Hormone-Induced Gene Expression During Gravicurvature of <Emphasis Type="Italic">Brassica</Emphasis> Roots

To investigate the spatial and temporal dependence of hormonal regulation during gravitropism, we compared the effects of root cap application of indole-3-acetic acid (IAA) and abscisic acid (ABA) with gene expression changes occurring naturally during gravitropic reaction of Brassica rapa roots. The expression of auxin, ABA, and metabolism-related genes in the tip, elongation zone, and maturation zone varied with time, location, and hormone concentration and characterized polar auxin transport. IAA was transported readily shootward and inhibited growth more than ABA. Expression of PIN3 and IAA5 in the elongation zone showed downregulation on the convex but upregulation on the concave side. Both PIN7 and IAA5 responded near maximally to 10^?8 M IAA within 30 min, suggesting that auxin activates its own transport system. Ubiquitin 1 (UBQ1) responded after a lag time of more than 1 h to IAA. The metabolic control gene Phosphoenolpyruvate carboxylase 1 (PEPC1) was more sensitive to ABA but upregulated by high concentrations of either hormone. The time course and duration of gene activation suggests that ABA is not involved in gravitropic curvature, differential elongation is not simply explained by IAA-induced upregulation, and that reference genes are sensitive to auxin.     

Carotenoid deficiency impairs ABA and IAA biosynthesis and differentially affects drought and cold tolerance in rice

Plant responses to abiotic stresses are coordinated by arrays of growth and developmental programs. Phytohormones such as abscisic acid (ABA) and indole-3-acetic acid (IAA) play critical roles in developmental progresses and environmental responses through complex signalling networks. However, crosstalk between the two hormones at the biosynthesis level remains largely unknown. Here, we report that carotenoid-deficient mutants (phs1, phs2, phs3-1, phs4, and PDS-RNAi transgenic rice) were impaired in the biosynthesis of ABA and IAA. Under drought conditions, phs3-1 and PDS-RNAi transgenic rice showed larger stomata aperture and earlier wilting compared to the wild type at both seedling and panicle developmental stage. Interestingly, these carotenoid-deficient lines showed increased cold resistance, which was likely due to the combined effects of reduced IAA content, alleviated oxidative damage and decreased membrane penetrability. Furthermore, we found that IAA content was significantly declined in rice treated with fluridone (a carotenoid and ABA biosynthesis inhibitor), and expression of auxin synthesis and metabolism-related genes were altered in the fluridone-treated rice similar to that in the carotenoid-deficient mutants. In addition, exogenous IAA, but not ABA, could restore the dwarf phenotype of phs3-1 and PDS-RNAi transgenic rice. These results support a crosstalk between ABA and IAA at the biosynthesis level, and this crosstalk is involved in development and differentially affects drought and cold tolerance in rice.     

AUXIN RESPONSE FACTOR 2 Intersects Hormonal Signals in the Regulation of Tomato Fruit Ripening

The involvement of ethylene in fruit ripening is well documented, though knowledge regarding the crosstalk between ethylene and other hormones in ripening is lacking. We discovered that AUXIN RESPONSE FACTOR 2A (ARF2A), a recognized auxin signaling component, functions in the control of ripening. ARF2A expression is ripening regulated and reduced in the rin, nor and nr ripening mutants. It is also responsive to exogenous application of ethylene, auxin and abscisic acid (ABA). Over-expressing ARF2A in tomato resulted in blotchy ripening in which certain fruit regions turn red and possess accelerated ripening. ARF2A over-expressing fruit displayed early ethylene emission and ethylene signaling inhibition delayed their ripening phenotype, suggesting ethylene dependency. Both green and red fruit regions showed the induction of ethylene signaling components and master regulators of ripening. Comprehensive hormone profiling revealed that altered ARF2A expression in fruit significantly modified abscisates, cytokinins and salicylic acid while gibberellic acid and auxin metabolites were unaffected. Silencing of ARF2A further validated these observations as reducing ARF2A expression let to retarded fruit ripening, parthenocarpy and a disturbed hormonal profile. Finally, we show that ARF2A both homodimerizes and interacts with the ABA STRESS RIPENING (ASR1) protein, suggesting that ASR1 might be linking ABA and ethylene-dependent ripening. These results revealed that ARF2A interconnects signals of ethylene and additional hormones to co-ordinate the capacity of fruit tissue to initiate the complex ripening process.     

PIF4 and PIF5 Transcription Factors Link Blue Light and Auxin to Regulate the Phototropic Response in Arabidopsis

Both blue light (BL) and auxin are essential for phototropism in Arabidopsis thaliana. However, the mechanisms by which light is molecularly linked to auxin during phototropism remain elusive. Here, we report that PHYTOCHROME INTERACTING FACTOR4 (PIF4) and PIF5 act downstream of the BL sensor PHOTOTROPIN1 (PHOT1) to negatively modulate phototropism in Arabidopsis. We also reveal that PIF4 and PIF5 negatively regulate auxin signaling. Furthermore, we demonstrate that PIF4 directly activates the expression of the AUXIN/INDOLE-3-ACETIC ACID (IAA) genes IAA19 and IAA29 by binding to the G-box (CACGTG) motifs in their promoters. Our genetic assays demonstrate that IAA19 and IAA29, which physically interact with AUXIN RESPONSE FACTOR7 (ARF7), are sufficient for PIF4 to negatively regulate auxin signaling and phototropism. This study identifies a key step of phototropic signaling in Arabidopsis by showing that PIF4 and PIF5 link light and auxin.     

Crosstalk between ABA and auxin signaling pathways in roots of <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> (L.) Heynh.

Studies of abscisic acid (ABA) and auxin have revealed that these pathways impinge on each other. The Daucus carota (L.) Dc3 promoter: uidA (-glucuronidase: GUS) chimaeric reporter (ProDc3:GUS) is induced by ABA, osmoticum, and the auxin indole-3-acetic acid (IAA) in vegetative tissues of transgenic Arabidopsis thaliana (L.) Heynh. Here, we describe the root tissue-specific expression of ProDc3:GUS in the ABA-insensitive-2 (abi2-1), auxin-insensitive-1 (aux1), auxin-resistant-4 (axr4), and rooty (rty1) mutants of Arabidopsis in response to ABA, IAA and synthetic auxins naphthalene acetic acid (NAA), and 2, 4-(dichlorophenoxy) acetic acid. Quantitative analysis of ProDc3:GUS expression showed that the abi2-1 mutant had reduced GUS activity in response to ABA, IAA, or 2, 4-d, but not to NAA. Similarly, chromogenic staining of ProDc3:GUS activity showed that the aux1 and axr4 mutants gave predictable hypomorphic ProDc3:GUS expression phenotypes in roots treated with IAA or 2, 4-d, but not the diffusible auxin NAA. Likewise the rty mutant, which accumulates auxin, showed elevated ProDc3:GUS expression in the absence or presence of hormones relative to wild type. Interestingly, the aux1 and axr4 mutants showed a hypomorphic effect on ABA-inducible ProDc3:GUS expression, demonstrating that ABA and IAA signaling pathways interact in roots. Possible mechanisms of crosstalk between ABA and auxin signaling are discussed.     

Cadmium interferes with maintenance of auxin homeostasis in Arabidopsis seedlings

Auxin and its homeostasis play key roles in many aspects of plant growth and development. Cadmium (Cd) is a phytotoxic heavy metal and its inhibitory effects on plant growth and development have been extensively studied. However, the underlying molecular mechanism of the effects of Cd stress on auxin homeostasis is still unclear. In the present study, we found that the root elongation, shoot weight, hypocotyl length and chlorophyll content in wild-type (WT) Arabidopsis seedlings were significantly reduced after exposure to Cd stress. However, the lateral root (LR) formation was markedly promoted by Cd stress. The level and distribution of auxin were both greatly altered in primary root tips and cotyledons of Cd-treated plants. The results also showed that after Cd treatment, the IAA content was significantly decreased, which was accompanied by increases in the activity of the IAA oxidase and alteration in the expression of several putative auxin biosynthetic and catabolic genes. Application of the auxin transport inhibitor, 1-naphthylphthalamic acid (NPA) and 1-naphthoxyacetic acid (1-NOA), reversed the effects of Cd on LR formation. Additionally, there was less promotion of LR formation by Cd treatment in aux1-7 and pin2 mutants than that in the WT. Meanwhile, Cd stress also altered the expression of PINs and AUX1 in Arabidopsis roots, implying that the auxin transport pathway is required for Cd-modulated LR development. Taken together, these findings suggest that Cd stress disturbs auxin homeostasis through affecting auxin level, distribution, metabolism, and transport in Arabidopsis seedling.     

Auxin Enhancement of mRNAs in Epidermis and Internal Tissues of the Pea Stem and Its Significance for Control of Elongation

The epidermis has been considered the site of auxin action on elongation of stems and coleoptiles. To try to identify mRNAs that might mediate auxin stimulation of cell enlargement, we compared, using in vitro translation assays, mRNA enhancement by indoleacetic acid (IAA) in the epidermis, with that in the internal tissues, of pea (Pisum sativum L., cv Alaska) third internode segments. We used seedlings that had been grown under red light, which enables the epidermis to be peeled efficiently from the internode. Most of the `early' IAA enhancements previously reported using etiolated peas, plus several hitherto undescribed enhancements, occur in both the epidermis and the internal tissue of the light-grown plants after 4 hours of IAA treatment. These enhancements, therefore, do not fulfill the expectation of elongation-specific mRNAs localized to the epidermis. One epidermis-specific IAA enhancement does occur, but begins only subsequent to 1 hour (but before 4 hours) of auxin treatment. Similarly, the previously mentioned IAA enhancements common to epidermis and internal tissue do not begin, in the light-grown plants, within 1 hour of IAA treatment. Since IAA stimulates elongation in light-grown internodes within 15 minutes, it appears that none of these mRNAs can be responsible for auxin induction of elongation. We confirmed, with our methods, the previous reports that some of these mRNAs are enhanced by IAA within 0.5 hour in etiolated internodes. This indicates that we could have detected an early enhancement in light-grown tissue had it occurred.     

Inhibitory action of auxin on root elongation not mediated by ethylene

The inhibitory effects of indole-3-acetic acid (IAA) and 1-aminocyclopropane-1-carboxylic acid (ACC) on elongation growth of pea (Pisum sativum L.) seedling roots were investigated in relation to the effects of these compounds on ethylene production by the root tips. When added to the growth solution both compounds caused a progressively increasing inhibition of growth within the concentration range of 0.01 to 1 micromolar. However, only ACC increased ethylene production in root tips excised from the treated seedlings after 24 hours. High auxin concentrations caused a transitory increase of ethylene production during a few hours in the beginning of the treatment period, but even in 1 micromolar IAA this increase was too low to have any appreciable effect on growth. ACC, but not IAA, caused growth curvatures, typical of ethylene treatment, in the root tips. IAA caused conspicuous swelling of the root tips while ACC did not. Cobalt and silver ions reversed the growth inhibitory effects induced by ACC but did not counteract the inhibition of elongation or swelling caused by IAA. The growth effects caused by the ACC treatments were obviously due to ethylene production. We found no evidence to indicate that the growth inhibition or swelling caused by IAA is mediated by ethylene. It is concluded that the inhibitory action of IAA on root growth is caused by this auxin per se.     

Rapid auxin-induced root growth inhibition requires the TIR and AFB auxin receptors

We investigated the relation between auxin-induced gene expression and the rapid auxin-induced growth inhibition in Arabidopsis thaliana roots. The natural auxin indole-3-acetic acid (IAA) induced a strong activation of gene expression as visualized by the DR5rev::GFP reporter gene technique. This effect was specific for active auxins and was abolished in knockout mutants of the F-box auxin receptors. We measured the IAA-induced growth inhibition at high time resolution and show that the F-box auxin receptor mutants failed to display this effect. We conclude that the F-box auxin receptors are needed for the response. In hypocotyls, auxin induces an increase in elongation growth, and this effect has been earlier shown to be independent of the F-box receptors. Based on these findings, we discuss differences in the growth control modes in roots and shoots. We demonstrate that the rapid auxin-induced root growth inhibition, unlike the induction of growth in hypocotyls, requires the presence of the F-box auxin receptors.     

Arabidopsis iba response5 suppressors separate responses to various hormones

Auxin controls numerous plant growth processes by directing cell division and expansion. Auxin-response mutants, including iba response5 (ibr5), exhibit a long root and decreased lateral root production in response to exogenous auxins. ibr5 also displays resistance to the phytohormone abscisic acid (ABA). We found that the sar3 suppressor of auxin resistant1 (axr1) mutant does not suppress ibr5 auxin-response defects, suggesting that screening for ibr5 suppressors might reveal new components important for phytohormone responsiveness. We identified two classes of Arabidopsis thaliana mutants that suppressed ibr5 resistance to indole-3-butyric acid (IBA): those with restored responses to both the auxin precursor IBA and the active auxin indole-3-acetic acid (IAA) and those with restored response to IBA but not IAA. Restored IAA sensitivity was accompanied by restored ABA responsiveness, whereas suppressors that remained IAA resistant also remained ABA resistant. Some suppressors restored sensitivity to both natural and synthetic auxins; others restored responsiveness only to auxin precursors. We used positional information to determine that one ibr5 suppressor carried a mutation in PLEIOTROPIC DRUG RESISTANCE9 (PDR9/ABCG37/At3g53480), which encodes an ATP-binding cassette transporter previously implicated in cellular efflux of the synthetic auxin 2,4-dichlorophenoxyacetic acid.