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

OsCYP22 Interacts With OsCSN5 to Affect Rice Root Growth and Auxin Signalling

Beyond structural support, plant root systems play crucial roles in the absorption of water and nutrients, fertiliser efficiency and crop yield. However, the molecular mechanism regulating root architecture in rice remains largely unknown. In this study, a short-root rice mutant was identified and named Oscyp22. Oscyp22 showed impairment in the growth of primary, adventitious and lateral roots. Histochemical and fluorescent staining analyses revealed reduced cell elongation and division activity in the root of Oscyp22. Further analysis showed that Oscyp22 displayed an impaired response to auxin treatment, indicating a disruption in the auxin signal transduction. Transcriptome analysis and auxin content measurement suggested that OsCYP22 might be involved in auxin synthesis and transport. Protein assays demonstrated that OsCYP22 could interact with OsCSN5 and induce its rapid degradation. Notably, Oscsn5 mutants also showed short root phenotypes and deficiencies in auxin response. These findings suggest that OsCYP22 plays a role in rice root growth potentially through auxin signalling and OsCSN5 stability.

Root Development of Tomato Plants Infected by the Cacao Pathogen Moniliophthora perniciosa Is Affected by Limited Sugar Availability

Moniliophthora perniciosa is the causal agent of the witches' broom disease of cacao (Theobroma cacao), and it can infect the tomato (Solanum lycopersicum) 'Micro-Tom' (MT) cultivar. Typical symptoms of infection are stem swelling and axillary shoot outgrowth, whereas reduction in root biomass is another side effect. Using infected MT, we investigated whether impaired root growth derives from hormonal imbalance or sink competition. Intense stem swelling coincided with a reduction in root biomass, predominantly affecting lateral roots. RNA-seq analyses of root samples identified only a few differentially expressed genes involved in hormone metabolism, and root hormone levels were not expressively altered. Inoculation of the auxin highly-sensitive entire mutant genotype maintained the impaired root phenotype; in contrast, the low-cytokinin MT transgenic line overexpressing CYTOKININ OXIDASE-2 (35S::AtCKX2) with fewer symptoms did not exhibit root growth impairment. Genes involved in cell wall, carbohydrate, and amino acid metabolism were downregulated, accompanied by lower levels of carbohydrate and amino acid in roots, suggesting a reduction in metabolite availability. 13CO2 was supplied to MT plants, and less 13C was detected in the roots of infected MT but not in those of 35S::AtCKX2 line plants, suggesting that cytokinin-mediated sugar sink establishment at the infection site may contribute to impaired root growth. Exogenous sucrose application to roots of infected MT plants partially restored root growth. We propose that the impairment of lateral root development is likely attributed to disrupted sugar signalling and photoassimilate supply by establishing a strong sugar sink at the infected stem.

Molecular, hormonal, and metabolic mechanisms of fruit set, the ovary-to-fruit transition, in horticultural crops

Fruit set is the process by which the ovary develops into a fruit and is an important factor in determining fruit yield. Fruit set is induced by two hormones, auxin and gibberellin, and the activation of their signaling pathways, partly by suppressing various negative regulators. Many studies have investigated the structural changes and gene networks in the ovary during fruit set, revealing the cytological and molecular mechanisms. In tomato (Solanum lycopersicum), SlIAA9 and SlDELLA/PROCERA act as auxin and gibberellin signaling repressors, respectively, and are important regulators of the activity of transcription factors and downstream gene expression involved in fruit set. Upon pollination, SlIAA9 and SlDELLA are degraded, which subsequently activates downstream cascades and mainly contributes to active cell division and cell elongation, respectively, in ovaries during fruit setting. According to current knowledge, the gibberellin pathway functions as the most downstream signal in fruit set induction, and therefore its role in fruit set has been extensively explored. Furthermore, multi-omics analysis has revealed the detailed dynamics of gene expression and metabolites downstream of gibberellins, highlighting the rapid activation of central carbon metabolism. This review will outline the relevant mechanisms at the molecular and metabolic levels during fruit set, particularly focusing on tomato.

SlIAA23-SlARF6 module controls arbuscular mycorrhizal symbiosis by regulating strigolactone biosynthesis in tomato

Auxins are a class of phytohormones with roles involved in the establishment and maintenance of the arbuscular mycorrhizal symbiosis (AMS). Auxin response factors (ARFs) and Auxin/Indole-acetic acids (AUX/IAAs), as two transcription factors of the auxin signaling pathway, coregulate the transcription of auxin response genes. However, the interrelation and regulatory mechanism of ARFs and AUX/IAAs in regulating AMS are still unclear. In this study, we found that the content of auxin in tomato roots increased sharply and revealed the importance of the auxin signaling pathway in the early stage of AMS. Notably, SlARF6 was found to play a negative role in AMF colonization. Silencing SlARF6 significantly increased the expression of AM-marker genes, as well as AMF-induced phosphorus uptake. SlIAA23 could interact with SlARF6 in vivo and in vitro, and promoted the AMS and phosphorus uptake. Interestingly, SlARF6 and SlIAA23 played a contrary role in strigolactone (SL) synthesis and accumulation in AMF-colonized roots of tomato plants. SlARF6 could directly bind to the AuxRE motif of the SlCCD8 promoter and inhibited its transcription, however, this effect was attenuated by SlIAA23 through interaction with SlARF6. Our results suggest that SlIAA23-SlARF6 coregulated tomato-AMS via an SL-dependent pathway, thus affecting phosphorus uptake in tomato plants.

Reduced auxin signalling through the cyclophilin gene DIAGEOTROPICA impacts tomato fruit development and metabolism during ripening

Auxin is an important hormone playing crucial roles during fruit growth and ripening; however, the metabolic impact of changes in auxin signalling during tomato (Solanum lycopersicum L.) ripening remains unclear. Here, we investigated the significance of changes in auxin signalling during different stages of fruit development by analysing changes in tomato fruit quality and primary metabolism using mutants with either lower or higher auxin sensitivity [diageotropica (dgt) and entire mutants, respectively]. Altered auxin sensitivity modifies metabolism, through direct impacts on fruit respiration and fruit growth. We verified that the dgt mutant plants exhibit reductions in fruit set, total fruit dry weight, fruit size, number of seeds per fruit, and fresh weight loss during post-harvest. Sugar accumulation was associated with delayed fruit ripening in dgt, probably connected with reduced ethylene levels and respiration, coupled with a lower rate of starch degradation. In contrast, despite exhibiting parthenocarpy, increased auxin perception (entire) did not alter fruit ripening, leading to only minor changes in primary metabolism. By performing a comprehensive analysis, our results connect auxin signalling and metabolic changes during tomato fruit development, indicating that reduced auxin signalling led to extensive changes in sugar concentration and starch metabolism during tomato fruit ripening.

Impaired auxin signaling increases vein and stomatal density but reduces hydraulic efficiency and ultimately net photosynthesis

Auxins are known to regulate xylem development in plants, but their effects on water transport efficiency are poorly known. Here we used tomato plants with the diageotropica mutation (dgt), which has impaired function of a cyclophilin 1 cis-trans isomerase involved in auxin signaling, and the corresponding wild type (WT) to explore the mutation's effects on plant hydraulics and leaf gas exchange. The xylem of the dgt mutant showed a reduced hydraulically weighted vessel diameter (Dh) (24-43%) and conduit number (25-58%) in petioles and stems, resulting in lower theoretical hydraulic conductivities (Kt); on the other hand, no changes in root Dh and Kt were observed. The measured stem and leaf hydraulic conductances of the dgt mutant were lower (up to 81%), in agreement with the Kt values; however, despite dgt and WT plants showing similar root Dh and Kt, the measured root hydraulic conductance of the dgt mutant was 75% lower. The dgt mutation increased the vein and stomatal density, which could potentially increase photosynthesis. Nevertheless, even though it had the same photosynthetic capacity as WT plants, the dgt mutant showed a photosynthetic rate c. 25% lower, coupled with a stomatal conductance reduction of 52%. These results clearly demonstrate that increases in minor vein and stomatal density only result in higher leaf gas exchange when accompanied by higher hydraulic efficiency.

Increased branching independent of strigolactone in cytokinin oxidase 2-overexpressing tomato is mediated by reduced auxin transport

Tomato production is influenced by shoot branching, which is controlled by different hormones. Here we produced tomato plants overexpressing the cytokinin-deactivating gene CYTOKININ OXYDASE 2 (CKX2). CKX2-overexpressing (CKX2-OE) plants showed an excessive growth of axillary shoots, the opposite phenotype expected for plants with reduced cytokinin content, as evidenced by LC-MS analysis and ARR5-GUS staining. The TCP transcription factor SlBRC1b was downregulated in the axillary buds of CKX2-OE and its excessive branching was dependent on a functional version of the GRAS-family gene LATERAL SUPPRESSOR (LS). Grafting experiments indicated that increased branching in CKX2-OE plants is unlikely to be mediated by root-derived signals. Crossing CKX2-OE plants with transgenic antisense plants for the strigolactone biosynthesis gene CAROTENOID CLEAVAGE DIOXYGENASE (CCD7-AS) produced an additive phenotype, indicating independent effects of cytokinin and strigolactones on increased branching. On the other hand, CKX2-OE plants showed reduced polar auxin transport and their bud outgrowth was reduced when combined with auxin mutants. Accordingly, CKX2-OE basal buds did not respond to auxin applied in the decapitated apex. Our results suggest that tomato shoot branching depends on a fine-tuning of different hormonal balances and that perturbations in the auxin status could compensate for the reduced cytokinin levels in CKX2-OE plants.

OsCyp2-P, an auxin-responsive cyclophilin, regulates Ca2+ calmodulin interaction for an ion-mediated stress response in rice

OsCYP2-P is an active cyclophilin (having peptidyl-prolyl cis/trans-isomerase activity, PPIase) isolated from the wild rice Pokkali having a natural capacity to grow and yield seeds in coastal saline regions of India. Transcript abundance analysis in rice seedlings showed the gene is inducible by multiple stresses, including salinity, drought, high temperature, and heavy metals. To dissect the role of OsCYP2-P gene in stress response, we raised overexpression (OE) and knockdown (KD) transgenic rice plants with >2-3 folds higher and approximately 2-fold lower PPIase activity, respectively. Plants overexpressing this gene had more favorable physiological and biochemical parameters (K⁺ /Na⁺ ratio, electrolytic leakage, membrane damage, antioxidant enzymes) than wild type, and the reverse was observed in plants that were knocked down for this gene. We propose that OsCYP2-P contributes to stress tolerance via maintenance of ion homeostasis and thus prevents toxic cellular ion buildup and membrane damage. OE plants were found to have a higher harvest index and higher number of filled grains under salinity and drought stress than wild type. OsCYP2-P interacts with calmodulin, indicating it functions via the Ca-CaM pathway. Compared to the WT, the germinating OE seeds exhibited a substantially higher auxin level, and this hormone was below the detection limits in the WT and KD lines. These observations strongly indicate that OsCyp2-P affects the signaling and transport of auxin in rice.

Missense mutation of a class B heat shock factor is responsible for the tomato bushy root-2 phenotype

The bushy root-2 (brt-2) tomato mutant has twisting roots, and slower plant development. Here we used whole genome resequencing and genetic mapping to show that brt-2 is caused by a serine to cysteine (S75C) substitution in the DNA binding domain (DBD) of a heat shock factor class B (HsfB) encoded by SolycHsfB4a. This gene is orthologous to the Arabidopsis SCHIZORIZA gene, also known as AtHsfB4. The brt-2 phenotype is very similar to Arabidopsis lines in which the function of AtHsfB4 is altered: a proliferation of lateral root cap and root meristematic tissues, and a tendency for lateral root cap cells to easily separate. The brt-2 S75C mutation is unusual because all other reported amino acid substitutions in the highly conserved DBD of eukaryotic heat shock factors are dominant negative mutations, but brt-2 is recessive. We further show through reciprocal grafting that brt-2 exerts its effects predominantly through the root genotype even through BRT-2 is expressed at similar levels in both root and shoot meristems. Since AtHsfB4 is induced by root knot nematodes (RKN), and loss-of-function mutants of this gene are resistant to RKNs, BRT-2 could be a target gene for RKN resistance, an important trait in tomato rootstock breeding.Gene & accession numbersSolycHsfB4a - Solyc04g078770.

Moniliophthora perniciosa, the causal agent of witches' broom disease of cacao, interferes with cytokinin metabolism during infection of Micro-Tom tomato and promotes symptom development

Moniliophthora perniciosa causes witches' broom disease of cacao and inflicts symptoms suggestive of hormonal imbalance. We investigated whether infection of the tomato (Solanum lycopersicum) model system Micro-Tom (MT) by the Solanaceae (S)-biotype of Moniliophthora perniciosa, which causes stem swelling and hypertrophic growth of axillary shoots, results from changes in host cytokinin metabolism. Inoculation of an MT-transgenic line that overexpresses the Arabidopsis CYTOKININ OXIDASE-2 gene (35S::AtCKX2) resulted in a reduction in disease incidence and stem diameter. RNA-sequencing analysis of infected MT and 35S::AtCKX2 revealed the activation of cytokinin-responsive marker genes when symptoms were conspicuous. The expression of an Moniliophthora perniciosa tRNA-ISOPENTENYL-TRANSFERASE suggests the production of isopentenyladenine (iP), detected in mycelia grown in vitro. Inoculated MT stems showed higher levels of dihydrozeatin and trans-zeatin but not iP. The application of benzyladenine induced symptoms similar to infection, whereas applying the cytokinin receptor inhibitors LGR-991 and PI55 decreased symptoms. Moniliophthora perniciosa produces iP that might contribute to cytokinin synthesis by the host, which results in vascular and cortex enlargement, axillary shoot outgrowth, reduction in root biomass and an increase in fruit locule number. This strategy may be associated with the manipulation of sink establishment to favour infection by the fungus.

The auxin-producing Bacillus thuringiensis RZ2MS9 promotes the growth and modifies the root architecture of tomato (Solanum lycopersicum cv. Micro-Tom)

Strains of Bacillus thuringiensis (Bt) are commonly commercialized as bioinoculants for insect pest control, but their benefits go beyond their insecticidal property: they can act as plant growth-promoters. Auxins play a major role in the plant growth promotion. However, the mechanism of auxin production by the Bacilli group, and more specifically by Bt strains, is unclear. In previous work, the plant growth-promoting rhizobacterium (PGPR) B. thuringiensis strain RZ2MS9 increased the corn roots. This drew our attention to the strain's auxin production trait, earlier detected in vitro. Here, we demonstrate that in its genome, RZ2MS9 harbours the complete set of genes required in two pathways that are used for Indole acetic acid (IAA) production. We also detected that the strain produces almost five times more IAA during the stationary phase. The bacterial application increased the shoot dry weight of the Micro-Tom (MT) tomato by 24%. The application also modified MT root architecture, with an increase of 26% in the average lateral root length and inhibition of the axial root. At the cellular level, RZ2MS9-treated MT plants presented elongated root cortical cells with intensified mitotic activity. Altogether, these are the best characterized auxin-associated phenotypes. Besides that, no growth alteration was detected in the auxin-insensitive diageotropic (dgt) plants either with or without the RZ2MS9 inoculation. Our results suggest that auxins play an important role in the ability of B. thuringiensis RZ2MS9 to promote MT growth and provide a better understanding of the auxin production mechanism by a Bt strain.

Adventitious root formation in crops-Potato as an example

The root system of potato is made up of adventitious roots (AR) that form at the base of a sprout once it emerges from the mother tuber. By definition, AR originate from dormant preformed meristems, or from cells neighboring vascular tissues in stems or leaves. This may occur as part of the developmental program of the plant (e.g., potato), or when replacing the embryonic primary roots in response to stress conditions, such as flooding, nutrient deprivation, or wounding. AR formation is studied mainly in cereals and model plants, and less is known about its developmental program in root and tuber crops. In this review, we summarize the recent data on AR development in potato and relate this knowledge to what is known from model plants. For example, AR formation following stem cutting in potato follows a pattern of initiation, expression, and emergence phases that are known for other plants and involves auxin, the master regulator of AR induction and development. Molecular regulation of AR formation and the effect of environmental stresses are discussed. Understanding the origin and nature of AR systems in important crops will contribute to increased production and improve global food security.

An auxin-mediated regulatory framework for wound-induced adventitious root formation in tomato shoot explants

Adventitious roots (ARs) are produced from non-root tissues in response to different environmental signals, such as abiotic stresses, or after wounding, in a complex developmental process that requires hormonal crosstalk. Here, we characterized AR formation in young seedlings of Solanum lycopersicum cv. 'Micro-Tom' after whole root excision by means of physiological, genetic and molecular approaches. We found that a regulated basipetal auxin transport from the shoot and local auxin biosynthesis triggered by wounding are both required for the re-establishment of internal auxin gradients within the vasculature. This promotes cell proliferation at the distal cambium near the wound in well-defined positions of the basal hypocotyl and during a narrow developmental window. In addition, a pre-established pattern of differential auxin responses along the apical-basal axis of the hypocotyl and an as of yet unknown cell-autonomous inhibitory pathway contribute to the temporal and spatial patterning of the newly formed ARs on isolated hypocotyl explants. Our work provides an experimental outline for the dissection of wound-induced AR formation in tomato, a species that is suitable for molecular identification of gene regulatory networks via forward and reverse genetics approaches.

A quick protocol for the identification and characterization of early growth mutants in tomato

Root system architecture (RSA) manipulation may improve water and nutrient capture by plants under normal and extreme climate conditions. With the aim of initiating the genetic dissection of RSA in tomato, we established a defined ontology that allowed the curated annotation of the observed phenotypes on 12 traits at four consecutive growth stages. In addition, we established a quick approach for the molecular identification of the mutations associated with the trait-of-interest by using a whole-genome sequencing approach that does not require the building of an additional mapping population. As a proof-of-concept, we screened 4543 seedlings from 300 tomato M₃ lines (Solanum lycopersicum L. cv. Micro-Tom) generated by chemical mutagenesis with ethyl methanesulfonate. We studied the growth and early development of both the root system (primary and lateral roots) and the aerial part of the seedlings as well as the wound-induced adventitious roots emerging from the hypocotyl. We identified 659 individuals (belonging to 203 M₃ lines) whose early seedling and RSA phenotypes differed from those of their reference background. We confirmed the genetic segregation of the mutant phenotypes affecting primary root length, seedling viability and early RSA in 31 M₄ families derived from 15 M₃ lines selected in our screen. Finally, we identified a missense mutation in the SlCESA3 gene causing a seedling-lethal phenotype with short roots. Our results validated the experimental approach used for the identification of tomato mutants during early growth, which will allow the molecular identification of the genes involved.

Low auxin sensitivity of diageotropica tomato mutant alters nitrogen deficiency response

Plant responses to nitrogen supply are dependent on auxin signaling, but much still remains to be elucidated regarding N deficiency in tomato. Thus, the objective of this work was to evaluate how low auxin sensitivity regulates the responses of tomato plants to N deficiency. For this purpose, we used the tomato diageotropica mutant, with low auxin sensitivity, and a near isogenic line cv. Micro-Tom grown in nutrient solutions under absence and presence of nitrogen. Plant height, stem diameter, root and shoot dry mass, area and root density, number of lateral roots, leaf area, chlorophylls and carotenoids content, nitrogen accumulation and nitrogen use efficiency were evaluated. We observed a clear interaction between the tomato genotype and nitrogen. When the plants were grown with nitrogen, 'Micro-Tom' showed higher growth than the diageotropica mutant. Under nitrogen deficiency condition, the mutant showed improved growth, nitrogen use efficiency and higher contents of pigments. In general, the low sensitivity to auxin in diageotropica caused reduced growth in both shoot and root. However, the diageotropica tomato showed a positive regulation of the nitrogen use efficiency under nitrogen deficiency. In general, our data revealed that the reduced sensitivity to auxin increased the adaptive capacity to the nitrogen deficiency.

RNA-Seq analysis of genes affected by Cyclophilin A/DIAGEOTROPICA (DGT) in tomato root development

Cyclophilin A/DIAGEOTROPICA (DGT) has been linked to auxin-regulated development in tomato and appears to affect multiple developmental pathways. Loss of DGT function results in a pleiotropic phenotype that is strongest in the roots, including shortened roots with no lateral branching. Here, we present an RNA-Seq dataset comparing the gene expression profiles of wildtype (‘Ailsa Craig’) and dgt tissues from three spatially separated developmental stages of the tomato root tip, with three replicates for each tissue and genotype. We also identify differentially expressed genes, provide an initial comparison of genes affected in each genotype and tissue, and provide the pipeline used to analyze the data. Further analysis of this dataset can be used to gain insight into the effects of DGT on various root developmental pathways in tomato.

Long-distance regulation of shoot gravitropism by Cyclophilin 1 in tomato (Solanum lycopersicum) plants

The phloem-mobile protein SlCyp1 traffics to distant parts of the shoot to regulate its gravitropic response. In addition, SlCyp1 targets specific cells in the root to promote lateral root development. The tomato (Solanum lycopersicum) Cyclophilin 1 (SlCyp1) gene encodes a peptidyl-prolyl isomerase required for auxin response, lateral root development and gravitropic growth. The SlCyp1 protein is a phloem-mobile signal that moves from shoot to root to regulate lateral root development (Spiegelman et al., Plant J 83:853-863, 2015; J Exp Bot 68:953-964, 2017a). Here, we explored the mechanism of SlCyp1 movement by fusing it to the fluorescent protein mCherry. We found that, once trafficked to the root, SlCyp1 is unloaded from the phloem to the surrounding tissues, including the pericycle and lateral root primordia. Interestingly, SlCyp1 not only moves to the root system, but also to distant parts of the shoot. Grafting of the SlCyp1 mutant diageotropica (dgt) scions on VFN8 control rootstocks resulted in recovery of dgt shoot gravitropism, which was associated with the restoration of auxin-response capacity. Application of the cyclophilin inhibitor cyclosporine A suppressed gravitropic recovery, indicating that SlCyp1 must be active in the target tissue to affect the gravitropic response. These results provide new insights on the mechanism of SlCyp1 transport and functioning as a long-distance signal regulating shoot gravitropism.

The auxin-resistant dgt tomato mutant grows less than the wild type but is less sensitive to ammonium toxicity and nitrogen deficiency

The low-auxin-sensitivity tomato mutant, dgt, despite displaying reduced plant growth, has been linked to greater resistance to N deficiency. This led us to test the role of auxin resistance of dgt in NH₄⁺ toxicity and N deficiency, compared to wild type tomato (cv. Micro-Tom, MT), grown in hydroponic media. A completely randomized design with three replications in a 2 × 4 factorial scheme was adopted, corresponding to the two tomato genotypes (MT and dgt), involving four nutritional treatments: NO₃^- (5 mM); NH₄⁺ (5 mM); NO₃^- (5 mM) plus exogenous auxin (10 μM IAA); and N omission. The results show that NH₄⁺ was toxic to MT but not to dgt. Under N deficiency, MT displayed a lower shoot NO₃^- content, a lower photosynthetic rate, and a decrease in both shoot and root dry weight. However, in dgt, no difference was observed in shoot NO₃^- content and photosynthetic rate between plants grown on NO₃^- or under N deficiency. In addition, dgt showed an increase in shoot dry weight under N deficiency. We highlight the role of auxin resistance in the adaptation of plants to NH₄⁺ toxicity and N deficiency.

Sequence characteristics of Medicago truncatula cyclophilin family members and function analysis of MsCYP20-3B involved in axillary shoot development

Cyclophilins (CYPs) belonging to the immunophilin family are present in all organisms and widely distributed in various cells associated with the activity of peptidyl-prolyl cis/trans isomerase. Plant CYPs are members of a multi-gene family and are involved in a series of biological processes. However, little is known about their structure, evolution, developmental expression and functional analysis in Medicago truncatula. In this study, a total of 33 CYP genes were identified and found to be unevenly distributed on eight chromosomes. Among them, 21 are single-domain and 12 are multi-domain proteins, and most were predicted to be localized in the cytosol, nucleus or chloroplast. Phylogenetic and gene structure analysis revealed seven segmental gene pairs, indicating that segmental duplication probably made a large contribution to the expansion of MtCYP gene family. Furthermore, gene expression analysis revealed that about 10 MtCYP genes (were) highly expressed involved in vegetative and reproduction tissues in M. truncatula, and MsCYP20-3B was mainly upregulated in stems, leaves and flower buds in alfalfa (Medicago sativa). Overexpression of MsCYP20-3B was shown to regulate axillary shoot development associated with higher jasmonic acid and abscisic acid contents in M. truncatula. Our study suggests the importance of the CYP genes family in development, reproduction and stress responses, and provides a reference for future studies and application of CYP genes for alfalfa genetic improvement.

Waterlogging-induced adventitious root formation in cucumber is regulated by ethylene and auxin through reactive oxygen species signalling

Development of adventitious roots (ARs) at the base of the shoot is an important adaptation of plants to waterlogging stress; however, its physiological mechanisms remain unclear. Here, we investigated the regulation of AR formation under waterlogged conditions by hormones and reactive oxygen species (ROS) in Cucumis sativus L., an agriculturally and economically important crop in China. We found that ethylene, auxin, and ROS accumulated in the waterlogged cucumber plants. On the other hand, application of the ethylene receptor inhibitor 1-methylcyclopropene (1-MCP), the auxin transport inhibitor 1-naphthylphthalamic acid (NPA), or the NADPH oxidase inhibitor diphenyleneiodonium (DPI) decreased the number of ARs induced by waterlogging. Auxin enhanced the expression of ethylene biosynthesis genes, which led to ethylene entrapment in waterlogged plants. Both ethylene and auxin induced the generation of ROS. Auxin-induced AR formation was inhibited by 1-MCP, although ethylene-induced AR formation was not inhibited by NPA. Both ethylene- and auxin-induced AR formation were counteracted by DPI. These results indicate that auxin-induced AR formation is dependent on ethylene, whereas ethylene-induced AR formation is independent of auxin. They also show that ROS signals mediate both ethylene- and auxin-induced AR formation in cucumber plants.

Gibberellins modulate auxin responses during tomato (Solanum lycopersicum L.) fruit development

In tomato, auxin and gibberellins (GAs) interact with each other to drive fruit growth and development. While the role of auxin in directing GA biosynthesis and signal is already known, very little information has been obtained about GA-mediated control of auxin signalling and response. Interestingly, we show that gibberellic acid (GA₃ ) is able to modify the expression of several auxin signalling genes in the partial auxin-insensitive diageotropica (dgt) mutant, suggesting that GAs may override the control of DGT on auxin signal. Procera (pro) mutation, which confers a constitutively active GA signal, enhances the effects of exogenous auxin, indicating that PRO may act as a negative effector of auxin responses in fruits. Indeed, transcript modulation of some auxin/indole acetic acid and auxin response factor genes in auxin-treated dgt/pro fruits suggests that PRO controls their expression possibly bypassing DGT. It was also shown that GA biosynthesis, in response to auxin treatment, is largely controlled by DGT. It is therefore conceivable that the DGT-mediated increase of active GAs in auxin-treated or pollinated fruits would promote PRO degradation, which in turn activates part of the auxin signalling cascade.

GhCyP3 improves the resistance of cotton to Verticillium dahliae by inhibiting the E3 ubiquitin ligase activity of GhPUB17

A U-box E3 ubiquitin ligase GhPUB17 is inhibited by GhCyP3 with antifungal activity and acts as a negative regulator involved in cotton resistance to Verticillium dahliae. E3 ubiquitin ligases, the key component enzymes of the ubiquitin-proteasome system, which contains the most diverse structural and functional members involved in the determination of target specificity and the regulation of metabolism, have been well documented in previous studies. Here, we identify GhPUB17, a U-box E3 ligase in cotton that has ubiquitination activity and is involved in the cotton immune response to Verticillium dahliae. The expression level of GhPUB17 is downregulated in the ssn mutant with a constitutively activated immune response (Sun et al., Nat Commun 5:5372, 2014). Infection with V. dahliae or exogenous hormone treatment, including jasmonic acid and salicylic acid, significantly upregulated GhPUB17 in cotton roots, which suggested a possible role for this E3 ligase in the plant immune response to pathogens. Moreover, GhPUB17-knockdown cotton plants are more resistant to V. dahliae, whereas GhPUB17-overexpressing plants are more susceptible to the pathogen, which indicated that GhPUB17 is a negative regulator of cotton resistance to V. dahliae. A yeast two-hybrid (Y2H) assay identified GhCyP3 as a protein that interacts with GhPUB17, and this finding was confirmed by further protein interaction assays. The downregulation of GhCyP3 in cotton seedlings attenuated the plants' resistance to V. dahliae. In addition, GhCyP3 showed antifungal activity against V. dahliae, and the E3 ligase activity of GhPUB17 was repressed by GhCyP3 in vitro. These results suggest that GhPUB17 negatively regulates cotton immunity to V. dahliae and that the antifungal protein GhCyP3 likely interacts with and inhibits the ligase activity of GhPUB17 and plays an important role in the cotton-Verticillium interaction.

Solanum lycopersicum GOLDEN 2-LIKE 2 transcription factor affects fruit quality in a light- and auxin-dependent manner

Plastids are organelles responsible for essential aspects of plant development, including carbon fixation and synthesis of several secondary metabolites. Chloroplast differentiation and activity are highly regulated by light, and several proteins involved in these processes have been characterised. Such is the case of the GOLDEN 2-LIKE (GLK) transcription factors, which induces the expression of genes related to chloroplast differentiation and photosynthesis. The tomato (Solanum lycopersicum) genome harbours two copies of this gene, SlGLK1 and SlGLK2, each with distinct expression patterns. While the former predominates in leaves, the latter is mainly expressed in fruits, precisely at the pedicel region. During tomato domestication, the selection of fruits with uniform ripening fixed the mutation Slglk2, nowadays present in most cultivated varieties, what penalised fruit metabolic composition. In this study, we investigated how SlGLK2 is regulated by light, auxin and cytokinin and determined the effect of SlGLK2 on tocopherol (vitamin E) and sugar metabolism, which are components of the fruit nutritional and industrial quality. To achieve this, transcriptional profiling and biochemical analysis were performed throughout fruit development and ripening from SlGLK2, Slglk2, SlGLK2-overexpressing genotypes, as well as from phytochrome and hormonal deficient mutants. The results revealed that SlGLK2 expression is regulated by phytochrome-mediated light perception, yet this gene can induce chloroplast differentiation even in a phytochrome-independent manner. Moreover, auxin was found to be a negative regulator of SlGLK2 expression, while SlGLK2 enhances cytokinin responsiveness. Additionally, SlGLK2 enhanced chlorophyll content in immature green fruits, leading to an increment in tocopherol level in ripe fruits. Finally, SlGLK2 overexpression resulted in higher total soluble solid content, possibly by the regulation of sugar metabolism enzyme-encoding genes. The results obtained here shed light on the regulatory network that interconnects SlGLK2, phytohormones and light signal, promoting the plastidial activity and consequently, influencing the quality of tomato fruit.

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.

LBD16 and LBD18 acting downstream of ARF7 and ARF19 are involved in adventitious root formation in Arabidopsis

BACKGROUND

Adventitious root (AR) formation is a complex genetic trait, which is controlled by various endogenous and environmental cues. Auxin is known to play a central role in AR formation; however, the mechanisms underlying this role are not well understood.

RESULTS

In this study, we showed that a previously identified auxin signaling module, AUXIN RESPONSE FACTOR(ARF)7/ARF19-LATERAL ORGAN BOUNDARIES DOMAIN(LBD)16/LBD18 via AUXIN1(AUX1)/LIKE-AUXIN3 (LAX3) auxin influx carriers, which plays important roles in lateral root formation, is involved in AR formation in Arabidopsis. In aux1, lax3, arf7, arf19, lbd16 and lbd18 single mutants, we observed reduced numbers of ARs than in the wild type. Double and triple mutants exhibited an additional decrease in AR numbers compared with the corresponding single or double mutants, respectively, and the aux1 lax3 lbd16 lbd18 quadruple mutant was devoid of ARs. Expression of LBD16 or LBD18 under their own promoters in lbd16 or lbd18 mutants rescued the reduced number of ARs to wild-type levels. LBD16 or LBD18 fused to a dominant SRDX repressor suppressed promoter activity of the cell cycle gene, Cyclin-Dependent Kinase(CDK)A1;1, to some extent. Expression of LBD16 or LBD18 was significantly reduced in arf7 and arf19 mutants during AR formation in a light-dependent manner, but not in arf6 and arf8. GUS expression analysis of promoter-GUS reporter transgenic lines revealed overlapping expression patterns for LBD16, LBD18, ARF7, ARF19 and LAX3 in AR primordia.

CONCLUSION

These results suggest that the ARF7/ARF19-LBD16/LBD18 transcriptional module via the AUX1/LAX3 auxin influx carriers plays an important role in AR formation in Arabidopsis.

Genome-Wide Identification of Cyclophilin Gene Family in Cotton and Expression Analysis of the Fibre Development in Gossypium barbadense

Cyclophilins (CYPs) are a member of the immunophilin superfamily (in addition to FKBPs and parvulins) and play a significant role in peptidyl-prolyl cis-trans isomerase (PPIase) activity. Previous studies have shown that CYPs have important functions in plants, but no genome-wide analysis of the cotton CYP gene family has been reported, and the specific biological function of this gene is still elusive. Based on the release of the cotton genome sequence, we identified 75, 78, 40 and 38 CYP gene sequences from G. barbadense, G. hirsutum, G. arboreum, and G. raimondii, respectively; 221 CYP genes were unequally located on chromosomes. Phylogenetic analysis showed that 231 CYP genes clustered into three major groups and eight subgroups. Collinearity analysis showed that segmental duplications played a significant role in the expansion of CYP members in cotton. There were light-responsiveness, abiotic-stress and hormone-response elements upstream of most of the CYPs. In addition, the motif composition analysis revealed that 49 cyclophilin proteins had extra domains, including TPR (tetratricopeptide repeat), coiled coil, U-box, RRM (RNA recognition motif), WD40 (RNA recognition motif) and zinc finger domains, along with the cyclophilin-like domain (CLD). The expression patterns based on qRT-PCR showed that six CYP expression levels showed greater differences between Xinhai21 (long fibres, G. barbadense) and Ashmon (short fibres, G. barbadense) at 10 and 20 days postanthesis (DPA). These results signified that CYP genes are involved in the elongation stage of cotton fibre development. This study provides a valuable resource for further investigations of CYP gene functions and molecular mechanisms in cotton.

Impairment of hormone pathways results in a general disturbance of fruit primary metabolism in tomato

Fruit metabolites are regulated by different phytohormones; however, this needs to be investigated. Dynamic metabolite profiling, based on gas chromatography-mass spectrometry, has been conducted on the fruit of tomato cultivar Micro-Tom and its five hormone mutants: dpy, not, dgt, epi and pro. In total, 48 metabolites were quantified, including sugars, organic acids and amino acids. The results demonstrated that ABA had a greater effect on the regulation of primary metabolism in tomato fruit, while ethylene can play an important role in the transition of primary to secondary metabolism. Besides, results from enzyme activities and transcript abundance involved in primary metabolism suggested that AIV and HXK4 could play key roles in the accumulation of the main sugars. To the best of our knowledge, this is the first comprehensive analysis of the link between hormone and metabolite change during fruit development in a collection of mutants with diverse hormone pathways.

Whole Root Transcriptomic Analysis Suggests a Role for Auxin Pathways in Resistance to Ralstonia solanacearum in Tomato

The soilborne pathogen Ralstonia solanacearum is the causal agent of bacterial wilt and causes significant crop loss in the Solanaceae family. The pathogen first infects roots, which are a critical source of resistance in tomato (Solanum lycopersicum L.). Roots of both resistant and susceptible plants are colonized by the pathogen, yet rootstocks can provide significant levels of resistance. Currently, mechanisms of this 'root-mediated resistance' remain largely unknown. To identify the molecular basis of this resistance, we analyzed the genome-wide transcriptional response of roots of resistant 'Hawaii 7996' and susceptible 'West Virginia 700' (WV) tomatoes at multiple timepoints after inoculation with R. solanacearum. We found that defense pathways in roots of the resistant Hawaii 7996 are activated earlier and more strongly than roots of susceptible WV. Further, auxin signaling and transport pathways are suppressed in roots of the resistant variety. Functional analysis of an auxin transport mutant in tomato revealed a role for auxin pathways in bacterial wilt. Together, our results suggest that roots mediate resistance to R. solanacearum through genome-wide transcriptomic changes that result in strong activation of defense genes and alteration of auxin pathways.

Comparative Analysis of Fruit Ripening-Related miRNAs and Their Targets in Blueberry Using Small RNA and Degradome Sequencing

MicroRNAs (miRNAs) play vital roles in the regulation of fruit development and ripening. Blueberry is an important small berry fruit crop with economical and nutritional value. However, nothing is known about the miRNAs and their targets involved in blueberry fruit ripening. In this study, using high-throughput sequencing of small RNAs, 84 known miRNAs belonging to 28 families and 16 novel miRNAs were identified in white fruit (WF) and blue fruit (BF) libraries, which represent fruit ripening onset and in progress, respectively. Among them, 41 miRNAs were shown to be differentially expressed during fruit maturation, and 16 miRNAs representing 16 families were further chosen to validate the sRNA sequencing data by stem-loop qRT-PCR. Meanwhile, 178 targets were identified for 41 known and 7 novel miRNAs in WF and BF libraries using degradome sequencing, and targets of miR160 were validated using RLM-RACE (RNA Ligase-Mediated (RLM)-Rapid Amplification of cDNA Ends) approach. Moreover, the expression patterns of 6 miRNAs and their targets were examined during fruit development and ripening. Finally, integrative analysis of miRNAs and their targets revealed a complex miRNA-mRNA regulatory network involving a wide variety of biological processes. The findings will facilitate future investigations of the miRNA-mediated mechanisms that regulate fruit development and ripening in blueberry.

Down-regulation of SlCyp1 in the phloem reduces auxin response and photosynthetic rate in tomato (Solanum lycopersicum) plants

The tomato dgt mutant, containing a single mutation in the Cyclophilin1 (SlCyp1) gene, is auxin insensitive and exhibits a pleotropic phenotype that includes lack of lateral roots, malformed xylem structure and reduced root-to-shoot ratio. Recently, we found that the SlCyp1 protein is phloem-mobile and traffic from shoot to root to induce lateral root formation. These processes are achieved through activation of auxin-mediated developmental programs. Inhibition of the trafficked SlCyp1 activity at the target site resulted in inhibition of the auxin response, supporting the hypothesis that this protein is indeed a mobile signal. Here, we show that partial silencing of SlCyp1 in the phloem only resulted in perturbed auxin response in the roots and reduced photosynthetic and transpiration rates. The presented data suggests that expression of SlCyp1 in the phloem is essential for proper auxin response at the whole plant level. We, therefore, propose that this protein acts as a long-distance signaling molecule acting as coordinator between roots and shoot activities.

Function of Cyclophilin1 as a long-distance signal molecule in the phloem of tomato plants

Tomato (Solanum lycopersicum) diageotropica (dgt) mutants, containing a single mutation in the Cyclophilin1 (SlCyp1) gene, are auxin-insensitive, exhibiting a pleiotropic phenotype including lack of geotropism, abnormal xylem structure, lack of lateral roots (LRs), and elevated shoot-to-root ratio. SlCyp1 is a putative peptidyl-prolyl isomerase that can traffic from shoot to root, where it induces changes in auxin response, LR formation, and xylem development, suggesting it has a role as a long-distance signaling molecule. Here, we explored the mechanism underlying SlCyp1 function in the phloem. Expression of SlCyp1 under a phloem-specific (AtSuc2) promoter in dgt plants partially restored the wild-type phenotype, including lateral root development, root branching, and xylem morphology. The observed developmental changes were associated with physiological alternations at the whole-plant level, including a reduction in shoot-to-root ratio, enhanced transpiration, and elevated photosynthetic rates. Conversely, phloem-specific expression of SlCyp1 active-site mutants did not restore the wild-type phenotype. Local inhibition of cyclophilin functioning in the target tissue reduced auxin sensitivity, suggesting that its enzymatic activity in the distant organ is required for its action as a long-distance signalling agent. The data presented suggest that SlCyp1 is a signal molecule trafficking from shoot to root where its activity is required for auxin-mediated lateral root development.

Master and servant: Regulation of auxin transporters by FKBPs and cyclophilins

Plant development and architecture are greatly influenced by the polar distribution of the essential hormone auxin. The directional influx and efflux of auxin from plant cells depends primarily on AUX1/LAX, PIN, and ABCB/PGP/MDR families of auxin transport proteins. The functional analysis of these proteins has progressed rapidly within the last decade thanks to the establishment of heterologous auxin transport systems. Heterologous co-expression allowed also for the testing of protein-protein interactions involved in the regulation of transporters and identified relationships with members of the FK506-Binding Protein (FKBP) and cyclophilin protein families, which are best known in non-plant systems as cellular receptors for the immunosuppressant drugs, FK506 and cyclosporin A, respectively. Current evidence that such interactions affect membrane trafficking, and potentially the activity of auxin transporters is reviewed. We also propose that FKBPs andcyclophilins might integrate the action of auxin transport inhibitors, such as NPA, on members of the ABCB and PIN family, respectively. Finally, we outline open questions that might be useful for further elucidation of the role of immunophilins as regulators (servants) of auxin transporters (masters).

Structural Biology of Nuclear Auxin Action

Auxin coordinates plant development largely via hierarchical control of gene expression. During the past decades, the study of early auxin genes paired with the power of Arabidopsis genetics have unraveled key nuclear components and molecular interactions that perceive the hormone and activate primary response genes. Recent research in the realm of structural biology allowed unprecedented insight into: (i) the recognition of auxin-responsive DNA elements by auxin transcription factors; (ii) the inactivation of those auxin response factors by early auxin-inducible repressors; and (iii) the activation of target genes by auxin-triggered repressor degradation. The biophysical studies reviewed here provide an impetus for elucidating the molecular determinants of the intricate interactions between core components of the nuclear auxin response module.

The Physiology of Adventitious Roots

Adventitious roots are plant roots that form from any nonroot tissue and are produced both during normal development (crown roots on cereals and nodal roots on strawberry [Fragaria spp.]) and in response to stress conditions, such as flooding, nutrient deprivation, and wounding. They are important economically (for cuttings and food production), ecologically (environmental stress response), and for human existence (food production). To improve sustainable food production under environmentally extreme conditions, it is important to understand the adventitious root development of crops both in normal and stressed conditions. Therefore, understanding the regulation and physiology of adventitious root formation is critical for breeding programs. Recent work shows that different adventitious root types are regulated differently, and here, we propose clear definitions of these classes. We use three case studies to summarize the physiology of adventitious root development in response to flooding (case study 1), nutrient deficiency (case study 2), and wounding (case study 3).

A tomato phloem-mobile protein regulates the shoot-to-root ratio by mediating the auxin response in distant organs

The plant vascular system serves as a conduit for delivery of both nutrients and signaling molecules to various distantly located organs. The anucleate sieve tube system of the angiosperm phloem delivers sugars and amino acids to developing organs, and has recently been shown to contain a unique population of RNA and proteins. Grafting studies have established that a number of these macromolecules are capable of moving long distances between tissues, thus providing support for operation of a phloem-mediated inter-organ communication network. Currently, our knowledge of the roles played by such phloem-borne macromolecules is in its infancy. Here, we show that, in tomato, translocation of a phloem-mobile cyclophilin, SlCyp1, from a wild-type scion into a mutant rootstock results in restoration of vascular development and lateral root initiation. This process occurs through reactivation of auxin response pathways and reprogramming of the root transcriptome. Moreover, we show that long-distance trafficking of SlCyp1 is associated with regulation of the shoot-to-root ratio in response to changing light intensities, by modulating root growth. We conclude that long-distance trafficking of SlCyp1 acts as a rheostat to control the shoot-to-root ratio, by mediating root development to integrate photosynthesis and light intensity with requirements for access to water and mineral nutrients.

Peptidyl-prolyl isomerization targets rice Aux/IAAs for proteasomal degradation during auxin signalling

In plants, auxin signalling is initiated by the auxin-promoted interaction between the auxin receptor TIR1, an E3 ubiquitin ligase, and the Aux/IAA transcriptional repressors, which are subsequently degraded by the proteasome. Gain-of-function mutations in the highly conserved domain II of Aux/IAAs abolish the TIR1-Aux/IAA interaction and thus cause an auxin-resistant phenotype. Here we show that peptidyl-prolyl isomerization of rice OsIAA11 catalysed by LATERAL ROOTLESS2 (LRT2), a cyclophilin-type peptidyl-prolyl cis/trans isomerase, directly regulates the stability of OsIAA11. NMR spectroscopy reveals that LRT2 efficiently catalyses the cis/trans isomerization of OsIAA11. The lrt2 mutation reduces OsTIR1-OsIAA11 interaction and consequently causes the accumulation of a higher level of OsIAA11 protein. Moreover, knockdown of the OsIAA11 expression partially rescues the lrt2 mutant phenotype in lateral root development. Together, these results illustrate cyclophilin-catalysed peptidyl-prolyl isomerization promotes Aux/IAA degradation, as a mechanism regulating auxin signalling.

DIAGEOTROPICA: news from the auxin swamp

Recently established links between the tomato cyclophilin A-type protein DIAGEOTROPICA and the regulation of polar auxin transport provide first mechanistic insights into the function of this enigmatic locus.

The cyclophilin A DIAGEOTROPICA gene affects auxin transport in both root and shoot to control lateral root formation

Cyclophilin A is a conserved peptidyl-prolyl cis-trans isomerase (PPIase) best known as the cellular receptor of the immunosuppressant cyclosporine A. Despite significant effort, evidence of developmental functions of cyclophilin A in non-plant systems has remained obscure. Mutations in a tomato (Solanum lycopersicum) cyclophilin A ortholog, DIAGEOTROPICA (DGT), have been shown to abolish the organogenesis of lateral roots; however, a mechanistic explanation of the phenotype is lacking. Here, we show that the dgt mutant lacks auxin maxima relevant to priming and specification of lateral root founder cells. DGT is expressed in shoot and root, and localizes to both the nucleus and cytoplasm during lateral root organogenesis. Mutation of ENTIRE/IAA9, a member of the auxin-responsive Aux/IAA protein family of transcriptional repressors, partially restores the inability of dgt to initiate lateral root primordia but not the primordia outgrowth. By comparison, grafting of a wild-type scion restores the process of lateral root formation, consistent with participation of a mobile signal. Antibodies do not detect movement of the DGT protein into the dgt rootstock; however, experiments with radiolabeled auxin and an auxin-specific microelectrode demonstrate abnormal auxin fluxes. Functional studies of DGT in heterologous yeast and tobacco-leaf auxin-transport systems demonstrate that DGT negatively regulates PIN-FORMED (PIN) auxin efflux transporters by affecting their plasma membrane localization. Studies in tomato support complex effects of the dgt mutation on PIN expression level, expression domain and plasma membrane localization. Our data demonstrate that DGT regulates auxin transport in lateral root formation.

Unraveling the signal scenario of fruit set

Long-term goals to impact or modify fruit quality and yield have been the target of researchers for many years. Different approaches such as traditional breeding,mutation breeding, and transgenic approaches have revealed a regulatory network where several hormones concur in a complex way to regulate fruit set and development,and these networks are shared in some way among species with different kinds of fruits. Understanding the molecular and biochemical networks of fruit set and development could be very useful for breeders to meet the current and future challenges of agricultural problems.

Mechanisms regulating auxin action during fruit development

Auxin controls many aspects of fruit development, including fruit set and growth, ripening and abscission. However, the mechanisms by which auxin regulates these processes are still poorly understood. While it is generally agreed that precise spatial and temporal control of auxin distribution and signaling are required for fruit development, the dynamics of auxin biosynthesis and the mechanisms for its transport to different fruit tissues are mostly unknown. Despite major advances in elucidating many aspects of auxin biology in vegetative tissues, until recently, the nature and importance of auxin metabolism, transport and signaling during fruit ontogeny remained obscure. In this review, we summarize recent research that has started to elucidate the molecular mechanisms by which auxin is produced and transported in the fruit and to unravel the complexity of auxin signaling during fruit development. We also discuss recent approaches used to reveal the genes and regulatory networks that mediate cell and tissue-specific control of auxin levels in the developing fruit.

Auxin increases the hydrogen peroxide (H2O2) concentration in tomato (Solanum lycopersicum) root tips while inhibiting root growth

BACKGROUND AND AIMS

The hormone auxin and reactive oxygen species (ROS) regulate root elongation, but the interactions between the two pathways are not well understood. The aim of this study was to investigate how auxin interacts with ROS in regulating root elongation in tomato, Solanum lycopersicum.

METHODS

Wild-type and auxin-resistant mutant, diageotropica (dgt), of tomato (S. lycopersicum 'Ailsa Craig') were characterized in terms of root apical meristem and elongation zone histology, expression of the cell-cycle marker gene Sl-CycB1;1, accumulation of ROS, response to auxin and hydrogen peroxide (H2O2), and expression of ROS-related mRNAs.

KEY RESULTS

The dgt mutant exhibited histological defects in the root apical meristem and elongation zone and displayed a constitutively increased level of hydrogen peroxide (H2O2) in the root tip, part of which was detected in the apoplast. Treatments of wild-type with auxin increased the H2O2 concentration in the root tip in a dose-dependent manner. Auxin and H2O2 elicited similar inhibition of cell elongation while bringing forth differential responses in terms of meristem length and number of cells in the elongation zone. Auxin treatments affected the expression of mRNAs of ROS-scavenging enzymes and less significantly mRNAs related to antioxidant level. The dgt mutation resulted in resistance to both auxin and H2O2 and affected profoundly the expression of mRNAs related to antioxidant level.

CONCLUSIONS

The results indicate that auxin regulates the level of H2O2 in the root tip, so increasing the auxin level triggers accumulation of H2O2 leading to inhibition of root cell elongation and root growth. The dgt mutation affects this pathway by reducing the auxin responsiveness of tissues and by disrupting the H2O2 homeostasis in the root tip.

Identification of genes involved in wild crucifer Rorippa indica resistance response on mustard aphid Lipaphis erysimi challenge

Mustard aphid, Lipaphis erysimi (L.) Kaltenbach is a perpetual annual threat in the cultivation of rapeseed- mustard (Brassica spp.) crop in tropical and sub-tropical climate. Cultivated Brassica germplasm has failed so far to provide any source of resistance. Wild germplasm is a potential source of resistance against many threatening herbivores. On wild germplasm screening, we noted that the wild crucifer Rorippa indica (L.) Hiern confers resistance against L. erysimi. In the present study L. erysimi challenged transcriptome of R. indica was compared to un-infested R. indica sample to get a molecular insight about the aphid resistance mechanism and identify the candidate defense response genes. Cloning, sequencing and in silico sequence analysis of complimentary DNA amplified fragment length polymorphism identified 116 differentially expressed transcript derived fragments revealed thirty candidates which are from different functional categories including redox regulation, signalling, photosynthesis, structure, metabolism, defense response as well as a few of unknown function. Twenty four identifications were then studied by quantitative real time RT PCR analysis at 6, 12, 24 and 48 hour time point post infestation to understand the early-to-late defense response through their relative gene expression profiles. Seventeen fragments showed significant up or down regulation at p<0.05 level. The response was influenced by different phytohormonal signalling pathways simultaneously. The candidate defense response expressed sequence tags specifically for the resistance genes identified in this study have implication in building desired mustard aphid resistance in susceptible rapeseed-mustard plants in future. This is the first molecular report on crucifer defense response against mustard aphid L. erysimi.

OsCYP2, a chaperone involved in degradation of auxin-responsive proteins, plays crucial roles in rice lateral root initiation

Auxin plays a pivotal role in many facets of plant development. It acts by inducing the interaction between auxin-responsive [auxin (AUX)/indole-3-acetic acid (IAA)] proteins and the ubiquitin protein ligase SCF(TIR) to promote the degradation of the AUX/IAA proteins. Other cofactors and chaperones that participate in auxin signaling remain to be identified. Here, we characterized rice (Oryza sativa) plants with mutations in a cyclophilin gene (OsCYP2). cyp2 mutants showed defects in auxin responses and exhibited a variety of auxin-related growth defects in the root. In cyp2 mutants, lateral root initiation was blocked after nuclear migration but before the first anticlinal division of the pericycle cell. Yeast two-hybrid and in vitro pull-down results revealed an association between OsCYP2 and the co-chaperone Suppressor of G2 allele of skp1 (OsSGT1). Luciferase complementation imaging assays further supported this interaction. Similar to previous findings in an Arabidopsis thaliana SGT1 mutant (atsgt1b), degradation of AUX/IAA proteins was retarded in cyp2 mutants treated with exogenous 1-naphthylacetic acid. Our results suggest that OsCYP2 participates in auxin signal transduction by interacting with OsSGT1.

Auxin influences strigolactones in pea mycorrhizal symbiosis

Hormone interactions are essential for the control of many developmental processes, including intracellular symbioses. The interaction between auxin and the new plant hormone strigolactone in the regulation of arbuscular mycorrhizal symbiosis was examined in one of the few auxin deficient mutants available in a mycorrhizal species, the auxin-deficient bsh mutant of pea (Pisum sativum). Mycorrhizal colonisation with the fungus Glomus intraradices was significantly reduced in the low auxin bsh mutant. The bsh mutant also exhibited a reduction in strigolactone exudation and the expression of a key strigolactone biosynthesis gene (PsCCD8). Strigolactone exudation was also reduced in wild type plants when the auxin content was reduced by stem girdling. Low strigolactone levels appear to be at least partially responsible for the reduced colonisation of the bsh mutant, as application of the synthetic strigolactone GR24 could partially rescue the mycorrhizal phenotype of bsh mutants. Data presented here indicates root auxin content was correlated with strigolactone exudation in both mutant and wild type plants. Mutant studies suggest that auxin may regulate early events in the formation of arbuscular mycorrhizal symbiosis by controlling strigolactone levels, both in the rhizosphere and possibly during early root colonisation.

A gain-of-function mutation in the ROC1 gene alters plant architecture in Arabidopsis

Plant architecture is an important agronomic trait and is useful for identification of plant species. The molecular basis of plant architecture, however, is largely unknown. Forward genetics was used to identify an Arabidopsis mutant with altered plant architecture. Using genetic and molecular approaches, we analyzed the roles of a mutated cyclophilin in the control of plant architecture. The Arabidopsis mutant roc1 has reduced stem elongation and increased shoot branching, and the mutant phenotypes are strongly affected by temperature and photoperiod. Map-based cloning and transgenic experiments demonstrated that the roc1 mutant phenotypes are caused by a gain-of-function mutation in a cyclophilin gene, ROC1. Besides, application of the plant hormone gibberellic acid (GA) further suppresses stem elongation in the mutant. GA treatment enhances the accumulation of mutated but not of wildtype (WT) ROC1 proteins. The roc1 mutation does not seem to interfere with GA biosynthesis or signaling. GA signaling, however, antagonizes the effect of the roc1 mutation on stem elongation. The altered plant architecture may result from the activation of an R gene by the roc1 protein. We also present a working model for the interaction between the roc1 mutation and GA signaling in regulating stem elongation.