biosynthetic pathway in iar4-3. We therefore suggest that the auxin phenotypes of iar4 mutants are the result of altered IAA homeostasis.Auxin regulates numerous aspects of plant development and physiology, including embryogenesis, vascular differentiation, organogenesis, tropic growth, and root and shoot architecture. In a simplified view, auxin biology can be broken down into three general areas: indole-3-acetic acid (IAA) biosynthesis and metabolism, the cell-to-cell transport of IAA, and the SCF TIR1

Quantitative Phosphoproteome Profiling of Iron-Deficient Arabidopsis Roots

“…In line with previous reports, both the number and length of root hairs are reduced in iar4 plants when compared with the wild type (Fig. 5, A-E; Quint et al, 2009). In addition, the length of the primary root is severely decreased in the mutant.…”

“…These results also indicate that the decrease in auxin response upon Fe deficiency is not part of a nonspecific stress response but rather associated with the growth type. Similar to what has been reported previously (Quint et al, 2009), GUS staining of iar4-3[DR5-GUS] plants was markedly lower when compared with the wild type, supporting a critical function of IAR4 in auxin homeostasis. The DR5 expression pattern in the mutant among the different growth types was generally similar to the wild type, but lower staining intensity was observed under all tested conditions (Fig.…”

Auxin Control in the Formation of Adventitious Roots

“…Great progress has been made in recent years in understanding the auxin response genes and auxin signaling (Parry and Estelle, 2006;Quint et al, 2009). The response to auxin includes a rapid initial cell growth response that may involve auxin-induced changes in pH, calcium and gene expression.…”

Natural Variation of Transcriptional Auxin Response Networks inArabidopsis thaliana

“…Agrobacterium tumefaciens-mediated (GV3101) transformation of Arabidopsis accessions was performed by floral dip (Clough and Bent, 1998), and transgenic seedlings were selected as described previously (Quint et al, 2009). For the histochemical glucuronidase assays, seedlings of eight (Fei-0), six (Sha), and seven (Col-0) independent and homozygous T3 lines were mock treated (0.1% ethanol in liquid ATS) or treated with 1 mM IAA in liquid ATS medium for 3 h and stained overnight at 378C, as described previously (Stomp, 1991).…”