High Auxin Concentrations Research Articles

OsPht1;8, a phosphate transporter, is involved in auxin and phosphate starvation response in rice.

The responses of plants to auxin and phosphate (Pi) starvation are closely linked. However, the underlying mechanisms connecting the Pi starvation (-Pi) responses to auxin are largely unclear. Here, we show that OsPht1;8 (OsPT8), a phosphate transporter, functions in both the auxin and -Pi responses in rice (Oryza sativa L.) and tobacco (Nicotiana tabacum). The overexpression of OsPT8 (OsPT8-Oe) led to the loss of sensitivity to auxin and -Pi in adventitious roots, lateral roots, and root hairs in rice. The expression levels of OsPT8 and pOsPT8::GUS staining in roots, root-shoot junctions and leaves of rice were induced by IAA treatments. The number of young lateral roots in the OsPT8-Oe transgenic rice, which had higher auxin concentrations, was distinctly more than that in the wild-type, possibly as a result of increased expression of auxin-related genes under normal Pi condition. Moreover, tobacco overexpressing OsPT8 had a similar root phenotype to OsPT8-Oe rice. These data reveal a novel biological function of OsPT8 in the cross-talk between Pi and auxin signaling, and provide new evidence for the linkage between auxin and -Pi responses.

Ectopic Expression of WINDING 1 Leads to Asymmetrical Distribution of Auxin and a Spiral Phenotype in Rice.

Ectopic expression of the rice WINDING 1 (WIN1) gene leads to a spiral phenotype only in shoots but not in roots. Rice WIN1 belongs to a specific class of proteins in cereal plants containing a Bric-a-Brac/Tramtrack/Broad (BTB) complex, a non-phototropic hypocotyl 3 (NPH3) domain and a coiled-coil motif. The WIN1 protein is predominantly localized to the plasma membrane, but is also co-localized to plasmodesmata, where it exhibits a punctate pattern. It is observed that WIN1 is normally expressed in roots and the shoot-root junction, but not in the rest of shoots. In roots, WIN1 is largely localized to the apical and basal sides of cells. However, upon ectopic expression, WIN1 appears on the longitudinal sides of leaf sheath cells, correlated with the appearance of a spiral phenotype in shoots. Despite the spiral phenotype, WIN1-overexpressing plants exhibit a normal phototropic response. Although treatments with exogenous auxins or a polar auxin transport inhibitor do not alter the spiral phenotype, the excurvature side has a higher auxin concentration than the incurvature side. Furthermore, actin filaments are more prominent in the excurvature side than in the incurvature side, which correlates with cell size differences between these two sides. Interestingly, ectopic expression of WIN1 does not cause either unequal auxin distribution or actin filament differences in roots, so a spiral phenotype is not observed in roots. The action of WIN1 appears to be different from that of other proteins causing a spiral phenotype, and it is likely that WIN1 is involved in 1-N-naphthylphthalamic acid-insensitive plasmodesmata-mediated auxin transport.

Loss of developmental pluripotency occurs in two stages during leaf aging in Arabidopsis thaliana

Aging in plants and animals can result in the loss of cellular pluripotency and the inability to regenerate multiple organs from differentiated, somatic tissues. Detached cotyledons from Arabidopsis thaliana (L.) Heynh. seedlings can proliferate to form callus as well as root and shoot organs in the presence of the phytohormones auxin and cytokinin or their synthetic analogs. In this study, detached cotyledons from the ecotype Landsberg erecta (Ler-0) demonstrated two separate developmental intervals of pluripotency loss. During a 48- to 60-h interval, occurring approximately 4 to 6 d after germination (DAG), detached cotyledons from Ler-0 could proliferate new cells (callus) but lost shoot regeneration competency, which the ecotype Nossen-0 (No-0) retained. In older cotyledons, the pluripotency loss was more severe, as detached cotyledons failed to proliferate callus. In the first interval, Ler-0 cotyledons lost the ability to respond to treatment with a high concentration of synthetic auxin, a level previously hypothesized to promote dedifferentiation. However, both young and old Ler-0 cotyledons were found to induce WUSCHEL::NLS-GUS, a marker for undifferentiated shoot apical meristem cells. The late-stage interval positively correlated to the timing of slowed growth in cotyledons still attached to intact seedlings. Also, cytokinin overexpression could rescue (or delay) this late-stage callus proliferation failure, but not the early-stage decline in shoot regeneration ability. Combined, the results demonstrate that loss of pluripotency in vitro occurs in distinct temporal stages and involves auxin and/or cytokinin signaling. Furthermore, there is genetic variation for this loss of pluripotency within Arabidopsis.

Differential TOR activation and cell proliferation in Arabidopsis root and shoot apexes

The developmental plasticity of plants relies on the remarkable ability of the meristems to integrate nutrient and energy availability with environmental signals. Meristems in root and shoot apexes share highly similar molecular players but are spatially separated by soil. Whether and how these two meristematic tissues have differential activation requirements for local nutrient, hormone, and environmental cues (e.g., light) remain enigmatic in photosynthetic plants. Here, we report that the activation of root and shoot apexes relies on distinct glucose and light signals. Glucose energy signaling is sufficient to activate target of rapamycin (TOR) kinase in root apexes. In contrast, both the glucose and light signals are required for TOR activation in shoot apexes. Strikingly, exogenously applied auxin is able to replace light to activate TOR in shoot apexes and promote true leaf development. A relatively low concentration of auxin in the shoot and high concentration of auxin in the root might be responsible for this distinctive light requirement in root and shoot apexes, because light is required to promote auxin biosynthesis in the shoot. Furthermore, we reveal that the small GTPase Rho-related protein 2 (ROP2) transduces light-auxin signal to activate TOR by direct interaction, which, in turn, promotes transcription factors E2Fa,b for activating cell cycle genes in shoot apexes. Consistently, constitutively activated ROP2 plants stimulate TOR in the shoot apex and cause true leaf development even without light. Together, our findings establish a pivotal hub role of TOR signaling in integrating different environmental signals to regulate distinct developmental transition and growth in the shoot and root.

Genetic stability of regenerated plants via indirect somatic embryogenesis and indirect shoot regeneration of Carum copticum L.

Ajowan (Carum copticum L.) is an important and endangered industrial medicinal plant that growing in some parts of Iran. Two efficient protocols, without somaclonal variation induction, were developed for indirect somatic embryogenesis and indirect shoot regeneration of three Iranian ecotypes of ajowan. In the first experiment, higher concentration of auxin than cytokinin (1, 1.5 and 2mg/L of 2,4-dichlorophenoxyacetic acid along with 0.25, 0.5 and 0.75mg/L kinetin) was used for callus induction in 5, 10 and 15days old hypocotyl explants. In the second experiment, higher concentration of cytokinin than auxin (1mg/L of kinetin along with 0.5mg/L of 2,4-dichlorophenoxyacetic acid) was used for callus induction in 15-d old hypocotyl explants. The higher frequency of somatic embryos was achieved from 15days old hypocotyl explants of Ghoom ecotype with 28.33 embryos when induced calli from MS medium supplemented with 1.5mg/L 2,4-dichlorophenoxyacetic acid in combination with 0.5mg/L kinetin transferred to free plant growth regulator MS medium. Momentary removing of 2,4-dichlorophenoxyacetic acid was successful for somatic embryogenesis in Iranian ecotypes of ajowan that it significantly reduce the time of the culture and thus reduce the risk of somaclonal variation. Maximum number of initiated shoots per explant was related to Shiraz ecotype with average 18.33 shoots per callus in MS medium supplemented with 1.5mg/L of specify type of cytokinin (3-methoxy [-6-benzylamino-9-tetrahydropyran-2-yl] purine) plus 0.25mg/L naphathalene acetic acid. The survival rate of rooted plantlets was 60.86% and 58.33% for indirect somatic embryogenesis and indirect shoot regeneration derived plants, respectively. The genetic stability of regenerated plants via indirect somatic embryogenesis and indirect shoot regeneration was proved through flow cytometry analysis.

Antioxidant response of Stevia rebaudiana (Bertoni) Bertoni (Angiosperms; Asteraceae) during developing phase of suspension cell culture

The present study established a protocol for suspension cell culture of Stevia rebaudiana and demonstrates the antioxidant enzymes activity during various developing phase of it. Higher concentration of Auxin and Cytokines (3.0 mg L -1 BAP and 4.0 mg L -1 NAA) with Ascorbic acid (1 mg L -1 ) considered as highly suitable growth regulator combination for growth and development of Stevia rebaudiana suspension culture. Maximum concentration of stevioside (~70 mg G -1 of dry weight of tissue) was obtained on 14 th day (exponential phase) which got reduced on 28 th day (apoptotic phase) upto ~21 mg G -1 of dry weight of tissue. The amount of MDA reduced initially upto stationary growth phase which showed the adaptation of suspension cells in the culture medium and culture environment. Reduction of Chlorophyll showed the enhancing phenolic content with progressive culture period, while enhanced amount of proline was indicating the generation of defense mechanism with depletion of nutrient (with increased culture duration). The amount of SOD got enhanced with elevating the concentration of H 2 O 2 as well. The concentration of CAT enhanced upto stationary growth phase of suspension cell while APX concentration showed continuous inhibition up to apoptotic phase from exponential phase.

Effects of auxin on the initiation and proliferation of Catharanthus roseus (L.) G. Don adventitious roots

The influence of exogenous auxins such as IBA and NAA on the initation and proliferation of Catharanthus roseus (L.) G. Don adventitious roots was investigated. The results showed that adventitious roots were successfully established from cotyledons on ½ MS medium supplemented with IBA 0,7 mg/L and from leaves of in vitro shoots with IBA 0,7 mg/L or NAA 0,5 mg/L. The reduction of auxin concentration was required for root proliferation. The growth of adventitious roots cultured in shake flasks was better with IBA 0,3 mg/L or NAA 0,1 mg/L, while higher auxin concentrations led to significant development of callus. Biomass after 28 days of proliferation was proved of ajmalicine accumulation.

In vitro plant regeneration of Aster scaber via somatic embryogenesis

We established an in vitro plant regeneration system via somatic embryogenesis of Aster scaber, an important source of various biologically active phytochemicals. We examined the callus induction and embryogenic capacities of three explants, including leaves, petioles, and roots, on 25 different media containing different combinations of α-naphthalene acetic acid (NAA) and 6-benzyladenine (BA). The optimum concentrations of NAA and BA for the production of embryogenic calli were 5.0 μM and 0.05 μM, respectively. Media containing higher concentrations of auxin and cytokinin (such as 25 μM NAA and 25 μM BA) were suitable for shoot regeneration, especially for leaf-derived calli, which are the most readily available calli and are highly competent. For root induction from regenerated shoots, supplemental auxin and/or cytokinin did not improve rooting, but instead caused unwanted callus induction or retarded growth of regenerated plants. Therefore, plant growth regulator-free medium was preferable for root induction. Normal plants were successfully obtained from calli under the optimized conditions described above. This is the first report of the complete process of in vitro plant regeneration of A. scaber via somatic embryogenesis.

DASH transcription factor impacts Medicago truncatula seed size by its action on embryo morphogenesis and auxin homeostasis.

The endosperm plays a pivotal role in the integration between component tissues of molecular signals controlling seed development. It has been shown to participate in the regulation of embryo morphogenesis and ultimately seed size determination. However, the molecular mechanisms that modulate seed size are still poorly understood especially in legumes. DASH (DOF Acting in Seed embryogenesis and Hormone accumulation) is a DOF transcription factor (TF) expressed during embryogenesis in the chalazal endosperm of the Medicago truncatula seed. Phenotypic characterization of three independent dash mutant alleles revealed a role for this TF in the prevention of early seed abortion and the determination of final seed size. Strong loss-of-function alleles cause severe defects in endosperm development and lead to embryo growth arrest at the globular stage. Transcriptomic analysis of dash pods versus wild-type (WT) pods revealed major transcriptional changes and highlighted genes that are involved in auxin transport and perception as mainly under-expressed in dash mutant pods. Interestingly, the exogenous application of auxin alleviated the seed-lethal phenotype, whereas hormonal dosage revealed a much higher auxin content in dash pods compared with WT. Together these results suggested that auxin transport/signaling may be affected in the dash mutant and that aberrant auxin distribution may contribute to the defect in embryogenesis resulting in the final seed size phenotype.

Protoplast Co-culture Bioassay for Allelopathy in Leguminous Plants, Leucaena leucocephala and Mucuna gigantea, Containing Allelochemical Amino Acids, Mimosine and L-DOPA

Protoplasts isolated from calluses of two woody leguminous plants, Leucaena leucocephala and Mucuna gigantea, which contain high levels of the allelochemical amino acids, mimosine and l-DOPA, respectively, were co-cultured to examine their allelopathic activities. The young leaves were first confirmed to have strong inhibitory activity by the sandwich method. Callus cultures were induced and sub-cultured from seeds of L. leucocephala and from leaves of M. gigantea in Murashige and Skoog’s (MS) basal media containing high concentrations of auxin and cytokinins. Protoplasts of both calluses were isolated using the separate well method under strong enzymatic conditions, in high osmotic, 0.8 M mannitol solution. Protoplasts of both species had strong inhibitory effects on the recipient protoplasts of cotyledons of Lactuca sativa (lettuce) co-cultured using 96 multi-well culture plates in the MS basal medium, containing 1 ?M 2,4-dichlorophenoxyacetic acid, 0.1 ?M benzyladenine and 0.8 M mannitol solution. Mimosine was less inhibitory than l-DOPA on the growth of lettuce protoplasts at the stage of colony formation. Amino acid profiles of calluses and protoplasts of the above two species and Mucuna pruriens showed very low contents of mimosine and l-DOPA, respectively. The cause of the high inhibitory allelopathic activities found in Leucaena and Mucuna protoplasts on the growth of lettuce protoplasts was discussed.

A Molecular framework for the differential responses of primary and lateral roots to auxin in Arabidopsis thaliana

The Arabidopsis thaliana root system, which comprises primary, lateral, and adventitious roots, responds differentially to environmental and hormonal signals. For example, exposure to high concentrations of the plant hormone auxin promotes lateral root (LR) initiation and emergence; however, high levels of auxin inhibit primary root (PR) growth. Here, we show that transacting small interfering RNA-dependent post-transcriptional gene silencing of AUXIN RESPONSE FACTOR 3 (tasiRNA-ARF PTGS) likely plays a role in promoting the differential responses of PR and LR to auxin. In the presence of 1-naphthaleneacetic acid (NAA), a synthetic auxin, the expression domain of ARF3 (a negative regulator in auxin signaling) expanded into the meristem zone (MZ) in PR. Moreover, we found a remarkable increase in the expression of a cleavage-resistant version of ARF3 (pARF3::ARF3(m)-GUS) in PR even without application of exogenous auxin, suggesting the involvement of posttranscriptional regulation. In addition, the expression levels of microRNA390 (miR390), which triggers biogenesis of tasiRNA-ARF, were substantially reduced in PR, but increased in LR. Taken together, our findings suggest that high auxin levels differentially regulate the growth and development of PR and LR via the tasiRNA-ARF pathway.

Effect of auxin, cytokinin and nitrogen on anthocyanin biosynthesis in callus cultures of red-fleshed apple (Malus sieversii f.niedzwetzkyana)

We have induced callus tissues from one R6/R6 homozygous genotype red-fleshed apple individual which was the hybrid offspring of Malus sieversii f.niedzwetzkyana and ‘Fuji’, and investigated the effect of auxin alone and auxin combined with cytokinin or nitrogen deficiency on anthocyanin synthesis. In callus culture, auxin alone significantly inhibited anthocyanin biosynthesis with the increase of auxin concentration. The inhibitory effect of 2,4-dichlorophenoxyacetic acid (2,4-D) on anthocyanin accumulation was about tenfold stronger than naphthalene acetic acid. Anthocyanin regulatory genes (MdMYB10 and MdbHLH3) and structural genes were dramatically suppressed by 0.6 mg/L 2,4-D. The inhibitory effect of auxin on anthocyanin biosynthesis was influenced by cytokinins 6-benzylaminopurine (BAP) and thidiazuron (TDZ) as well as nitrogen deficiency. Auxin and cytokinin displayed the interaction in controlling anthocyanin biosynthesis. Co-treatment of auxin and cytokinin (BAP or TDZ) significantly enhanced the cytokinin-induced increase in anthocyanin levels but too high auxin concentration strongly inhibited anthocyanin synthesis even in the presence of cytokinin. Nitrogen deficiency could reverse the inhibition of anthocyanin accumulation by auxin.

HISTORY AND PERSPECTIVES ON THE ROLE OF ETHYLENE IN PINEAPPLE FLOWERING

Pineapple is an ancient Amerindian crop that was spread throughout the tropics by early Spanish and Portuguese explorers. They were grown in heated greenhouses in Europe and Britain by 1720 and a greenhouse-based commercial pineapple industry began in the Azores in about 1872. It was discovered by accident there in 1874 that plants could be forced into flower (forced) with smoke and growers adopted the practice as a way to schedule fruiting when prices were high. By the 1920s, growers in Puerto Rico had adapted the practice for in-field use by building smudge fires beneath blocks of pineapple covered with muslin cloth. It was known that distillation of wood produced ethylene and it was shown in 1932 that smoke and ethylene gas were both effective in forcing pineapple. That discovery led the Hawaiian pineapple industry, then the largest and most technologically advanced in the world, to improve on the new forcing technology. Hawaiian Pineapple Company patented a method of forcing pineapple with calcium carbide in 1936 and the industry-supported Pineapple Research Institute (PRI) patented a method of forcing pineapple with acetylene or ethylene as a gas or as a water solution. In 1941, California Packing Corporation patented an apparatus that could mix and apply a water-oil-colloidal earth emulsion containing acetylene or ethylene to field-grown pineapple plants to force them. In 1942, the PRI reported that pineapple plants could be forced with a low concentration of 1-naphthaleneacetic acid (NAA) while higher concentrations inhibited natural induction (NI) of flowering. It was shown in 1966 that auxins forced pineapple by stimulating the plant to produce ethylene. Over a ten year period, the PRI screened thousands of compounds as potential forcing agents and in 1956 reported that 2-hydroxyethylhydrazine (BOH) would force pineapple; in 1967 it was shown that BOH was an ethylene delivery agent. In 1968, pineapple plants were forced with 2-chloroethylphosphonic acid (ethephon), another ethylene delivery agent. Currently, pineapples on large plantations typically are forced approximately weekly with ethephon or ethylene. On smaller farms, forcing pineapples with calcium carbide or acetylene is more common. Natural induction, which is believed to be an ethylene-driven event, disrupts this orderly process of fruit production and is a serious management problem for growers of 'MD2-(Gold') pineapple. 'MD-2' is considered sensitive to NI in most environments where the winter photoperiod is less than about 11.5 h. High concentrations of auxins inhibit flowering, but with inconsistent results. A 1983 study showed that aminoethoxyvinylglycine (AVG), an inhibitor of ethylene biosynthesis, prevented NI of ornamental bromeliads, which like pineapple can be forced with ethephon. When AVG was commercialized in 1997, field trials were established to explore its potential to control NI of pineapple. Research over about a 10-year period showed that AVG could consistently control NI with an acceptable cost/benefit in commercial pineapple fields. AVG is registered for that use in Hawaii and a number of other countries where large commercial pineapple industries are located. From a historical perspective, when it comes to natural or forced induction of flowering, or its inhibition, all paths eventually lead to ethylene.

Structural basis for oligomerization of auxin transcriptional regulators

The plant hormone auxin is a key morphogenetic regulator acting from embryogenesis onwards. Transcriptional events in response to auxin are mediated by the auxin response factor (ARF) transcription factors and the Aux/IAA (IAA) transcriptional repressors. At low auxin concentrations, IAA repressors associate with ARF proteins and recruit corepressors that prevent auxin-induced gene expression. At higher auxin concentrations, IAAs are degraded and ARFs become free to regulate auxin-responsive genes. The interaction between ARFs and IAAs is thus central to auxin signalling and occurs through the highly conserved domain III/IV present in both types of proteins. Here, we report the crystal structure of ARF5 domain III/IV and reveal the molecular determinants of ARF-IAA interactions. We further provide evidence that ARFs have the potential to oligomerize, a property that could be important for gene regulation in response to auxin.

Adventitious rooting adjuvant activity of 1,3-di(benzo[d]oxazol-5-yl)urea and 1,3-di(benzo[d]oxazol-6-yl)urea: new insights and perspectives

Here we report new insights on the adventitious rooting adjuvant activity of 1,3-di(benzo[d]oxazol-5-yl)urea (5-BDPU) and 1,3-di(benzo[d]oxazol-6-yl)urea (6-BDPU), both symmetrically substituted urea derivatives that do not show either auxin- or cytokinin-like activity per se. Our data demonstrate that these synthetic molecules enhance adventitious rooting in distantly-related herbaceous and woody species, in the presence of endogenous or exogenous auxin. For the first time, we report that BDPUs enhance adventitious rooting in the presence of either indole-3-butyric acid (IBA) or 1-naphtalene acetic acid and that their optimal concentration depends on the strength of the exogenous auxin. Trying to understand the mode of action of BDPUs, we also show that their adventitious rooting adjuvant activity correlates with high mRNA levels of auxin-responsive genes related to the adventitious rooting process at the very early stages of adventitious rooting, before the activation of cell divisions in pine hypocotyls cuttings. The high mRNA levels are measured in the presence of low auxin concentrations and BDPUs. The mRNA levels quantified in these conditions are similar to those measured in the presence of high auxin concentrations but in the absence of BDPUs. In addition, the spatial distribution of endogenous auxin is localized in globular-shaped structures of cell divisions located centrifugal to the resin canals, at the positions of adventitious root formation, in the presence of urea derivatives and IBA after 6 days of the root induction process.

Factors affecting morphogenetic potential in oilseed rape roots of the Skrzeszowicki and Start cultivars

The effect of the origin of root segments, seedling age, growth substances and gelled or liquid media were tested in respect to the morphogenetic potential of rape root segments of Skrzeszowicki (high glucosinolate content) and Start (low glucosinolate content) cultivars. Callus and roots were formed on all root segments after an approximately 2 week growth period; buds were formed after ca. 4 weeks only on segments adjacent to the hypocotyl. Higher concentrations of auxin and cytokinins were required for bud induction. Cultivar differences in the morphogenetic responses of the root segments were found. They were manifested by the more abundant callus formation (BAP+NAA) and more numerous lateral roots and buds (KIN+IBA) on segments from the Skrzeszowicki cultivar than from the Start cultivar.

Optimization of multiple shoot induction and plant regeneration in Indian barley (Hordeum vulgare) cultivars using mature embryos

Barley is the fourth most important crop in the world. Development of a regeneration system using immature embryos is both time consuming and laborious. The present study was initiated with a view to develop a regeneration system in six genotypes of Indian barley (Hordeum vulgare) cultivars as a prerequisite to transformation. The mature embryos were excised from seeds and cultured on MS medium supplemented with high and low concentrations of cytokinins and auxins respectively. The MS medium containing 3mg/L N6-benzylaminopurine (BA) and 0.5mg/L 2,4-dichlorophenoxyacetic acid (2,4-D) was found to be the most effective for multiple shoot formation in HOR7231 cultivar that could produce 12 shoots per explant. The other cultivars HOR4409 and HOR3844 produced a minimum number of adventitious shoots (1.33 and 1.67 respectively) on MS medium supplemented with 1mg/L BA and 0.3mg/L 2,4-D. The elongated shoots were separated and successfully rooted on MS medium containing 1mg/L indole-3-acetic acid (IAA). The response of different barley cultivars was found to be varying with respect to multiple shoot production. This is the first report of multiple shoot induction and plantlet regeneration in Indian cultivar of barley which would be useful for genetic transformation.

Unifoliata-Afila interactions in pea leaf morphogenesis.

Processes of leaf morphogenesis provide the basis for the great diversity of leaf form among higher plants. The common garden pea (Pisum sativum) offers a developmental model system for understanding how gene and hormone interactions impart a large array of mutant leaf phenotypes. • To understand the role of auxin in AF and UNI gene function and their interaction, we compared the range of leaf phenotypes on afila (af) and unifoliata (uni) double mutants, examined the effects of these mutations on auxin levels, auxin transport, auxin response via DR5::GUS, and expression of auxin-regulated genes. • The adult leaves of af uni double mutants have leaflets and tendrils and typically possess two lateral pinna pairs and a terminal leaflet. The af mutants have higher auxin content, stronger auxin response, and higher expression of auxin responsive genes than wildtype. The uni mutant has reduced auxin content and transport, whereas the uni-tac mutant has higher auxin content and transport and reduced auxin response compared to wildtype. • Auxin concentration and response differences characterize the antagonistic relationship between AF and UNI in pea leaf development. The mechanism involves modulation of auxin mediated by one or both genes; UNI is expressed in and promotes high auxin levels, and AF suppresses auxin levels.

SEPALLATA1/2-suppressed mature apples have low ethylene, high auxin and reduced transcription of ripening-related genes

Fruit ripening is an important developmental trait in fleshy fruits, making the fruit palatable for seed dispersers. In some fruit species, there is a strong association between auxin concentrations and fruit ripening. We investigated the relationship between auxin concentrations and the onset of ethylene-related ripening in Malus × domestica (apples) at both the hormone and transcriptome levels. Transgenic apples suppressed for the SEPALLATA1/2 (SEP1/2) class of gene (MADS8/9) that showed severely reduced ripening were compared with untransformed control apples. In each apple type, free indole-3-acetic acid (IAA) concentrations were measured during early ripening. The changes observed in auxin were assessed in light of global changes in gene expression. It was found that mature MADS8/9-suppressed apples had a higher concentration of free IAA. This was associated with increased expression of the auxin biosynthetic genes in the indole-3-acetamide pathway. Additionally, in the MADS8/9-suppressed apples, there was less expression of the GH3 auxin-conjugating enzymes. A number of genes involved in the auxin-regulated transcription (AUX/IAA and ARF classes of genes) were also observed to change in expression, suggesting a mechanism for signal transduction at the start of ripening. The delay in ripening observed in MADS8/9-suppressed apples may be partly due to high auxin concentrations. We propose that, to achieve low auxin associated with fruit maturation, the auxin homeostasis is controlled in a two-pronged manner: (i) by the reduction in biosynthesis and (ii) by an increase in auxin conjugation. This is associated with the change in expression of auxin-signalling genes and the up-regulation of ripening-related genes.

Initiation und Etablierung von Keimblättern im Arabidopsis-Embryo

Cotyledons are specialised embryonic leaves whose initiation starts when the activities of the Ser/Thr-kinase PID and the NPH3-like protein ENP polarise the auxin transporter PIN1 in epidermal cells of the Arabidopsis embryo apex. The resulting directional auxin flux generates two opposing cell groups with high concentrations of the hormone auxin. These auxin maxima are essential for initiation and maintenance of cotyledons. We describe the interplay of factors participating in this process.