MASSUGU2 Encodes Aux/IAA19, an Auxin-Regulated Protein That Functions Together with the Transcriptional Activator NPH4/ARF7 to Regulate Differential Growth Responses of Hypocotyl and Formation of Lateral Roots in Arabidopsis thaliana

Abstract

We have isolated a dominant, auxin-insensitive mutant of Arabidopsis thaliana, massugu2 (msg2), that displays neither hypocotyl gravitropism nor phototropism, fails to maintain an apical hook as an etiolated seedling, and is defective in lateral root formation. Yet other aspects of growth and development of msg2 plants are almost normal. These characteristics of msg2 are similar to those of another auxin-insensitive mutant, non-phototropic hypocotyl4 (nph4), which is a loss-of-function mutant of AUXIN RESPONSE FACTOR7 (ARF7) (Harper et al., 2000). Map-based cloning of the MSG2 locus reveals that all four mutant alleles result in amino acid substitutions in the conserved domain II of an Auxin/Indole-3-Acetic Acid protein, IAA19. Interestingly, auxin inducibility of MSG2/IAA19 gene expression is reduced by 65% in nph4/arf7. Moreover, MSG2/IAA19 protein binds to the C-terminal domain of NPH4/ARF7 in a Saccharomyces cerevisiae (yeast) twohybrid assay and to the whole latter protein in vitro by pull-down assay. These results suggest that MSG2/IAA19 and NPH4/ ARF7 may constitute a negative feedback loop to regulate differential growth responses of hypocotyls and lateral root formation.

The Ectomycorrhizal FungusLaccaria bicolorStimulates Lateral Root Formation in Poplar and Arabidopsis through Auxin Transport and Signaling

“…Quantitative real-time PCR analysis of these differentially expressed targets at various time points during the poplar-L. bicolor interaction identified genes, such as PtaIAA19.3 and PtaIAA28.1, whose transcript levels were modified early but only transiently. When defective or absent, homologs of the Arabidopsis auxin transcription regulators PtaIAA19.3 and PtaIAA28.1 are known to cause a strong LR phenotype (Tian and Reed, 1999;Rogg et al, 2001;Tatematsu et al, 2004;Muto et al, 2007). Strikingly, when NPA was applied during the poplar-L. bicolor interaction, PtaIAA19.3 transcript accumulation was repressed in the presence of the fungus and no LR stimulation was observed.…”

An anionic class III peroxidase from zucchini may regulate hypocotyl elongation through its auxin oxidase activity

“…In A. thaliana several genes involved in auxin homeostasis are known to contain auxin response elements in their promoters (Barlier et al 2000). Furthermore recent studies have revealed that speciWc auxin-mediated responses are controlled via a negative (auto)feedback loop (Tatematsu et al 2004). As APRX gene is auxin-inducible but that the protein is also an auxin oxidase, APRX may participate in the local negative feedback regulation of auxin levels.…”

The Rice COLEOPTILE PHOTOTROPISM1 Gene Encoding an Ortholog of Arabidopsis NPH3 Is Required for Phototropism of Coleoptiles and Lateral Translocation of Auxin

“…The only exception was the report of Iino (1991), who found an asymmetry in maize coleoptiles. On the other hand, more recent studies with Arabidopsis have provided genetic evidence that auxin is involved in the process of phototropism (Harper et al, 2000;Blakeslee et al, 2004;Tatematsu et al, 2004). For resolving the signaling pathway for phototropism, it is required that the Cholodny-Went hypothesis is firmly established.…”