Opposite Root Growth Phenotypes of hy5 versus hy5 hyh Mutants Correlate with Increased Constitutive Auxin Signaling

The Arabidopsis transcription factor HY5 controls light-induced gene expression downstream of photoreceptors and plays an important role in the switch of seedling shoots from dark-adapted to light-adapted development. In addition, HY5 has been implicated in plant hormone signaling, accounting for the accelerated root system growth phenotype of hy5 mutants. Mutants in the close HY5 homolog HYH resemble wild-type, despite the largely similar expression patterns and levels of HY5 and HYH, and the functional equivalence of the respective proteins. Moreover, the relative contribution of HYH to the overall activity of the gene pair is increased by an alternative HYH transcript, which encodes a stabilized protein. Consistent with the enhanced root system growth observed in hy5 loss-of-function mutants, constitutively overexpressed alternative HYH inhibits root system growth. Paradoxically, however, in double mutants carrying hy5 and hyh null alleles, the hy5 root growth phenotype is suppressed rather than enhanced. Even more surprisingly, compared to wild-type, root system growth is diminished in hy5 hyh double mutants. In addition, the double mutants display novel shoot phenotypes that are absent from either single mutant. These include cotyledon fusions and defective vasculature, which are typical for mutants in genes involved in the transcriptional response to the plant hormone auxin. Indeed, many auxin-responsive and auxin signaling genes are misexpressed in hy5 mutants, and at a higher number and magnitude in hy5 hyh mutants. Therefore, auxin-induced transcription is constitutively activated at different levels in the two mutant backgrounds. Our data support the hypothesis that the opposite root system phenotypes of hy5 single and hy5 hyh double mutants represent the morphological response to a quantitative gradient in the same molecular process, that is gradually increased constitutive auxin signaling. The data also suggest that HY5 and HYH are important negative regulators of auxin signaling amplitude in embryogenesis and seedling development

Involvement of auxin pathways in modulating root architecture during beneficial plantmicroorganism interactions

A wide variety of microorganisms known to produce auxin and auxin precursors form beneficial relationships with plants and alter host root development. Moreover, other signals produced by microorganisms affect auxin pathways in host plants. However, the precise role of auxin and auxin-signalling pathways in modulating plantmicrobe interactions is unknown. Dissecting out the auxin synthesis, transport and signalling pathways resulting in the characteristic molecular, physiological and developmental response in plants will further illuminate upon how these intriguing inter-species interactions of environmental, ecological and economic significance occur. The present review seeks to survey and summarize the scattered evidence in support of known host root modifications brought about by beneficial microorganisms and implicate the role of auxin synthesis, transport and signal transduction in modulating beneficial effects in plants. Finally, through a synthesis of the current body of work, we present outstanding challenges and potential future research directions on studies related to auxin signalling in plantmicrobe interactions

Novel Shoot Phenotypes in <i>hy5 hyh</i> Double Mutants

<div><p>Phenotypic analyses of wt, <i>hy5</i>, <i>hyh,</i> and <i>hy5 hyh</i> seedlings.</p><p>(A) Darkfield microscopy images of cotyledons cleared for visualization of the vasculature. The values for each genotype correspond to the average expanded cotyledon size 7 dag in cm². The respective standard errors of the mean are 0.012, 0.011, 0.019, and 0.023 cm².</p><p>(B) Wild-type cotyledons, before and after clearing.</p><p>(C and D) As in (B), for representative fused cotyledons of <i>hy5 hyh</i> seedlings. Note the true leaf opposing the fused cotyledon in (D).</p><p>(E) Representative shoots of 12-d-old light-grown seedlings.</p><p>(F) Representative first leaves.</p><p>(G) Darkfield microscopy images of first leaves, cleared for visualization of the vasculature.</p></div

Complementation of <i>hy5</i> Phenotypes by <i>HYH</i> Transcripts

<div><p>Analyses of independent transgenic lines constitutively overexpressing <i>altHYH</i> or <i>flHYH</i> under control of the <i>35S</i> promoter in a <i>hy5</i> background.</p><p>(A) Hypocotyl length.</p><p>(B) Lateral root phenotypes of representative plantlets.</p><p>(C) Root greening phenotypes of representative plants.</p><p>(D) Primary root length. Plant age: (A and D) = 8 dag (days after germination); (B) = 13 dag; (C) = 30 dag. Error bars represent standard error of the mean.</p></div

Root System Phenotypes in <i>hy5 hyh</i> Double Mutants

<div><p>Phenotypic analyses of wild-type, <i>hy5</i>, <i>hyh,</i> and <i>hy5 hyh</i> seedlings.</p><p>(A) Gravitropic response of the root tip (curvature). Time points refer to time after change of the gravity vector by 90 degrees.</p><p>(B) Representative seedlings at 8 dag.</p><p>(C) Progression of lateral root emergence.</p><p>(D) Progression of primary root growth of the seedlings in (C).</p><p>(E) Lateral root density of the seedlings in (C) and (D).</p><p>(F) Progression of adventitious root formation from the hypocotyl of 5-d-old dark-grown seedlings after shift into the light for the indicated number of days.</p><p>(G) Lateral root growth rate in the different genotypes, recorded between 8 and 10 dag.</p><p>(H) Quantification of primary root meristem size. Error bars represent standard error of the mean.</p></div

Correlation of Root System Growth and Auxin Signaling

<p>Graphical presentation of a model explaining the opposite root system phenotypes of <i>hy5</i> and <i>hy5 hyh</i> mutants. The strength of (constitutive) auxin signaling in the respective genotype determines the extent of root system growth. Root system growth is maximal at a certain optimum auxin signaling level but decreases if this level is either lower or higher. This assumption is supported by the phenotypic response of wild-type seedling root systems to the external application of increasing amounts of auxin: whereas low auxin concentrations promote root system growth, high concentrations inhibit growth.</p