Changes in root development of Arabidopsis promoted by organic matter from oxisols

The presence of ramified and abundant lateral roots is fundamental to plant growth on highly weathered soils. In this work, the effects of humic acid (HA), fulvic acid and hexanic–methanolic (HM) extract, after alkaline extraction from topsoil of seven different oxisols, on the development of roots of Arabidopsis thaliana was evaluated. Furthermore, we used another emergent plant model, that is, micro‐tom (MT) tomatoes with size similar to that of Arabidopsis to test the effects of HA. It was observed that both humic fractions and the HM extract were able to change the root development, improving the number of lateral roots and their development in comparison to control plants. The promotion of root growth by the three organic matter fractions was higher than that observed by 10^?6 mol L^?1 indole acetic acid. The treatment of MT tomato mutant, less sensitive to auxin, with HA did not promote the emergence of lateral roots, being an additional indication of auxin‐like activities of HA. However, some organic matter fractions exhibited, besides promotion of lateral roots number, increase in the length of principal root, which is not a typical auxin effect, indicating that these substances could contain other physiologically active substances.     

OsPIN2, which encodes a member of the auxin efflux carrier proteins,is involved in root elongation growth and lateral root formation patterns via the regulation of auxin distribution in rice

Auxin flow is important for different root developmental processes such as root formation, emergence, elongation and gravitropism. However, the detailed information about the mechanisms regulating the auxin flow is less well understood in rice. We characterized the auxin transport‐related mutants, Ospin‐formed2‐1 (Ospin2‐1) and Ospin2‐2, which exhibited curly root phenotypes and altered lateral root formation patterns in rice. The OsPIN2 gene encodes a member of the auxin efflux carrier proteins that possibly regulates the basipetal auxin flow from the root tip toward the root elongation zone. According to DR5‐driven GUS expression, there is an asymmetric auxin distribution in the mutants that corresponded with the asymmetric cell elongation pattern in the mutant root tip. Auxin transport inhibitor, N‐1‐naphthylphthalamic acid and Ospin2‐1 Osiaa13 double mutant rescued the curly root phenotype indicating that this phenotype results from a defect in proper auxin distribution. The typical curly root phenotype was not observed when Ospin2‐1 was grown in distilled water as an alternative to tap water, although higher auxin levels were found at the root tip region of the mutant than that of the wild‐type. Therefore, the lateral root formation zone in the mutant was shifted basipetally compared with the wild‐type. These results reflect that an altered auxin flow in the root tip region is responsible for root elongation growth and lateral root formation patterns in rice.     

Shoot-supplied ammonium targets the root auxin influx carrier AUX1 and inhibits lateral root emergence in Arabidopsis

Deposition of ammonium (NH₄⁺) from the atmosphere is a substantial environmental problem. While toxicity resulting from root exposure to NH₄⁺ is well studied, little is known about how shoot‐supplied ammonium (SSA) affects root growth. In this study, we show that SSA significantly affects lateral root (LR) development. We show that SSA inhibits lateral root primordium (LRP) emergence, but not LRP initiation, resulting in significantly impaired LR number. We show that the inhibition is independent of abscisic acid (ABA) signalling and sucrose uptake in shoots but relates to the auxin response in roots. Expression analyses of an auxin‐responsive reporter, DR5:GUS, and direct assays of auxin transport demonstrated that SSA inhibits root acropetal (rootward) auxin transport while not affecting basipetal (shootward) transport or auxin sensitivity of root cells. Mutant analyses indicated that the auxin influx carrier AUX1, but not the auxin efflux carriers PIN‐FORMED (PIN)1 or PIN2, is required for this inhibition of LRP emergence and the observed auxin response. We found that AUX1 expression was modulated by SSA in vascular tissues rather than LR cap cells in roots. Taken together, our results suggest that SSA inhibits LRP emergence in Arabidopsis by interfering with AUX1‐dependent auxin transport from shoot to root.     

Characterization of the growth and auxin physiology of roots of the tomato mutant,diageotropica

Roots of the tomato (Lycopersicon esculentum, Mill.) mutant diageotropica (dgt) exhibit an altered phenotype. These roots are agravitropic and lack lateral roots. Relative to wild-type (VFN8) roots, dgt roots are less sensitive to growth inhibition by exogenously applied IAA and auxin transport inhibitors (phytotropins), and the roots exhibit a reduction in maximal growth inhibition in response to ethylene. However, IAA transport through roots, binding of the phytotropin, tritiated naphthylphthalamic acid ([³H]NPA), to root microsomal membranes, NPA-sensitive IAA uptake by root segments, and uptake of [³H]NPA into root segments are all similar in mutant and wild-type roots. We speculate that the reduced sensitivity of dgt root growth to auxin-transport inhibitors and ethylene is an indirect result of the reduction in sensitivity to auxin in this single gene, recessive mutant. We conclude that dgt roots, like dgt shoots, exhibit abnormalities indicating they have a defect associated with or affecting a primary site of auxin perception or action.Abbreviations BCA bicinchoninic acid - IAA indole 3-acetic acid - dgt diageotropica - IC₅₀ concentration for 50% inhibition of growth - NPA N-1-naphthylphthalamic acid - SCB-1 semicarbazone 1 This research was supported by grants from Sandoz Agro, Inc. (G.K.M), the National Aeronautics and Space Administration (NASA) and the National Science Foundation (T.L.L), and NASA (D.L.R.).     

The short-rooted vitamin B<Subscript>6</Subscript>-deficient mutant <Emphasis Type="Italic">pdx1</Emphasis> has impaired local auxin biosynthesis

The phytohormone auxin regulates many aspects of plant growth and development. Auxin often acts distantly from the site of its biosynthesis and this long-distance-transported auxin is well known to play a critical role in eliciting physiological responses including regulating root development. Auxin can be produced in roots, yet the function of locally synthesized auxin in root growth is unclear. The major auxin in plants, indole 3-acetic acid (IAA), is mainly synthesized through tryptophan (Trp)-dependent pathways that require pyridoxal phosphate (an active form of vitamin B₆)-dependent enzymes. We previously reported that the Arabidopsis vitamin B₆ biosynthesis mutant pdx1 has stunted root growth although the underlying cause is unknown. Here we showed that the pdx1 root is deficient in auxin biosynthesis. By reciprocal grafting of pdx1 and the wild type, we demonstrated that the stunted root growth in pdx1 is caused by a locally generated signal(s) in roots. To test whether auxin might be one such signal, the auxin responsive DR5::GUS reporter was introduced into the mutant. The DR5::GUS activity in pdx1 root tips was greatly reduced compared with that in the wild type although the auxin response was unaltered. pdx1 also suppresses the root hair growth defects in the auxin overproduction mutant yucca. These data indicate that pdx1 is impaired in Trp-dependent auxin biosynthesis, which may contribute to the short-root phenotype of pdx1. We suggest that locally synthesized auxin may play a critical role in postembryonic root growth.     

The diageotropica gene of tomato encodes a cyclophilin: a novel player in auxin signaling

The single gene, auxin-resistant diageotropica (dgt) mutant of tomato displays a pleiotropic auxin-related phenotype that includes a slow gravitropic response, lack of lateral roots, reduced apical dominance, altered vascular development, and reduced fruit growth. Some auxin responses are unaltered in dgt plants, however, and the levels, metabolism, and transport of auxin appear normal, indicating that the Dgt gene encodes a component of a specific auxin signaling pathway. By combining map-based cloning with comparative microsynteny, we determined that the Dgt gene encodes a cyclophilin (CYP) (LeCYP1; gi:170439) that has not previously been identified as a component of auxin signaling and plant development. Each of the three known dgt alleles contains a unique mutation in the coding sequence of LeCyp1. Alleles dgt ^1-1 and dgt ^1-2 contain single nucleotide point mutations that generate an amino acid change (G₁₃₇R) and a stop codon (W₁₂₈stop), respectively, while dgt ^dp has an amino acid change (W₁₂₈CΔ129–133) preceding a 15 bp deletion. Complementation of dgt plants with the wild-type LeCyp1 gene restored the wild-type phenotype. Each dgt mutation reduced or nullified the peptidyl–prolyl isomerase activity of the GST–LeCYP1 fusion proteins in vitro. RT-PCR and immunoblot analyses indicated that the dgt mutations do not affect the expression of LeCyp1 mRNA, but the accumulation of LeCYP1 protein is greatly reduced for all three mutant alleles. The CYP inhibitor, cyclosporin A, partially mimics the effects of the dgt mutation in inhibiting auxin-induced adventitious root initiation in tomato hypocotyl sections and reducing the auxin-induced expression of the early auxin response genes, LeIAA10 and 11. These observations confirm that the PPIase activity of the tomato CYP, LeCYP1, encoded by the Dgt gene is important for specific aspects of auxin regulation of plant growth, development, and environmental responses.     

OsORC3 is required for lateral root development in rice

The origin recognition complex (ORC) is a pivotal element in DNA replication, heterochromatin assembly, checkpoint regulation and chromosome assembly. Although the functions of the ORC have been determined in yeast and model animals, they remain largely unknown in the plant kingdom. In this study, Oryza sativa Origin Recognition Complex subunit 3 (OsORC3) was cloned using map‐based cloning procedures, and functionally characterized using a rice (Oryza sativa) orc3 mutant. The mutant showed a temperature‐dependent defect in lateral root (LR) development. Map‐based cloning showed that a G→A mutation in the 9th exon of OsORC3 was responsible for the mutant phenotype. OsORC3 was strongly expressed in regions of active cell proliferation, including the primary root tip, stem base, lateral root primordium, emerged lateral root primordium, lateral root tip, young shoot, anther and ovary. OsORC3 knockdown plants lacked lateral roots and had a dwarf phenotype. The root meristematic zone of ORC3 knockdown plants exhibited increased cell death and reduced vital activity compared to the wild‐type. CYCB1;1::GUS activity and methylene blue staining showed that lateral root primordia initiated normally in the orc3 mutant, but stopped growing before formation of the stele and ground tissue. Our results indicate that OsORC3 plays a crucial role in the emergence of lateral root primordia.     

Modulation of auxin signalling through DIAGETROPICA and ENTIRE differentially affects tomato plant growth via changes in photosynthetic and mitochondrial metabolism

Auxin modulates a range of plant developmental processes including embryogenesis, organogenesis, and shoot and root development. Recent studies have shown that plant hormones also strongly influence metabolic networks, which results in altered growth phenotypes. Modulating auxin signalling pathways may therefore provide an opportunity to alter crop performance. Here, we performed a detailed physiological and metabolic characterization of tomato (Solanum lycopersicum) mutants with either increased (entire) or reduced (diageotropica—dgt) auxin signalling to investigate the consequences of altered auxin signalling on photosynthesis, water use, and primary metabolism. We show that reduced auxin sensitivity in dgt led to anatomical and physiological modifications, including altered stomatal distribution along the leaf blade and reduced stomatal conductance, resulting in clear reductions in both photosynthesis and water loss in detached leaves. By contrast, plants with higher auxin sensitivity (entire) increased the photosynthetic capacity, as deduced by higher V_cmax and J_max coupled with reduced stomatal limitation. Remarkably, our results demonstrate that auxin‐sensitive mutants (dgt) are characterized by impairments in the usage of starch that led to lower growth, most likely associated with decreased respiration. Collectively, our findings suggest that mutations in different components of the auxin signalling pathway specifically modulate photosynthetic and respiratory processes.     

The auxin-resistant diageotropica mutant of tomato responds to gravity via an auxin-mediated pathway

 Hypocotyls of the diageotropica (dgt) mutant of tomato (Lycopersicon esculentum Mill.) do not elongate in response to exogenous auxin, but can respond to gravity. This appears paradoxical in light of the Cholodny-Went hypothesis, which states that shoot gravicurvature results from asymmetric stimulation of elongation by auxin. While light-grown dgt seedlings can achieve correct gravitropic reorientation, the response is slow compared to wild-type seedlings. The sensitivity of dgt seedlings to inhibition of gravicurvature by immersion in auxin or auxin-transport inhibitors is similar to that of wild-type plants, indicating that both an auxin gradient and auxin transport are required for the gravitropic response and that auxin uptake, efflux, and at least one auxin receptor are functional in dgt. Furthermore, dgt gravicurvature is the result of asymmetrically increased elongation as would be expected for an auxin-mediated response. Our results suggest differences between elongation in response to exogenous auxin (absent in dgt) and elongation in response to gravistimulation (present but attenuated in dgt) and confirm the presence of two phases during the gravitropic response, both of which are dependent on functional auxin transport. Received: 16 July 1999 / Accepted: 24 September 1999     

The Diageotropica Gene Differentially Affects Auxin and Cytokinin Responses throughout Development in Tomato

The interactions between the plant hormones auxin and cytokinin throughout plant development are complex, and genetic investigations of the interdependency of auxin and cytokinin signaling have been limited. We have characterized the cytokinin sensitivity of the auxin-resistant diageotropica (dgt) mutant of tomato (Lycopersicon esculentum Mill.) in a range of auxin- and cytokinin-regulated responses. Intact, etiolated dgt seedlings showed cross-resistance to cytokinin with respect to root elongation, but cytokinin effects on hypocotyl growth and ethylene synthesis in these seedlings were not impaired by the dgt mutation. Seven-week-old, green wild-type and dgt plants were also equally sensitive to cytokinin with respect to shoot growth and hypocotyl and internode elongation. The effects of cytokinin and the dgt mutation on these processes appeared additive. In tissue culture organ regeneration from dgt hypocotyl explants showed reduced sensitivity to auxin but normal sensitivity to cytokinin, and the effects of cytokinin and the mutation were again additive. However, although callus induction from dgt hypocotyl explants required auxin and cytokinin, dgt calli did not show the typical concentration-dependent stimulation of growth by either auxin or cytokinin observed in wild-type calli. Cross-resistance of the dgt mutant to cytokinin thus was found to be limited to a small subset of auxin- and cytokinin-regulated growth processes affected by the dgt mutation, indicating that auxin and cytokinin regulate plant growth through both shared and separate signaling pathways.     

The auxin transporter,OsAUX1, is involved in primary root and root hair elongation and in Cd stress responses in rice (Oryza sativa L.)

Auxin and cadmium (Cd) stress play critical roles during root development. There are only a few reports on the mechanisms by which Cd stress influences auxin homeostasis and affects primary root (PR) and lateral root (LR) development, and almost nothing is known about how auxin and Cd interfere with root hair (RH) development. Here, we characterize rice osaux1 mutants that have a longer PR and shorter RHs in hydroponic culture, and that are more sensitive to Cd stress compared to wild‐type (Dongjin). OsAUX1 expression in root hair cells is different from that of its paralogous gene, AtAUX1, which is expressed in non‐hair cells. However, OsAUX1, like AtAUX1, localizes at the plasma membrane and appears to function as an auxin tranporter. Decreased auxin distribution and contents in the osaux1 mutant result in reduction of OsCyCB1;1 expression and shortened PRs, LRs and RHs under Cd stress, but may be rescued by treatment with the membrane‐permeable auxin 1‐naphthalene acetic acid. Treatment with the auxin transport inhibitors 1‐naphthoxyacetic acid and N‐1‐naphthylphthalamic acid increased the Cd sensitivity of WT rice. Cd contents in the osaux1 mutant were not altered, but reactive oxygen species‐mediated damage was enhanced, further increasing the sensitivity of the osaux1 mutant to Cd stress. Taken together, our results indicate that OsAUX1 plays an important role in root development and in responses to Cd stress.     

The<Emphasis Type="Italic"> diageotropica</Emphasis> mutation of tomato disrupts a signalling chain using extracellular auxin binding protein 1 as a receptor

The diageotropica (dgt) mutant of tomato (Lycopersicon esculentum Mill.) is known to lack a number of typical auxin responses. Here we show that rapid auxin-induced growth of seedling hypocotyls is completely abolished by the mutation over the full range of auxin concentrations tested, and also in early phases of the time course. Protoplasts isolated from wild-type hypocotyls respond to auxin by a rapid increase in cell volume, which we measured by image analysis at a high temporal resolution. A similar swelling could be triggered by antibodies directed against a part of the putative auxin-binding domain (box-a) of the auxin-binding protein 1 (ABP1). Induction of swelling both by auxin and by the antibody was not observed in the protoplasts isolated from the dgt mutant. However, dgt protoplasts are able to respond to the stimulator of the H⁺-ATPase, fusicoccin, with normal swelling. We propose that dgt is a signal-transduction mutation interfering with an auxin-signalling pathway that uses ABP1 as a receptor.Abbreviations ABP auxin-binding protein - CCD charge-coupled device - 2,4-D 2,4-dichlorophenoxyacetic acid - dgt diageotropica - FC fusicoccin     

Inhibition of primary roots and stimulation of lateral root development in Arabidopsis thaliana by the rhizobacterium Serratia marcescens 90–166 is through both auxin-dependent and -independent signaling pathways

The rhizobacterium Serratia marcescens strain 90–166 was previously reported to promote plant growth and induce resistance in Arabidopsis thaliana. In this study, the influence of strain 90-166 on root development was studied in vitro. We observed inhibition of primary root elongation, enhanced lateral root emergence, and early emergence of second order lateral roots after inoculation with strain 90–166 at a certain distance from the root. Using the DR5::GUS transgenic A. thaliana plant and an auxin transport inhibitor, N-1-naphthylphthalamic acid, the altered root development was still elicited by strain 90–166, indicating that this was not a result of changes in plant auxin levels. Intriguingly, indole-3-acetic acid, a major auxin chemical, was only identified just above the detection limit in liquid culture of strain 90–166 using liquid chromatography-mass spectrometry. Focusing on bacterial determinants of the root alterations, we found that primary root elongation was inhibited in seedlings treated with cell supernatant (secreted compounds), while lateral root formation was induced in seedlings treated with lysate supernatant (intracellular compounds). Further study revealed that the alteration of root development elicited by strain 90–166 involved the jasmonate, ethylene, and salicylic acid signaling pathways. Collectively, our results suggest that strain 90–166 can contribute to plant root development via multiple signaling pathways.     

Callus,shoot and hairy root formation in vitro as affected by the sensitivity to auxin and ethylene in tomato mutants

We analyzed the impact of ethylene and auxin disturbances on callus, shoots and Agrobacterium rhizogenes-induced hairy root formation in tomato (Solanum lycopersicum L.). The auxin low-sensitivity dgt mutation showed little hairy root initiation, whereas the ethylene low-sensitivity Nr mutation did not differ from the control Micro-Tom cultivar. Micro-Tom and dgt hairy roots containing auxin sensitivity/biosynthesis rol and aux genes formed prominent callus onto media supplemented with cytokinin. Under the same conditions, Nr hairy roots did not form callus. Double mutants combining Rg1, a mutation conferring elevated shoot formation capacity, with either dgt or Nr produced explants that formed shoots with little callus proliferation. The presence of rol + aux genes in Rg1 hairy roots prevented shoot formation. Taken together, the results suggest that although ethylene does not affect hairy root induction, as auxin does, it may be necessary for auxin-induced callus formation in tomato. Moreover, excess auxin prevents shoot formation in Rg1.