Interaction of auxin and ERECTA in elaborating Arabidopsis inflorescence architecture revealed by the activation tagging of a new member of the YUCCA family putative flavin monooxygenases

Auxin biosynthesis and storage forms

The plant hormone auxin drives plant growth and morphogenesis. The levels and distribution of the active auxin indole-3-acetic acid (IAA) are tightly controlled through synthesis, inactivation, and transport. Many auxin precursors and modified auxin forms, used to regulate auxin homeostasis, have been identified; however, very little is known about the integration of multiple auxin biosynthesis and inactivation pathways. This review discusses the many ways auxin levels are regulated through biosynthesis, storage forms, and inactivation, and the potential roles modified auxins play in regulating the bioactive pool of auxin to affect plant growth and development.

The jasmonic acid signaling pathway is linked to auxin homeostasis through the modulation of YUCCA8 and YUCCA9 gene expression

Interactions between phytohormones play important roles in the regulation of plant growth and development, but knowledge of the networks controlling hormonal relationships, such as between oxylipins and auxins, is just emerging. Here, we report the transcriptional regulation of two Arabidopsis YUCCA genes, YUC8 and YUC9, by oxylipins. Similar to previously characterized YUCCA family members, we show that both YUC8 and YUC9 are involved in auxin biosynthesis, as demonstrated by the increased auxin contents and auxin-dependent phenotypes displayed by gain-of-function mutants as well as the significantly decreased indole-3-acetic acid (IAA) levels in yuc8 and yuc8/9 knockout lines. Gene expression data obtained by qPCR analysis and microscopic examination of promoter-reporter lines reveal an oxylipin-mediated regulation of YUC9 expression that is dependent on the COI1 signal transduction pathway. In support of these findings, the roots of the analyzed yuc knockout mutants displayed a reduced response to methyl jasmonate (MeJA). The similar response of the yuc8 and yuc9 mutants to MeJA in cotyledons and hypocotyls suggests functional overlap of YUC8 and YUC9 in aerial tissues, while their function in roots shows some specificity, probably in part related to different spatio-temporal expression patterns of the two genes. These results provide evidence for an intimate functional relationship between oxylipin signaling and auxin homeostasis.

Overexpression of Arabidopsis YUCCA6 in potato results in high-auxin developmental phenotypes and enhanced resistance to water deficit

Indole-3-acetic acid (IAA), a major plant auxin, is produced in both tryptophan-dependent and tryptophan-independent pathways. A major pathway in Arabidopsis thaliana generates IAA in two reactions from tryptophan. Step one converts tryptophan to indole-3-pyruvic acid (IPA) by tryptophan aminotransferases followed by a rate-limiting step converting IPA to IAA catalyzed by YUCCA proteins. We identified eight putative StYUC (Solanum tuberosum YUCCA) genes whose deduced amino acid sequences share 50%-70% identity with those of Arabidopsis YUCCA proteins. All include canonical, conserved YUCCA sequences: FATGY motif, FMO signature sequence, and FAD-binding and NADP-binding sequences. In addition, five genes were found with ~50% amino acid sequence identity to Arabidopsis tryptophan aminotransferases. Transgenic potato (Solanum tuberosum cv. Jowon) constitutively overexpressing Arabidopsis AtYUC6 displayed high-auxin phenotypes such as narrow downward-curled leaves, increased height, erect stature, and longevity. Transgenic potato plants overexpressing AtYUC6 showed enhanced drought tolerance based on reduced water loss. The phenotype was correlated with reduced levels of reactive oxygen species in leaves. The results suggest a functional YUCCA pathway of auxin biosynthesis in potato that may be exploited to alter plant responses to the environment.

Temperature-dependent shade avoidance involves the receptor-like kinase ERECTA

Plants detect the presence of neighbouring vegetation by monitoring changes in the ratio of red (R) to far-red (FR) wavelengths (R:FR) in ambient light. Reductions in R:FR are perceived by the phytochrome family of plant photoreceptors and initiate a suite of developmental responses termed the shade avoidance syndrome. These include increased elongation growth of stems and petioles, enabling plants to overtop competing vegetation. The majority of shade avoidance experiments are performed at standard laboratory growing temperatures (>20°C). In these conditions, elongation responses to low R:FR are often accompanied by reductions in leaf development and accumulation of plant biomass. Here we investigated shade avoidance responses at a cooler temperature (16°C). In these conditions, Arabidopsis thaliana displays considerable low R:FR-mediated increases in leaf area, with reduced low R:FR-mediated petiole elongation and leaf hyponasty responses. In Landsberg erecta, these strikingly different shade avoidance phenotypes are accompanied by increased leaf thickness, increased biomass and an altered metabolite profile. At 16°C, low R:FR treatment results in the accumulation of soluble sugars and metabolites associated with cold acclimation. Analyses of natural genetic variation in shade avoidance responses at 16°C have revealed a regulatory role for the receptor-like kinase ERECTA.

A MAPK cascade downstream of ERECTA receptor-like protein kinase regulates Arabidopsis inflorescence architecture by promoting localized cell proliferation

Spatiotemporal-specific cell proliferation and cell differentiation are critical to the formation of normal tissues, organs, and organisms. The highly coordinated cell differentiation and proliferation events illustrate the importance of cell-cell communication during growth and development. In Arabidopsis thaliana, ERECTA (ER), a receptor-like protein kinase, plays important roles in promoting localized cell proliferation, which determines inflorescence architecture, organ shape, and size. However, the downstream signaling components remain unidentified. Here, we report a mitogen-activated protein kinase (MAPK; or MPK) cascade that functions downstream of ER in regulating localized cell proliferation. Similar to an er mutant, loss of function of MPK3/MPK6 or their upstream MAPK kinases (MAPKKs; or MKKs), MKK4/MKK5, resulted in shortened pedicels and clustered inflorescences. Epistasis analysis demonstrated that the gain of function of MKK4 and MKK5 transgenes could rescue the loss-of-function er mutant phenotype at both morphological and cellular levels, suggesting that the MPK3/MPK6 cascade functions downstream of the ER receptor. Furthermore, YODA (YDA), a MAPKK kinase, was shown to be upstream of MKK4/MKK5 and downstream of ER in regulating inflorescence architecture based on both gain- and loss-of-function data. Taken together, these results suggest that the YDA-MKK4/MKK5-MPK3/MPK6 cascade functions downstream of the ER receptor in regulating localized cell proliferation, which further shapes the morphology of plant organs.

WOX2 and STIMPY-LIKE/WOX8 promote cotyledon boundary formation in Arabidopsis

One of the key events in dicot plant embryogenesis is the emergence of the two cotyledon primordia, which marks the transition from radial symmetry to bilateral symmetry. In Arabidopsis thaliana, the three CUP-SHAPED COTYLEDON (CUC) genes are responsible for determining the boundary region between the cotyledons. However, the mechanisms controlling their transcription activation are not well understood. Previous studies found that several WOX family homeobox transcription factors are involved in embryo apical patterning and cotyledon development. Here we show that WOX2 and STIMPY-LIKE (STPL/WOX8) act redundantly to differentially regulate the expression of the CUC genes in promoting the establishment of the cotyledon boundary, without affecting the primary shoot meristem. Loss of both WOX2 and STPL results in reduced CUC2 and CUC3 expression in one side of the embryo, but an expansion of the CUC1 domain. Furthermore, we found that STPL is expressed in the embryo proper, and its activation is enhanced by the removal of WOX2, providing an explanation for the functional redundancy between WOX2 and STPL. Additional evidence also showed that WOX2 and STPL function independently in regulating different aspects of local auxin gradient formation during early embryogenesis.

Arabidopsis thaliana AUCSIA-1 regulates auxin biology and physically interacts with a kinesin-related protein

Aucsia is a green plant gene family encoding 44–54 amino acids long miniproteins. The sequenced genomes of most land plants contain two Aucsia genes. RNA interference of both tomato (Solanum lycopersicum) Aucsia genes (SlAucsia-1 and SlAucsia-2) altered auxin sensitivity, auxin transport and distribution; it caused parthenocarpic development of the fruit and other auxin-related morphological changes. Here we present data showing that the Aucsia-1 gene of Arabidopsis thaliana alters, by itself, root auxin biology and that the AtAUCSIA-1 miniprotein physically interacts with a kinesin-related protein. The AtAucsia-1 gene is ubiquitously expressed, although its expression is higher in roots and inflorescences in comparison to stems and leaves. Two allelic mutants for AtAucsia-1 gene did not display visible root morphological alterations; however both basipetal and acropetal indole-3-acetic acid (IAA) root transport was reduced as compared with wild-type plants. The transcript steady state levels of the auxin efflux transporters ATP BINDING CASSETTE subfamily B (ABCB) ABCB1, ABCB4 and ABCB19 were reduced in ataucsia-1 plants. In ataucsia-1 mutant, lateral root growth showed an altered response to i) exogenous auxin, ii) an inhibitor of polar auxin transport and iii) ethylene. Overexpression of AtAucsia-1 inhibited primary root growth. In vitro and in vivo protein-protein interaction experiments showed that AtAUCSIA-1 interacts with a 185 amino acids long fragment belonging to a 2712 amino acids long protein of unknown function (At4g31570). Bioinformatics analysis indicates that the AtAUCSIA-1 interacting protein (AtAUCSIA-1IP) clusters with a group of CENP-E kinesin-related proteins. Gene ontology predictions for the two proteins are consistent with the hypothesis that the AtAUCSIA-1/AtAUCSIA-1IP complex is involved in the regulation of the cytoskeleton dynamics underlying auxin biology.

Activation of a flavin monooxygenase gene YUCCA7 enhances drought resistance in Arabidopsis

Auxin regulates diverse molecular and physiological events at the cellular and organismal levels during plant growth and development in response to environmental stimuli. It acts either through distinct signaling pathways or in concert with other growth hormones. Its biological functions are adjusted by modulating biosynthesis, conjugate formation, and polar transport and distribution. Several tryptophan-dependent and -independent auxin biosynthetic pathways have been proposed. Recent studies have shown that a few flavin monooxygenase enzymes contribute to the tryptophan-dependent auxin biosynthesis. Here, we show that activation of a flavin monooxygenase gene YUCCA7 (YUC7), which belongs to the tryptophan-dependent auxin biosynthetic pathway, enhances drought resistance. An Arabidopsis activation-tagged mutant yuc7-1D exhibited phenotypic changes similar to those observed in auxin-overproducing mutants, such as tall, slender stems and curled, narrow leaves. Accordingly, endogenous levels of total auxin were elevated in the mutant. The YUC7 gene was induced by drought, primarily in the roots, in an abscisic acid (ABA)-dependent manner. The yuc7-1D mutant was resistant to drought, and drought-responsive genes, such as RESPONSIVE TO DESSICATION 29A (RD29A) and COLD-REGULATED 15A (COR15A), were up-regulated in the mutant. Interestingly, whereas stomatal aperture and production of osmoprotectants were not discernibly altered, lateral root growth was significantly promoted in the yuc7-1D mutant when grown under drought conditions. These observations support that elevation of auxin levels in the roots enhances drought resistance possibly by promoting root growth.

Regulation of inflorescence architecture by intertissue layer ligand-receptor communication between endodermis and phloem

Multicellular organisms achieve final body shape and size by coordinating cell proliferation, expansion, and differentiation. Loss of function in the Arabidopsis ERECTA (ER) receptor-kinase gene confers characteristic compact inflorescence architecture, but its underlying signaling pathways remain unknown. Here we report that the expression of ER in the phloem is sufficient to rescue compact er inflorescences. We further identified two Epidermal Patterning Factor-like (EPFL) secreted peptide genes, EPFL4 and EPFL6/CHALLAH (CHAL), as redundant, upstream components of ER-mediated inflorescence growth. The expression of EPFL4 or EPFL6 in the endodermis, a layer adjacent to phloem, is sufficient to rescue the er-like inflorescence of epfl4 epfl6 plants. EPFL4 and EPFL6 physically associate with ER in planta. Finally, transcriptome analysis of er and epfl4 epfl6 revealed a potential downstream component as well as a role for plant hormones in EPFL4/6- and ER-mediated inflorescence growth. Our results suggest that intercell layer communication between the endodermis and phloem mediated by peptide ligands and a receptor kinase coordinates proper inflorescence architecture in Arabidopsis.

Activation tagging

Insertional mutagenesis is one of the most effective approaches to determine the function of plant genes. However, due to genetic redundancy, loss-of-function mutations often fail to reveal the function of a member of gene families. Activation tagging is a powerful gain-of-function approach to reveal the functions of genes, especially those with high sequence similarity recalcitrant to loss-of-function genetic analyses. Activation tagging randomly inserts a T-DNA fragment containing engineered four copies of enhancer element into a plant genome to activate transcription of flanking genes. We recently generated a new binary vector, pBASTA-AT2, which has been efficiently used to discover genes involved in BR biosynthesis, metabolism, and signal transduction. Compared to pSKI015, a commonly used activation tagging vector, pBASTA-AT2, contains a smaller size of T-DNA and a bigger number of unique restriction sites within the T-DNA region, making cloning of the flanking sequence a lot easier. Our analysis indicated that pBASTA-AT2 gives dramatically improved transformation efficiency relative to pSKI015. In this article, detailed information about this activation tagging vector and the protocol for its application are provided. Three recommended gene cloning approaches based on the use of pBASTA-AT2, including inverse PCR, thermal asymmetric interlaced PCR, and adaptor ligation-mediated PCR, are described to identify T-DNA insertion sites after selection of activation-tagged mutant plants.

Auxin-responsive OsMGH3, a common downstream target of OsMADS1 and OsMADS6, controls rice floret fertility

GH3 proteins control auxin homeostasis by inactivating excess auxin as conjugates of amino acids and sugars and thereby controlling cellular bioactive auxin. Since auxin regulates many aspects of plant growth and development, regulated expression of these genes offers a mechanism to control various developmental processes. OsMGH3/OsGH3-8 is expressed abundantly in rice florets and is regulated by two related and redundant transcription factors, OsMADS1 and OsMADS6, but its contribution to flower development is not known. We functionally characterize OsMGH3 by overexpression and knock-down analysis and show a partial overlap in these phenotypes with that of mutants in OsMADS1 and OsMADS6. The overexpression of OsMGH3 during the vegetative phase affects the overall plant architecture, whereas its inflorescence-specific overexpression creates short panicles with reduced branching, resembling in part the effects of OsMADS1 overexpression. In contrast, the down-regulation of endogenous OsMGH3 caused phenotypes consistent with auxin overproduction or activated signaling, such as ectopic rooting from aerial nodes. Florets in OsMGH3 knock-down plants were affected in carpel development and pollen viability, both of which reduced fertility. Some of these floret phenotypes are similar to osmads6 mutants. Taken together, we provide evidence for the functional significance of auxin homeostasis and its transcriptional regulation during rice panicle branching and floret organ development.

YUCCA6 over-expression demonstrates auxin function in delaying leaf senescence in Arabidopsis thaliana

The Arabidopsis thaliana YUCCA family of flavin monooxygenase proteins catalyses a rate-limiting step in de novo auxin biosynthesis. A YUCCA6 activation mutant, yuc6-1D, has been shown to contain an elevated free IAA level and to display typical high-auxin phenotypes. It is reported here that Arabidopsis plants over-expressing YUCCA6, such as the yuc6-1D activation mutant and 35S:YUC6 transgenic plants, displayed dramatic longevity. In addition, plants over-expressing YUCCA6 exhibited classical, delayed dark-induced and hormone-induced senescence in assays using detached rosette leaves. However, plants over-expressing an allele of YUCCA6, that carries mutations in the NADPH cofactor binding site, exhibited neither delayed leaf senescence phenotypes nor phenotypes typical of auxin overproduction. When the level of free IAA was reduced in yuc6-1D by conjugation to lysine, yuc6-1D leaves senesced at a rate similar to the wild-type leaves. Dark-induced senescence in detached leaves was accompanied by a decrease in their free IAA content, by the reduced expression of auxin biosynthesis enzymes such as YUCCA1 and YUCCA6 that increase cellular free IAA levels, and by the increased expression of auxin-conjugating enzymes encoded by the GH3 genes that reduce the cellular free auxin levels. Reduced transcript abundances of SAG12, NAC1, and NAC6 during senescence in yuc6-1D compared with the wild type suggested that auxin delays senescence by directly or indirectly regulating the expression of senescence-associated genes.

The ERECTA gene controls spatial and temporal patterns of epidermal cell number and size in successive developing leaves of Arabidopsis thaliana

BACKGROUND AND AIMS

ERECTA has been identified as a pleiotropic regulator of developmental and physiological processes in Arabidopsis thaliana. Previous work demonstrated a role for ERECTA in the control of compensation between epidermal cell expansion and division in leaves.

METHODS

In this work, spatial and temporal analyses of epidermal cell division and expansion were performed on successive developing vegetative leaves of Arabidopsis thaliana in both ERECTA and erecta lines, LER and Ler, respectively, to understand how the ERECTA gene regulates compensation between these two processes.

KEY RESULTS

The loss of ERECTA function leads to a low cell expansion rate in all zones of a leaf and in all successive leaves of a plant. This low cell expansion rate is counterbalanced by an increase in the duration of cell division. As a consequence, the ERECTA mutation eliminates the tip to base cellular gradient generally observed in the leaf epidermis and also flattens the heteroblastic changes in epidermal cell area and number within a rosette. Ablation of floral buds eliminates the heteroblastic changes in cellular patterns in an ERECTA-dependent manner.

CONCLUSIONS

The results provide a detailed description of changes in leaf growth dynamics and cellular variables in both LER and Ler. Altogether they suggest that ERECTA influences leaf cellular development in relation to whole plant ontogeny.

Arabidopsis ERECTA-family receptor kinases mediate morphological alterations stimulated by activation of NB-LRR-type UNI proteins

Shoot apical meristems (SAMs), which maintain stem cells at the tips of stems, and axillary meristems (AMs), which arise at leaf axils for branch formation, play significant roles in the establishment of plant architecture. Previously, we showed that, in Arabidopsis thaliana, activation of NB-LRR (nucleotide-binding site-leucine-rich repeat)-type UNI proteins affects plant morphology through modulation of the regulation of meristems. However, information about genes involved in the processes was still lacking. Here, we report that ERECTA (ER) receptor kinase family members cooperatively mediate the morphological alterations that are stimulated by activation of UNI proteins. uni-1D is a gain-of-function mutation in the UNI gene and uni-1D mutants exhibit early termination of inflorescence stem growth and also formation of extra AMs at leaf axils. The former defect involves modulation of the SAM activity and is suppressed by er mutation. Though the AM phenotype is not affected by a single er mutation, it is suppressed by simultaneous mutations of ER-family members. It was previously shown that trans-zeatin (tZ)-type cytokinins were involved in the morphological phenotypes of uni-1D mutants and that expression of CYP735A2, which is essential for biosynthesis of tZ-type cytokinins, was modulated in uni-1D mutants. We show that this modulation of CYP735A2 expression requires activities of ER-family members. Moreover, the ER activity in UNI-expressing cells contributes to all morphological phenotypes of uni-1D mutants, suggesting that a cross-talk between ER-family-dependent and UNI-triggered signaling pathways plays a significant role in the morphological alterations observed in uni-1D mutants.

TFL2/LHP1 is involved in auxin biosynthesis through positive regulation of YUCCA genes

TERMINAL FLOWER2 (TFL2) is the plant homologue of metazoan HETEROCHROMATIN PROTEIN1 (HP1) protein family. It is known that, unlike most HP1 proteins, TFL2 does not primarily localize to heterochromatin; instead it functions in regulation of specific genes in euchromatic regions. We show that the tfl2 mutant has a lower rate of auxin biosynthesis, resulting in low levels of auxin. In line with this, tfl2 mutants have lower levels of expression of auxin response genes and retain an auxin response. The reduced rate of auxin biosynthesis in tfl2 is correlated to the down-regulation of specific genes in the tryptophan-dependent auxin biosynthesis pathway, a sub-set of the YUCCA genes. In vivo, TFL2 is targeted to a number of the YUCCA genes in an auxin-dependent fashion revealing a role of TFL2 in auxin regulation, probably as a component of protein complexes affecting transcriptional control.

The DOF transcription factor Dof5.1 influences leaf axial patterning by promoting Revoluta transcription in Arabidopsis

Dof proteins are transcription factors that have a conserved single zinc finger DNA-binding domain. In this study, we isolated an activation tagging mutant Dof5.1-D exhibiting an upward-curling leaf phenotype due to enhanced expression of the REV gene that is required for establishing adaxial-abaxial polarity. Dof5.1-D plants also had reduced transcript levels for IAA6 and IAA19 genes, indicating an altered auxin biosynthesis in Dof5.1-D. An electrophoretic mobility shift assay using the Dof5.1 DNA-binding motif and the REV promoter region indicated that the DNA-binding domain of Dof5.1 binds to a TAAAGT motif located in the 5'-distal promoter region of the REV promoter. Further, transient and chromatin immunoprecipitation assays verified binding activity of the Dof5.1 DNA-binding motif with the REV promoter. Consistent with binding assays, constitutive over-expression of the Dof5.1 DNA-binding domain in wild-type plants caused a downward-curling phenotype, whereas crossing Dof5.1-D to a rev mutant reverted the upward-curling phenotype of the Dof5.1-D mutant leaf to the wild-type. These results suggest that the Dof5.1 protein directly binds to the REV promoter and thereby regulates adaxial-abaxial polarity.

ERECTA controls low light intensity-induced differential petiole growth independent of phytochrome B and cryptochrome 2 action in Arabidopsis thaliana

Plants can respond quickly and profoundly to changes in their environment. Several species, including Arabidopsis thaliana, are capable of differential petiole growth driven upward leaf movement (hyponastic growth) to escape from detrimental environmental conditions. Recently, we demonstrated that the leucine-rich repeat receptor-like Ser/Thr kinase gene ERECTA, explains a major effect Quantitative Trait Locus (QTL) for ethylene-induced hyponastic growth in Arabidopsis. Here, we demonstrate that ERECTA controls the hyponastic growth response to low light intensity treatment in a genetic background dependent manner. Moreover, we show that ERECTA affects low light-induced hyponastic growth independent of Phytochrome B and Cryptochrome 2 signaling, despite that these photoreceptors are positive regulators of low light-induced hyponastic growth.

Auxin biosynthesis and its role in plant development

Indole-3-acetic acid (IAA), the main auxin in higher plants, has profound effects on plant growth and development. Both plants and some plant pathogens can produce IAA to modulate plant growth. Although the genes and biochemical reactions for auxin biosynthesis in some plant pathogens are well understood, elucidation of the mechanisms by which plants produce auxin has proven to be difficult. So far, no single complete pathway of de novo auxin biosynthesis in plants has been firmly established. However, recent studies have led to the discoveries of several genes in tryptophan-dependent auxin biosynthesis pathways. Recent findings have also determined that local auxin biosynthesis plays essential roles in many developmental processes including gametogenesis, embryogenesis, seedling growth, vascular patterning, and flower development. In this review, I summarize the recent advances in dissecting auxin biosynthetic pathways and how the understanding of auxin biosynthesis provides a crucial angle for analyzing the mechanisms of plant development.

High-throughput characterization of plant gene functions by using gain-of-function technology

Gain-of-function approaches have been used as an alternative or complementary method to loss-of-function approaches as well as to confer new functions to plants. Gain-of-function is achieved by increasing gene expression levels through the random activation of endogenous genes by transcriptional enhancers or the expression of individual transgenes by transformation. The advantages of gain-of-function approaches compared to loss-of-function approaches for the characterization of gene functions include the abilities to (a) analyze individual gene family members, (b) characterize the function of genes from nonmodel plants using a heterologous expression system, and (c) identify genes that confer stress tolerance to plants that result from the introduction of transgenes. In this review, we describe the current status of gain-of-function mutagenesis and provide several examples of how gene functions have been characterized via high-throughput screening using gain-of-function technology.

Auxin biosynthesis and its role in plant development

Indole-3-acetic acid (IAA), the main auxin in higher plants, has profound effects on plant growth and development. Both plants and some plant pathogens can produce IAA to modulate plant growth. Although the genes and biochemical reactions for auxin biosynthesis in some plant pathogens are well understood, elucidation of the mechanisms by which plants produce auxin has proven to be difficult. So far, no single complete pathway of de novo auxin biosynthesis in plants has been firmly established. However, recent studies have led to the discoveries of several genes in tryptophan-dependent auxin biosynthesis pathways. Recent findings have also determined that local auxin biosynthesis plays essential roles in many developmental processes including gametogenesis, embryogenesis, seedling growth, vascular patterning, and flower development. In this review, I summarize the recent advances in dissecting auxin biosynthetic pathways and how the understanding of auxin biosynthesis provides a crucial angle for analyzing the mechanisms of plant development.

Activation-tagged suppressors of a weak brassinosteroid receptor mutant

Brassinosteroids (BRs) are important plant hormones that act synergistically with auxin to regulate a variety of plant developmental and physiological processes. In the past decade, genetic and biochemical studies have revealed a linear signaling pathway that relies on protein phosphorylation to transmit the BR signal into the nucleus, altering expression of hundreds of genes to promote plant growth. We conducted an activation-tagging based suppressor screen to look for Arabidopsis genes that, when overexpressed by inserted 35S enhancer elements, could suppress the dwarf phenotype of a weak BR receptor mutant bri1-301. This screen identified a total of six dominant activation-tagged bri1 suppressors (atbs-Ds). Using a plasmid rescue approach, we discovered that the bri1-301 suppression effect in four atbs-D mutants (atbs3-D to atbs6-D) was caused by overexpression of a YUCCA gene thought to be involved in tryptophan-dependent auxin biosynthesis. Interestingly, the three activation-tagged YUCCA genes belong to the YUCCA IIA subfamily that includes two other members out of 11 known Arabidopsis YUCCA genes. In addition, our molecular studies revealed a T-DNA insertion near a basic helix-loop-helix gene in atbs1-D and a T-DNA insertion in a region carrying a BR biosynthetic gene in atbs2-D. Further studies of these atbs-D mutants could lead to better understanding of the BR signaling process and the BR-auxin interaction.

Hormonal regulation of temperature-induced growth in Arabidopsis

Successful plant survival depends upon the proper integration of information from the environment with endogenous cues to regulate growth and development. We have investigated the interplay between ambient temperature and hormone action during the regulation of hypocotyl elongation, and we have found that gibberellins (GAs) and auxin are quickly and independently recruited by temperature to modulate growth rate, whereas activity of brassinosteroids (BRs) seems to be required later on. Impairment of GA biosynthesis blocked the increased elongation caused at higher temperatures, but hypocotyls of pentuple DELLA knockout mutants still reduced their response to higher temperatures when BR synthesis or auxin polar transport were blocked. The expression of several key genes involved in the biosynthesis of GAs and auxin was regulated by temperature, which indirectly resulted in coherent variations in the levels of accumulation of nuclear GFP-RGA (repressor of GA1) and in the activity of the DR5 reporter. DNA microarray and genetic analyses allowed the identification of the transcription factor PIF4 (phytochrome-interacting factor 4) as a major target in the promotion of growth at higher temperature. These results suggest that temperature regulates hypocotyl growth by individually impinging on several elements of a pre-existing network of signaling pathways involving auxin, BRs, GAs, and PIF4.

The ERECTA Receptor-Like Kinase Regulates Cell Wall-Mediated Resistance to Pathogens in Arabidopsis thaliana

Some receptor-like kinases (RLK) control plant development while others regulate immunity. The Arabidopsis ERECTA (ER) RLK regulates both biological processes. To discover specific components of ER-mediated immunity, a genetic screen was conducted to identify suppressors of erecta (ser) susceptibility to Plectosphaerella cucumerina fungus. The ser1 and ser2 mutations restored disease resistance to this pathogen to wild-type levels in the er-1 background but failed to suppress er-associated developmental phenotypes. The deposition of callose upon P. cucumerina inoculation, which was impaired in the er-1 plants, was also restored to near wild-type levels in the ser er-1 mutants. Analyses of er cell walls revealed that total neutral sugars were reduced and uronic acids increased relative to those of wild-type walls. Interestingly, in the ser er-1 walls, neutral sugars were elevated and uronic acids were reduced relative to both er-1 and wild-type plants. The cell-wall changes found in er-1 and the ser er-1 mutants are unlikely to contribute to their developmental alterations. However, they may influence disease resistance, as a positive correlation was found between uronic acids content and resistance to P. cucumerina. We propose a specific function for ER in regulating cell wall-mediated disease resistance that is distinct from its role in development.

The short-rooted vitamin B6-deficient mutant pdx1 has impaired local auxin biosynthesis

The phytohormone auxin regulates many aspects of plant growth and development. Auxin often acts distantly from the site of its biosynthesis and this long-distance-transported auxin is well known to play a critical role in eliciting physiological responses including regulating root development. Auxin can be produced in roots, yet the function of locally synthesized auxin in root growth is unclear. The major auxin in plants, indole 3-acetic acid (IAA), is mainly synthesized through tryptophan (Trp)-dependent pathways that require pyridoxal phosphate (an active form of vitamin B(6))-dependent enzymes. We previously reported that the Arabidopsis vitamin B(6) biosynthesis mutant pdx1 has stunted root growth although the underlying cause is unknown. Here we showed that the pdx1 root is deficient in auxin biosynthesis. By reciprocal grafting of pdx1 and the wild type, we demonstrated that the stunted root growth in pdx1 is caused by a locally generated signal(s) in roots. To test whether auxin might be one such signal, the auxin responsive DR5::GUS reporter was introduced into the mutant. The DR5::GUS activity in pdx1 root tips was greatly reduced compared with that in the wild type although the auxin response was unaltered. pdx1 also suppresses the root hair growth defects in the auxin overproduction mutant yucca. These data indicate that pdx1 is impaired in Trp-dependent auxin biosynthesis, which may contribute to the short-root phenotype of pdx1. We suggest that locally synthesized auxin may play a critical role in postembryonic root growth.

The NGATHA genes direct style development in the Arabidopsis gynoecium

The gynoecium is the most complex floral organ, designed to protect the ovules and ensure their fertilization. Correct patterning and tissue specification in the developing gynoecium involves the concerted action of a host of genetic factors. In addition, apical-basal patterning into different domains, stigma and style, ovary and gynophore, appears to depend on the establishment and maintenance of asymmetric auxin distribution, with an auxin maximum at the apex. Here, we show that a small subfamily of the B3 transcription factor superfamily, the NGATHA (NGA) genes, act redundantly to specify style development in a dosage-dependent manner. Characterization of the NGA gene family is based on an analysis of the activation-tagged mutant named tower-of-pisa1 (top1), which was found to overexpress NGA3. Quadruple nga mutants completely lack style and stigma development. This mutant phenotype is likely caused by a failure to activate two auxin biosynthetic enzymes, YUCCA2 and YUCCA4, in the apical gynoecium domain. The NGA mutant phenotypes are similar to those caused by multiple combinations of mutations in STYLISH1 (STY1) and additional members of its family. NGA3/TOP1 and STY1 share almost identical patterns of expression, but they do not appear to regulate each other at the transcriptional level. Strong synergistic phenotypes are observed when nga3/top1 and sty1 mutants are combined. Furthermore, constitutive expression of both NGA3/TOP1 and STY1 induces the conversion of the ovary into style tissue. Taken together, these data suggest that the NGA and STY factors act cooperatively to promote style specification, in part by directing YUCCA-mediated auxin synthesis in the apical gynoecium domain.

The many functions of ERECTA

The Arabidopsis thaliana accession Landsberg erecta contains an induced mutation in the leucine-rich repeat receptor-like Ser/Thr kinase gene ERECTA. Landsberg erecta is commonly used as a genetic background in mutant screens and in natural variation studies. Therefore, the erecta mutation is present in many loss-of-function mutants and recombinant inbred lines. Information on how the absence of functional ERECTA affects the interpretation of obtained phenotypic results is scattered. In this report we inventoried ERECTA functions and highlight ERECTA as a pleiotropic regulator of developmental and physiological processes, as well as a modulator of responses to environmental stimuli.

Genome-wide analysis of gene expression in soybean shoot apical meristem

The shoot apical meristem (SAM) contains undifferentiated stem cells that are responsible for the initiation of above-ground organs. The nature of genetic programs and the regulatory networks underlying SAM function in a major legume crop, soybean was investigated here. We used soybean GeneChip (containing 37,744 probe sets) to examine the transcript profiles associated with micro-dissected, actively growing SAMs or growth arrested axillary meristems (AMs) experiencing apical dominance, in comparison to that of non-meristem (NM) tissue. A total of 1,090 and 1,523 transcripts were identified to be significantly up- or down-regulated in the SAM in comparison to the NM. RT-PCR and in situ hybridization analysis were also carried out to verify the experimental approach. The resulting gene expression profiles point to the combinatorial role of diverse regulatory pathways including those associated with cell division and proliferation, epigenetic regulation, auxin-mediated responses and microRNA regulation in meristem function. In situ hybridization analysis on selected transcripts has implicated their roles in SAM maintenance and the establishment of organ polarity. We also identified a gene, ANGUSITFOLIA3 that could potentially serve as a novel marker for differentiating cells in the meristem. Computational analysis on the promoter regions of Arabidopsis thaliana orthologs of genes with high expression in the soybean SAM revealed a conserved over-representation of three cis-acting regulatory motifs. Our data show that plant meristems possess a unique transcriptional profile, with shared "molecular signatures" in apical and axillary meristems providing a rich source of novel target genes for further studies into a fundamental process that impacts plant growth and crop productivity.

sparse inflorescence1, barren inflorescence1 and barren stalk1 promote cell elongation in maize inflorescence development

The sparse inflorescence1 (spi1), Barren inflorescence1 (Bif1), barren inflorescence2 (bif2), and barren stalk1 (ba1) mutants produce fewer branches and spikelets in the inflorescence due to defects in auxin biosynthesis, transport, or response. We report that spi1, bif1, and ba1, but not bif2, also function in promoting cell elongation in the inflorescence.

NARROW LEAF 7 controls leaf shape mediated by auxin in rice

Elucidation of the genetic basis of the control of leaf shape could be of use in the manipulation of crop traits, leading to more stable and increased crop production. To improve our understanding of the process controlling leaf shape, we identified a mutant gene in rice that causes a significant decrease in the width of the leaf blade, termed narrow leaf 7 (nal7). This spontaneous mutation of nal7 occurred during the process of developing advanced back-crossed progeny derived from crosses of rice varieties with wild type leaf phenotype. While the mutation resulted in reduced leaf width, no significant morphological changes at the cellular level in leaves were observed, except in bulli-form cells. The NAL7 locus encodes a flavin-containing monooxygenase, which displays sequence homology with YUCCA. Inspection of a structural model of NAL7 suggests that the mutation results in an inactive enzyme. The IAA content in the nal7 mutant was altered compared with that of wild type. The nal7 mutant overexpressing NAL7 cDNA exhibited overgrowth and abnormal morphology of the root, which was likely to be due to auxin overproduction. These results indicate that NAL7 is involved in auxin biosynthesis.

Heterologous expression, and biochemical and cellular characterization of CaPLA1 encoding a hot pepper phospholipase A1 homolog

Phospholipid signaling has been recently implicated in diverse cellular processes in higher plants. We identified a cDNA encoding the phospholipase A1 homolog (CaPLA1) from 5-day-old early roots of hot pepper. The deduced amino acid sequence showed that the lipase-specific catalytic triad is well conserved in CaPLA1. In vitro lipase assays and site-directed mutagenesis revealed that CaPLA1 possesses PLA1 activity, which catalyzes the hydrolysis of phospholipids at the sn-1 position. CaPLA1 was selectively expressed in young roots, at days 4-5 after germination, and rapidly declined thereafter, suggesting that the expression of CaPLA1 is subject to control by a development-specific mechanism in roots. Because transgenic work was extremely difficult in hot peppers, in this study we overexpressed CaPLA1 in Arabidopsis so as to provide cellular information on the function of this gene. CaPLA1 overexpressors had significantly longer roots, leaves and petioles, and grew more rapidly than the wild-type plants, leading to an early bolting phenotype with prolonged inflorescence. Microscopic analysis showed that the vegetative tissues of 35S:CaPLA1 plants contained an increased number of small-sized cells, which resulted in highly populated cell layers. In addition, mRNAs for cell cycle-controlled proteins and fatty acid catabolizing enzymes were coordinately upregulated in CaPLA1-overexpressing plants. These results suggest that CaPLA1 is functionally relevant in heterologous Arabidopsis cells, and hence might participate in a subset of positive control mechanisms of cell and tissue growth in transgenic lines. We discuss possible biochemical and cellular functions of CaPLA1 in relation to the phospholipid signaling pathway in hot pepper and transgenic Arabidopsis plants.

The role of local biosynthesis of auxin and cytokinin in plant development

Plant hormones are tightly regulated in response to environmental and developmental signals. It has long been speculated that biosynthesis of hormones occurs broadly in plant organs and that intricate, spatiotemporal regulation of hormones in developing organ primordia is achieved through transport and signal perception. However, recent identification of genes crucial for biosynthesis of auxin and cytokinin reveals that localized hormone biosynthesis also plays an important role in organ growth and patterning.

yucca6, a dominant mutation in Arabidopsis, affects auxin accumulation and auxin-related phenotypes

Auxin plays critical roles in many aspects of plant growth and development. Although a number of auxin biosynthetic pathways have been identified, their overlapping nature has prevented a clear elucidation of auxin biosynthesis. Recently, Arabidopsis (Arabidopsis thaliana) mutants with supernormal auxin phenotypes have been reported. These mutants exhibit hyperactivation of genes belonging to the YUCCA family, encoding putative flavin monooxygenase enzymes that result in increased endogenous auxin levels. Here, we report the discovery of fertile dominant Arabidopsis hypertall1-1D and hypertall1-2D (yucca6-1D, -2D) mutants that exhibit typical auxin overproduction phenotypic alterations, such as epinastic cotyledons, increased apical dominance, and curled leaves. However, unlike other auxin overproduction mutants, yucca6 plants do not display short or hairy root phenotypes and lack morphological changes under dark conditions. In addition, yucca6-1D and yucca6-2D have extremely tall (>1 m) inflorescences with extreme apical dominance and twisted cauline leaves. Microarray analyses revealed that expression of several indole-3-acetic acid-inducible genes, including Aux/IAA, SMALL AUXIN-UP RNA, and GH3, is severalfold higher in yucca6 mutants than in the wild type. Tryptophan (Trp) analog feeding experiments and catalytic activity assays with recombinant YUCCA6 indicate that YUCCA6 is involved in a Trp-dependent auxin biosynthesis pathway. YUCCA6:GREEN FLUORESCENT PROTEIN fusion protein indicates YUCCA6 protein exhibits a nonplastidial subcellular localization in an unidentified intracellular compartment. Taken together, our results identify YUCCA6 as a functional member of the YUCCA family with unique roles in growth and development.

CRM1/BIG-mediated auxin action regulates Arabidopsis inflorescence development

The shape of the inflorescence in Arabidopsis thaliana ecotype Columbia is a raceme with individual flowers developing acropetally. The ecotype Landsberg harboring the erecta (er) mutation shows a corymb-like inflorescence, namely a compact inflorescence with a flattened arrangement of flower buds at the tip. To gain insight into inflorescence development, we previously isolated corymb-like inflorescence mutants, named corymbosa1 (crm1), and found that the corymb-like inflorescence in crm1-1 was due to reduced cell elongation of pedicels and stem internodes. Double mutants of crm1 with er and crm2, and crm1-1 crm2-1 er-105 triple mutants show an additive phenotype. crm1-1 is caused by a mutation in BIG, which is required for polar auxin transport. CRM1/BIG is expressed in inflorescence meristems, floral meristems and vascular tissues. We analyzed a collection of 12 reduced lateral root formation (rlr) mutants, which are allelic to crm1-1, and categorized the mutants into three classes, depending on the plant developmental defects. Although all 12 alleles had new stop codons, the phenotype of heterozygous crm1-1/doc1-1 and Northern blotting suggest that new crm1/big mutant alleles are hypomorphic. Auxin-responsive DR5rev::GFP expression was decreased in crm1-1 vasculature of pedicels and stem internodes. PINFORMED1 (PIN1) and CRM1/BIG are expressed in vasculature of pedicels and stem internodes. The severity of corymb-like inflorescence in crm1/big mutants correlated with increased levels of PIN1. Our results suggest that CRM1/BIG controls the elongation of the pedicels and stem internodes through auxin action.

Flavin-containing monooxygenases in plants: looking beyond detox

Flavin-containing monooxygenases (FMOs) are known in bacteria, yeast and mammals where they catalyze the transfer of one atom of molecular O(2) to low molecular weight substrates. The predominant physiological function of animal FMOs appears to be detoxification of a vast spectrum of xenobiotics but until recently very little was known about the function of FMOs in plants. In the last two to three years, genetic and biochemical characterization has shown that plant FMOs can catalyze specific steps in the biosynthesis of auxin or in the metabolism of glucosinolates, and, furthermore, have a role in pathogen defence. Thus, plant FMOs hint that further FMO functions might be identified also in non-plant organisms and could stimulate novel research in this area.

Constitutively wilted 1, a member of the rice YUCCA gene family, is required for maintaining water homeostasis and an appropriate root to shoot ratio

Increasing its root to shoot ratio is a plant strategy for restoring water homeostasis in response to the long-term imposition of mild water stress. In addition to its important role in diverse fundamental processes, indole-3-acetic acid (IAA) is involved in root growth and development. Recent extensive characterizations of the YUCCA gene family in Arabidopsis and rice have elucidated that member's function in a tryptophan-dependent IAA biosynthetic pathway. Through forward- and reverse-genetics screening, we have isolated Tos17 and T-DNA insertional rice mutants in a CONSTITUTIVELY WILTED1 (COW1) gene, which encodes a new member of the YUCCA protein family. Homozygous plants with either a Tos17 or T-DNA-inserted allele of OsCOW1 exhibit phenotypes of rolled leaves, reduced leaf widths, and lower root to shoot ratios. These phenotypes are evident in seedlings as early as 7-10 d after germination, and remain until maturity. When oscow1 seedlings are grown under low-intensity light and high relative humidity, the rolled-leaf phenotype is greatly alleviated. For comparison, in such conditions, the transpiration rate for WT leaves decreases approx. 5- to 10-fold, implying that this mutant trait results from wilting rather than being a morphogenic defect. Furthermore, a lower turgor potential and transpiration rate in their mature leaves indicates that oscow1 plants are water-deficient, due to insufficient water uptake that possibly stems from that diminished root to shoot ratio. Thus, our observations suggest that OsCOW1-mediated IAA biosynthesis plays an important role in maintaining root to shoot ratios and, in turn, affects water homeostasis in rice.

A barley activation tagging system

Activation tagging, as the result of random genomic insertion of either promoter or enhancer sequences, can produce novel, dominant mutations by over-expression of endogenous genes. This powerful genomics tool has been used extensively in dicot species such as Arabidopsis, while rice is the only cereal for which an equivalent system exists. In this study we describe an activation tagging system in barley based upon the maize Ac/Ds transposable element system. A modified Ds element (UbiDs) containing two maize polyubiquitin promoters, transposed in families derived from multiple independent UbiDs transformants and generated new Ds insertion events at frequencies ranging from 0% to 52% per family. The majority of transposed UbiDs elements activated high levels of adjacent flanking sequence transcription. Transposon-mediated expression was detected in all barley cell and tissue types analysed suggesting that this system is applicable to all aspects of plant development and biogenesis. In addition to transcriptional activation, this system is also capable of generating insertional knockout mutants and a UbiDs inactivated allele of the granule bound starch synthase I gene (waxy) was recovered that lead to reduced amylose accumulation. The recovery and analysis of dominant over-expression phenotypes generated by this system will provide a novel approach to understanding gene function in large cereal genomes where gene redundancy may mask conventional loss-of-function mutations.

Identification of a flavin-monooxygenase as the S-oxygenating enzyme in aliphatic glucosinolate biosynthesis in Arabidopsis

The cancer-preventive activity of cruciferous vegetables is commonly attributed to isothiocyanates resulting from the breakdown of the natural products glucosinolates (GSLs). Sulforaphane, the isothiocyanate derived from 4-methylsulfinylbutyl GSL, is thought to be the major agent conferring cancer-preventive properties, whereas the isothiocyanate of 4-methylthiobutyl GSL does not have the same activity. We report the identification of an Arabidopsis flavin-monooxygenase (FMO) enzyme, FMO(GS-OX1), which catalyzes the conversion of methylthioalkyl GSLs into methylsulfinylalkyl GSLs. This is evidenced by biochemical characterization of the recombinant protein, and analyses of the GSL content in FMO(GS-OX1) overexpression lines and an FMO(GS-OX1) knock-out mutant of Arabidopsis. The FMO(GS-OX1) overexpression lines show almost complete conversion of methylthioalkyl into methylsulfinylalkyl GSLs, with an approximately fivefold increase in 4-methylsulfinylbutyl GSL in seeds. Identification of FMO(GS-OX1) provides a molecular tool for breeding of Brassica vegetable crops with increased levels of this important GSL, which has implications for production of functional foods enriched with the cancer-preventive sulforaphane.

Auxin biosynthesis by the YUCCA genes in rice

Although indole-3-acetic acid (IAA), the predominant auxin in plants, plays a critical role in various plant growth and developmental processes, its biosynthesis and regulation have not been clearly elucidated. To investigate the molecular mechanisms of IAA synthesis in rice (Oryza sativa), we identified seven YUCCA-like genes (named OsYUCCA1-7) in the rice genome. Plants overexpressing OsYUCCA1 exhibited increased IAA levels and characteristic auxin overproduction phenotypes, whereas plants expressing antisense OsYUCCA1 cDNA displayed defects that are similar to those of rice auxin-insensitive mutants. OsYUCCA1 was expressed in almost all of the organs tested, but its expression was restricted to discrete areas, including the tips of leaves, roots, and vascular tissues, where it overlapped with expression of a beta-glucuronidase reporter gene controlled by the auxin-responsive DR5 promoter. These observations are consistent with an important role for the rice enzyme OsYUCCA1 in IAA biosynthesis via the tryptophan-dependent pathway.

Activation tagging in plants—generation of novel, gain-of-function mutations

Activation tagging is a mutagenesis strategy that generates dominant, gain-of-function mutations as a consequence of gene over-expression. These mutations cause a class of mutant previously unobtainable by conventional mutagenesis. Unlike most mutant phenotypes, which are generally a consequence of gene inactivation, activation tagged phenotypes arise from excess functional gene product. Gene over-expression mutations are obtained by randomly inserting regulatory sequences throughout the genome, using either high-throughput plant transformation or mobile transposable elements to distribute these regulatory elements. Since the sequence of the regulatory element vector is known, it acts as a molecular tag, making isolation of the over-expressed gene a relatively straightforward process using standard molecular biological techniques. Activation tagged phenotypes have been generated by the over-expression of genes encoding a diverse range of protein and RNA products that are involved in all aspects of plant biogenesis. This mutation approach has been used extensively in Arabidopsis and to a lesser extent in several other species. In this review we summarise activation tagging in plants and suggest that the development of this mutagenesis strategy in more plants of agronomic significance is highly desirable.

Autonomy of cell proliferation and developmental programs during Arabidopsis aboveground organ morphogenesis

Elaboration of size and shape in multicellular organisms involves coordinated cell division and cell growth. In higher plants, continuity of cell layer structures exists from the shoot apical meristem (SAM), where organ primordia arise, to mature aboveground organs. To unravel the extent of inter-cell layer coordination during SAM and aboveground organ development, cell division in the epidermis was selectively restricted by expressing two cyclin-dependent kinase inhibitor genes, KRP1/ICK1 and KRP4, driven by the L1 layer-specific AtML1 promoter. The transgenes conferred reduced plant size with striking, distorted lateral organ shape. While epidermal cell division was severely inhibited with compensatory cell size enlargement, the underlying mesophyll/cortex layer kept normal cell numbers and resulted in small, packed cells with disrupted cell files. Our results demonstrate the autonomy of cell number checkpoint in the underlying tissues when epidermal cell division is restricted. Finally, the L1 layer-specific expression of both KRP1/ICK1 and KRP4 showed no effects on the structure and function of the SAM, suggesting that the effects of these cyclin-dependent kinase inhibitors are context dependent.

Involvement of HLS1 in Sugar and Auxin Signaling in Arabidopsis Leaves

Sugar regulates a variety of genes and controls plant growth and development similarly to phytohormones. As part of a screen for Arabidopsis mutants with defects in sugar-responsive gene expression, we identified a loss-of-function mutation in the HOOKLESS1 (HLS1) gene. HLS1 was originally identified to regulate apical hook formation of dark-grown seedlings (Lehman et al., 1996, Cell 85: 183-194). In hls1, sugar-induced gene expression in excised leaf petioles was more sensitive to exogenous sucrose than that in the wild type. Exogenous IAA partially repressed sugar-induced gene expression and concomitantly activated some auxin response genes such as AUR3 encoding GH3-like protein. The repression and the induction of gene expression by auxin were attenuated and enhanced, respectively, by the hls1 mutation. These results suggest that HLS1 plays a negative role in sugar and auxin signaling. Because AUR3 GH3-like protein conjugates free IAA to amino acids (Staswick et al., 2002, Plant Cell 14: 1405-1415; Staswick et al., 2005, Plant Cell 17: 616-627), enhanced expression of GH3-like genes would result in a decrease in the free IAA level. Indeed, hls1 leaves accumulated a reduced level of free IAA, suggesting that HLS1 may be involved in negative feedback regulation of IAA homeostasis through the control of GH3-like genes. We discuss the possible mechanisms by which HLS1 is involved in auxin signaling for sugar- and auxin-responsive gene expression and in IAA homeostasis.

Auxin biosynthesis by the YUCCA flavin monooxygenases controls the formation of floral organs and vascular tissues in Arabidopsis

Auxin biosynthesis in plants has remained obscure although auxin has been known for decades as a key regulator for plant growth and development. Here we define the YUC gene family and show unequivocally that four of the 11 predicted YUC flavin monooxygenases (YUC1, YUC2, YUC4, and YUC6) play essential roles in auxin biosynthesis and plant development. The YUC genes are mainly expressed in meristems, young primordia, vascular tissues, and reproductive organs. Overexpression of each YUC gene leads to auxin overproduction, whereas disruption of a single YUC gene causes no obvious developmental defects. However, yuc1yuc4, yuc2yuc6, all of the triple and quadruple mutants of the four YUC genes, display severe defects in floral patterning, vascular formation, and other developmental processes. Furthermore, inactivation of the YUC genes leads to dramatically reduced expression of the auxin reporter DR5-GUS in tissues where the YUC genes are expressed. Moreover, the developmental defects of yuc1yuc4 and yuc1yuc2yuc6 are rescued by tissue-specific expression of the bacterial auxin biosynthesis gene iaaM, but not by exogenous auxin, demonstrating that spatially and temporally regulated auxin biosynthesis by the YUC genes is essential for the formation of floral organs and vascular tissues.

A role for a flavin-containing mono-oxygenase in resistance against microbial pathogens in Arabidopsis

Using activation tagging in the Arabidopsis Col-0 rps2-101C background, we identified a mutant (FMO1-3D) that showed virtually no symptoms after inoculation with virulent Pseudomonas syringae pv. tomato DC3000 bacteria. The dominant, gain-of-function phenotype of the FMO1-3D mutant is due to over-expression of a class 3 flavin-containing mono-oxygenase (FMO). We recapitulated the FMO1-3D mutant phenotype in independent transgenic Col-0 lines over-expressing the FMO1 cDNA under the control of the 35S CaMV promoter. The increased basal resistance observed in the FMO1-3D mutant was also effective against the taxonomically unrelated downy mildew-causing pathogen Hyaloperonospora parasitica. By investigating the progeny from crosses of the FMO1-3D mutant with the NahG transgenic line, we showed that the enhanced basal resistance phenotype was dependent on the accumulation of salicylic acid. FMO1-3D plants showed wild-type resistant reactions after inoculation with avirulent bacteria, indicating that the R-gene-mediated defence physiology was not compromised by FMO1 over-expression. Transcripts of the class 3 FMO1 gene accumulated within 6 h after inoculation of wild-type Col-0 plants with avirulent Pst + avrRpt2 cells. Moreover, a T-DNA insertion into the FMO1 gene resulted in enhanced susceptibility to virulent Pseudomonas and Hyaloperonospora parasitica, suggesting that expression of the FMO1 gene is a hitherto undescribed component of the plant's resistance repertoire. We discuss the possibility that the FMO may participate in the detoxification of virulence factors produced by pathogens.

Auxin biosynthesis by the YUCCA flavin monooxygenases controls the formation of floral organs and vascular tissues in Arabidopsis

Auxin biosynthesis in plants has remained obscure although auxin has been known for decades as a key regulator for plant growth and development. Here we define the YUC gene family and show unequivocally that four of the 11 predicted YUC flavin monooxygenases (YUC1, YUC2, YUC4, and YUC6) play essential roles in auxin biosynthesis and plant development. The YUC genes are mainly expressed in meristems, young primordia, vascular tissues, and reproductive organs. Overexpression of each YUC gene leads to auxin overproduction, whereas disruption of a single YUC gene causes no obvious developmental defects. However, yuc1yuc4, yuc2yuc6, all of the triple and quadruple mutants of the four YUC genes, display severe defects in floral patterning, vascular formation, and other developmental processes. Furthermore, inactivation of the YUC genes leads to dramatically reduced expression of the auxin reporter DR5-GUS in tissues where the YUC genes are expressed. Moreover, the developmental defects of yuc1yuc4 and yuc1yuc2yuc6 are rescued by tissue-specific expression of the bacterial auxin biosynthesis gene iaaM, but not by exogenous auxin, demonstrating that spatially and temporally regulated auxin biosynthesis by the YUC genes is essential for the formation of floral organs and vascular tissues.