ELONGATED UPPERMOST INTERNODE encodes a cytochrome P450 monooxygenase that epoxidizes gibberellins in a novel deactivation reaction in rice

Improving panicle exsertion of rice cytoplasmic male sterile line by combination of artificial microRNA and artificial target mimic

The adoption of hybrid rice caused the second leap in rice yield after the 'green revolution' and contributes substantially to food security of China and the world. However, almost all cytoplasmic male sterile lines (A lines) as females of hybrid rice have a natural deficiency of 'panicle enclosure', which blocks pollination between the A line and the fertility restorer line as the male (R line) of hybrid rice and decreases seed yield. In hybrid rice seed production, exogenous '920' (the active ingredient is gibberellin A3 ) must be applied to eliminate or alleviate panicle enclosure of the A line; however, this not only increases production cost and pollutes the environment, it also decreases seed quality. In this study, we designed a transgenic approach to improve plant height and panicle exsertion of the A line to facilitate hybrid rice production and maintain the semi-dwarf plant type of the hybrid. This approach comprising two components-artificial microRNA (amiRNA) and artificial target mimicry-can manipulate the differential expression of the endogenous Eui1 gene that is associated with rice internode elongation in the A line and the hybrid. amiRNA is a recently developed gene silencing method with high specificity, while target mimicry is a natural mechanism inhibiting the miRNA function that was also recently characterized. This approach provides a paradigm to tune the expression of endogenous genes to achieve the desired phenotype by combining amiRNA and artificial target mimicry technologies.

The U-box E3 ubiquitin ligase TUD1 functions with a heterotrimeric G α subunit to regulate Brassinosteroid-mediated growth in rice

Heterotrimeric G proteins are an important group of signaling molecules found in eukaryotes. They function with G-protein-coupled-receptors (GPCRs) to transduce various signals such as steroid hormones in animals. Nevertheless, their functions in plants are not well-defined. Previous studies suggested that the heterotrimeric G protein α subunit known as D1/RGA1 in rice is involved in a phytohormone gibberellin-mediated signaling pathway. Evidence also implicates D1 in the action of a second phytohormone Brassinosteroid (BR) and its pathway. However, it is unclear how D1 functions in this pathway, because so far no partner has been identified to act with D1. In this study, we report a D1 genetic interactor Taihu Dwarf1 (TUD1) that encodes a functional U-box E3 ubiquitin ligase. Genetic, phenotypic, and physiological analyses have shown that tud1 is epistatic to d1 and is less sensitive to BR treatment. Histological observations showed that the dwarf phenotype of tud1 is mainly due to decreased cell proliferation and disorganized cell files in aerial organs. Furthermore, we found that D1 directly interacts with TUD1. Taken together, these results demonstrate that D1 and TUD1 act together to mediate a BR-signaling pathway. This supports the idea that a D1-mediated BR signaling pathway occurs in rice to affect plant growth and development.

Gibberellin 20-oxidase gene OsGA20ox3 regulates plant stature and disease development in rice

Gibberellin (GA) 20-oxidase (GA20ox) catalyses consecutive steps of oxidation in the late part of the GA biosynthetic pathway. A T-DNA insertion mutant (17S-14) in rice, with an elongated phenotype, was isolated. Analysis of the flanking sequences of the T-DNA insertion site revealed that an incomplete T-DNA integration resulted in enhanced constitutively expression of downstream OsGA20ox3 in the mutant. The accumulation of bioactive GA(1) and GA(4) were increased in the mutant in comparison with the wild-type plant. Transgenic plants overexpressing OsGA20ox3 showed phenotypes similar to those of the 17S-14 mutant, and the RNA interference (RNAi) lines that had decreased OsGA20ox3 expression exhibited a semidwarf phenotype. Expression of OsGA20ox3 was detected in the leaves and roots of young seedlings, immature panicles, anthers, and pollens, based on β-glucuronidase (GUS) activity staining in transgenic plants expressing the OsGA20ox3 promoter fused to the GUS gene. The OsGA20ox3 RNAi lines showed enhanced resistance against rice pathogens Magnaporthe oryzae (causing rice blast) and Xanthomonas oryzae pv. oryzae (causing bacterial blight) and increased expression of defense-related genes. Conversely, OsGA20ox3-overexpressing plants were more susceptible to these pathogens comparing with the wild-type plants. The susceptibility of wild-type plants to X. oryzae pv. oryzae was increased by exogenous application of GA(3) and decreased by S-3307 treatment. Together, the results provide direct evidence for a critical role of OsGA20ox3 in regulating not only plant stature but also disease resistance in rice.

An efficient rice mutagenesis system based on suspension-cultured cells

Plant mutants are important bio-resources for crop breeding and gene functional studies. Conventional methods for generating mutant libraries by mutagenesis of seeds with physical or chemical agents are of low efficiency. Here, we developed a highly-efficient ethyl methanesulfonate (EMS) mutagenesis system based on suspension-cultured cells, with rice (Oryza sativa L.) as an example. We show that treatment of suspension-cultured tiny cell clusters with 0.4% EMS for 18-22 h followed by differentiation and regeneration produced as high as 29.4% independent mutant lines with visible phenotypic variations, including a number of important agronomic traits such as grain size, panicle size, grain or panicle shape, tiller number and angle, heading date, male sterility, and disease sensitivity. No mosaic mutant was observed in the mutant lines tested. In this mutant library, we obtained a mutant with an abnormally elongated uppermost internode. Sequencing and functional analysis revealed that this is a new allelic mutant of eui (elongated uppermost internode) caused by two point mutations in the first exon of the EUI gene, representing a successful example of this mutagenesis system.

ZmGA3ox2, a candidate gene for a major QTL, qPH3.1, for plant height in maize

Maize plant height is closely associated with biomass, lodging resistance and grain yield. Determining the genetic basis of plant height by characterizing and cloning plant height genes will guide the genetic improvement of crops. In this study, a quantitative trait locus (QTL) for plant height, qPH3.1, was identified on chromosome 3 using populations derived from a cross between Zong3 and its chromosome segment substitution line, SL15. The plant height of the two lines was obviously different, and application of exogenous gibberellin A(3) removed this difference. QTL mapping placed qPH3.1 within a 4.0 cM interval, explaining 32.3% of the phenotypic variance. Furthermore, eight homozygous segmental isolines (SILs) developed from two larger F(2) populations further narrowed down qPH3.1 to within a 12.6 kb interval. ZmGA3ox2, an ortholog of OsGA3ox2, which encodes a GA3 β-hydroxylase, was positionally cloned. Association mapping identified two polymorphisms in ZmGA3ox2 that were significantly associated with plant height across two experiments. Quantitative RT-PCR showed that SL15 had higher ZmGA3ox2 expression relative to Zong3. The resultant higher GA(1) accumulation led to longer internodes in SL15 because of increased cell lengths. Moreover, a large deletion in the coding region of ZmGA3ox2 is responsible for the dwarf mutant d1-6016. The successfully isolated qPH3.1 enriches our knowledge on the genetic basis of plant height in maize, and provides an opportunity for improvement of plant architecture in maize breeding.

CYP714B1 and CYP714B2 encode gibberellin 13-oxidases that reduce gibberellin activity in rice

Bioactive gibberellins (GAs) control many aspects of growth and development in plants. GA(1) has been the most frequently found bioactive GA in various tissues of flowering plants, but the enzymes responsible for GA(1) biosynthesis have not been fully elucidated due to the enzymes catalyzing the 13-hydroxylation step not being identified. Because of the lack of mutants defective in this enzyme, biological significance of GA 13-hydroxylation has been unknown. Here, we report that two cytochrome P450 genes, CYP714B1 and CYP714B2, encode GA 13-oxidase in rice. Transgenic Arabidopsis plants that overexpress CYP714B1 or CYP714B2 show semidwarfism. There was a trend that the levels of 13-OH GAs including GA(1) were increased in these transgenic plants. Functional analysis using yeast or insect cells shows that recombinant CYP714B1 and CYP714B2 proteins can convert GA(12) into GA(53) (13-OH GA(12)) in vitro. Moreover, the levels of 13-OH GAs including GA(1) were decreased, whereas those of 13-H GAs including GA(4) (which is more active than GA(1)) were increased, in the rice cyp714b1 cyp714b2 double mutant. These results indicate that CYP714B1 and CYP714B2 play a predominant role in GA 13-hydroxylation in rice. The double mutant plants appear phenotypically normal until heading, but show elongated uppermost internode at the heading stage. Moreover, CYP714B1 and CYP714B2 expression was up-regulated by exogenous application of bioactive GAs. Our results suggest that GA 13-oxidases play a role in fine-tuning plant growth by decreasing GA bioactivity in rice and that they also participate in GA homeostasis.

SUI-family genes encode phosphatidylserine synthases and regulate stem development in rice

In vascular plants, the regulation of stem cell niche determines development of aerial shoot which consists of stems and lateral organs. Intercalary meristem (IM) controls internode elongation in rice and other grasses, however little attention has been paid to the underlying mechanism of stem cell maintenance. Here, we investigated the stem development in rice and showed that the Shortened Uppermost Internode 1 (SUI1) family of genes are pivotal for development of rice stems. We demonstrated that SUI-family genes regulate the development of IM for internode elongation and also the cell expansion of the panicle stem rachis in rice. The SUI-family genes encoded base-exchange types of phosphatidylserine synthases (PSSs), which possessed enzymatic activity in a yeast complementary assay. Overexpression of SUI1 and SUI2 caused outgrowths of internodes during vegetative development, and we showed that expression patterns of Oryza Sativa Homeobox 15 (OSH15) and Histone4 were impaired. Furthermore, genome-wide gene expression analysis revealed that overexpression and RNA knockdown of SUI-family genes affected downstream gene expression related to phospholipid metabolic pathways. Moreover, using Ultra-performance liquid chromatography-quadrupole time of flight-mass spectrometry, we analyzed PS contents in different genetic backgrounds of rice and showed that the quantity of very long chain fatty acids PS is affected by transgene of SUI-family genes. Our study reveals a new mechanism conveyed by the SUI1 pathway and provides evidence to link lipid metabolism with plant stem cell maintenance.

Re-sequencing and genetic variation identification of a rice line with ideal plant architecture

BACKGROUND

The ideal plant architecture (IPA) includes several important characteristics such as low tiller numbers, few or no unproductive tillers, more grains per panicle, and thick and sturdy stems. We have developed an indica restorer line 7302R that displays the IPA phenotype in terms of tiller number, grain number, and stem strength. However, its mechanism had to be clarified.

FINDINGS

We performed re-sequencing and genome-wide variation analysis of 7302R using the Solexa sequencing technology. With the genomic sequence of the indica cultivar 9311 as reference, 307 627 SNPs, 57 372 InDels, and 3 096 SVs were identified in the 7302R genome. The 7302R-specific variations were investigated via the synteny analysis of all the SNPs of 7302R with those of the previous sequenced none-IPA-type lines IR24, MH63, and SH527. Moreover, we found 178 168 7302R-specific SNPs across the whole genome and 30 239 SNPs in the predicted mRNA regions, among which 8 517 were Non-syn CDS. In addition, 263 large-effect SNPs that were expected to affect the integrity of encoded proteins were identified from the 7302R-specific SNPs. SNPs of several important previously cloned rice genes were also identified by aligning the 7302R sequence with other sequence lines.

CONCLUSIONS

Our results provided several candidates account for the IPA phenotype of 7302R. These results therefore lay the groundwork for long-term efforts to uncover important genes and alleles for rice plant architecture construction, also offer useful data resources for future genetic and genomic studies in rice.

Impacts of diversification of cytochrome P450 on plant metabolism

Cytochrome P450 monooxygenases (P450s) catalyze a wide variety of monooxygenation reactions in primary and secondary metabolism in plants. The share of P450 genes in each plant genome is estimated to be up to 1%. This implies that the diversification of P450 has made a significant contribution to the ability to acquire the emergence of new metabolic pathways during land plant evolution. The P450 families conserved universally in land plants contribute to their chemical defense mechanisms. Several P450s are involved in the biosynthesis and catabolism of plant hormones. Species-specific P450 families are essential for the biosynthetic pathways of phytochemicals such as terpenoids and alkaloids. Genome wide analysis of the gene clusters including P450 genes will provide a clue to defining the metabolic roles of orphan P450s. Metabolic engineering with plant P450s is an important technology for large-scale production of valuable phytochemicals such as medicines.

The rice hydroperoxide lyase OsHPL3 functions in defense responses by modulating the oxylipin pathway

As important signal molecules, jasmonates (JAs) and green leaf volatiles (GLVs) play diverse roles in plant defense responses against insect pests and pathogens. However, how plants employ their specific defense responses by modulating the levels of JA and GLVs remains unclear. Here, we describe identification of a role for the rice HPL3 gene, which encodes a hydroperoxide lyase (HPL), OsHPL3/CYP74B2, in mediating plant-specific defense responses. The loss-of-function mutant hpl3-1 produced disease-resembling lesions spreading through the whole leaves. A biochemical assay revealed that OsHPL3 possesses intrinsic HPL activity, hydrolyzing hydroperoxylinolenic acid to produce GLVs. The hpl3-1 plants exhibited enhanced induction of JA, trypsin proteinase inhibitors and other volatiles, but decreased levels of GLVs including (Z)-3-hexen-1-ol. OsHPL3 positively modulates resistance to the rice brown planthopper [BPH, Nilaparvata lugens (Stål)] but negatively modulates resistance to the rice striped stem borer [SSB, Chilo suppressalis (Walker)]. Moreover, hpl3-1 plants were more attractive to a BPH egg parasitoid, Anagrus nilaparvatae, than the wild-type, most likely as a result of increased release of BPH-induced volatiles. Interestingly, hpl3-1 plants also showed increased resistance to bacterial blight (Xanthomonas oryzae pv. oryzae). Collectively, these results indicate that OsHPL3, by affecting the levels of JA, GLVs and other volatiles, modulates rice-specific defense responses against different invaders.

Photocontrol of bud burst involves gibberellin biosynthesis in Rosa sp

Light is a critical determinant of plant shape by controlling branching patterns and bud burst in many species. To gain insight into how light induces bud burst, we investigated whether its inductive effect in rose was related to gibberellin (GA) biosynthesis. In axillary buds of beheaded plants subject to light, the expression of two GA biosynthesis genes (RoGA20ox and RoGA3ox) was promptly and strongly induced, while that of a GA-catabolism genes (RoGA2ox) was reduced. By contrast, lower expression levels of these two GA biosynthesis genes were found in darkness, and correlated with a total inhibition of bud burst. This effect was dependent on both light intensity and quality. In in vitro cultured buds, the inductive effect of light on the growth of preformed leaves and SAM organogenic activity was inhibited by ancymidol and paclobutrazol, two effectors of GA biosynthesis. This effect was concentration-dependent, and negated by GA(3). However, GA(3) alone could not rescue bud burst in the dark. GA biosynthesis was also required for the expression and activity of a vacuolar invertase, and therefore for light-induced sugar metabolism within buds. These findings are evidence that GA biosynthesis contributes to the light effect on bud burst and lay the foundations of a better understanding of its exact role in plant branching.

Gibberellin biosynthesis and its regulation

The GAs (gibberellins) comprise a large group of diterpenoid carboxylic acids that are ubiquitous in higher plants, in which certain members function as endogenous growth regulators, promoting organ expansion and developmental changes. These compounds are also produced by some species of lower plants, fungi and bacteria, although, in contrast to higher plants, the function of GAs in these organisms has only recently been investigated and is still unclear. In higher plants, GAs are synthesized by the action of terpene cyclases, cytochrome P450 mono-oxygenases and 2-oxoglutarate-dependent dioxygenases localized, respectively, in plastids, the endomembrane system and the cytosol. The concentration of biologically active GAs at their sites of action is tightly regulated and is moderated by numerous developmental and environmental cues. Recent research has focused on regulatory mechanisms, acting primarily on expression of the genes that encode the dioxygenases involved in biosynthesis and deactivation. The present review discusses the current state of knowledge on GA metabolism with particular emphasis on regulation, including the complex mechanisms for the maintenance of GA homoeostasis.

Arabidopsis acetyl-amido synthetase GH3.5 involvement in camalexin biosynthesis through conjugation of indole-3-carboxylic acid and cysteine and upregulation of camalexin biosynthesis genes

Camalexin (3-thiazol-2'-yl-indole) is the major phytoalexin found in Arabidopsis thaliana. Several key intermediates and corresponding enzymes have been identified in camalexin biosynthesis through mutant screening and biochemical experiments. Camalexin is formed when indole-3-acetonitrile (IAN) is catalyzed by the cytochrome P450 monooxygenase CYP71A13. Here, we demonstrate that the Arabidopsis GH3.5 protein, a multifunctional acetyl-amido synthetase, is involved in camalexin biosynthesis via conjugating indole-3-carboxylic acid (ICA) and cysteine (Cys) and regulating camalexin biosynthesis genes. Camalexin levels were increased in the activation-tagged mutant gh3.5-1D in both Col-0 and cyp71A13-2 mutant backgrounds after pathogen infection. The recombinant GH3.5 protein catalyzed the conjugation of ICA and Cys to form a possible intermediate indole-3-acyl-cysteinate (ICA(Cys)) in vitro. In support of the in vitro reaction, feeding with ICA and Cys increased camalexin levels in Col-0 and gh3.5-1D. Dihydrocamalexic acid (DHCA), the precursor of camalexin and the substrate for PAD3, was accumulated in gh3.5-1D/pad3-1, suggesting that ICA(Cys) could be an additional precursor of DHCA for camalexin biosynthesis. Furthermore, expression of the major camalexin biosynthesis genes CYP79B2, CYP71A12, CYP71A13 and PAD3 was strongly induced in gh3.5-1D. Our study suggests that GH3.5 is involved in camalexin biosynthesis through direct catalyzation of the formation of ICA(Cys), and upregulation of the major biosynthetic pathway genes.

Plant hormone jasmonate prioritizes defense over growth by interfering with gibberellin signaling cascade

Plants must effectively defend against biotic and abiotic stresses to survive in nature. However, this defense is costly and is often accompanied by significant growth inhibition. How plants coordinate the fluctuating growth-defense dynamics is not well understood and remains a fundamental question. Jasmonate (JA) and gibberellic acid (GA) are important plant hormones that mediate defense and growth, respectively. Binding of bioactive JA or GA ligands to cognate receptors leads to proteasome-dependent degradation of specific transcriptional repressors (the JAZ or DELLA family of proteins), which, at the resting state, represses cognate transcription factors involved in defense (e.g., MYCs) or growth [e.g. phytochrome interacting factors (PIFs)]. In this study, we found that the coi1 JA receptor mutants of rice (a domesticated monocot crop) and Arabidopsis (a model dicot plant) both exhibit hallmark phenotypes of GA-hypersensitive mutants. JA delays GA-mediated DELLA protein degradation, and the della mutant is less sensitive to JA for growth inhibition. Overexpression of a selected group of JAZ repressors in Arabidopsis plants partially phenocopies GA-associated phenotypes of the coi1 mutant, and JAZ9 inhibits RGA (a DELLA protein) interaction with transcription factor PIF3. Importantly, the pif quadruple (pifq) mutant no longer responds to JA-induced growth inhibition, and overexpression of PIF3 could partially overcome JA-induced growth inhibition. Thus, a molecular cascade involving the COI1-JAZ-DELLA-PIF signaling module, by which angiosperm plants prioritize JA-mediated defense over growth, has been elucidated.

Cloning of a Zoysia ZjLsL and its overexpression to induce axillary meristem initiation and tiller formation in Arabidopsis and bentgrass

Zoysia grass and creeping bentgrass are important turf grasses used in parks, gardens and playing fields. Development of grasses with increased tiller formation will enhance their commercial cultivation. To investigate the regulatory mechanism of tiller formation, we cloned the Zoysia japonica Lateral suppressor-like (ZjLsL) gene. The Lateral suppressor (Ls) gene encodes a transcriptional regulator belonging to the plant-specific GRAS protein family of putative transcription factors, and regulates axillary meristem initiation. A full-length DNA of the ZjLsL gene was isolated by 5'/3' DNA walking. Phylogenetic analysis showed that ZjLsL is closely related to Ls genes. Southern blot analysis revealed that zoysia grass has two copies of the ZjLsL gene. ZjLsL expression was detected in all organs of zoysia grass but was most highly expressed in culms. Overexpression of ZjLsL in creeping bentgrass and Arabidopsis plants promoted axillary bud formation. These results suggest that ZjLsL plays an important role in axillary meristem initiation and tiller formation.

Plant hormone jasmonate prioritizes defense over growth by interfering with gibberellin signaling cascade

Plants must effectively defend against biotic and abiotic stresses to survive in nature. However, this defense is costly and is often accompanied by significant growth inhibition. How plants coordinate the fluctuating growth-defense dynamics is not well understood and remains a fundamental question. Jasmonate (JA) and gibberellic acid (GA) are important plant hormones that mediate defense and growth, respectively. Binding of bioactive JA or GA ligands to cognate receptors leads to proteasome-dependent degradation of specific transcriptional repressors (the JAZ or DELLA family of proteins), which, at the resting state, represses cognate transcription factors involved in defense (e.g., MYCs) or growth [e.g. phytochrome interacting factors (PIFs)]. In this study, we found that the coi1 JA receptor mutants of rice (a domesticated monocot crop) and Arabidopsis (a model dicot plant) both exhibit hallmark phenotypes of GA-hypersensitive mutants. JA delays GA-mediated DELLA protein degradation, and the della mutant is less sensitive to JA for growth inhibition. Overexpression of a selected group of JAZ repressors in Arabidopsis plants partially phenocopies GA-associated phenotypes of the coi1 mutant, and JAZ9 inhibits RGA (a DELLA protein) interaction with transcription factor PIF3. Importantly, the pif quadruple (pifq) mutant no longer responds to JA-induced growth inhibition, and overexpression of PIF3 could partially overcome JA-induced growth inhibition. Thus, a molecular cascade involving the COI1-JAZ-DELLA-PIF signaling module, by which angiosperm plants prioritize JA-mediated defense over growth, has been elucidated.

Identification of genome-wide variations among three elite restorer lines for hybrid-rice

Rice restorer lines play an important role in three-line hybrid rice production. Previous research based on molecular tagging has suggested that the restorer lines used widely today have narrow genetic backgrounds. However, patterns of genetic variation at a genome-wide scale in these restorer lines remain largely unknown. The present study performed re-sequencing and genome-wide variation analysis of three important representative restorer lines, namely, IR24, MH63, and SH527, using the Solexa sequencing technology. With the genomic sequence of the Indica cultivar 9311 as the reference, the following genetic features were identified: 267,383 single-nucleotide polymorphisms (SNPs), 52,847 insertion/deletion polymorphisms (InDels), and 3,286 structural variations (SVs) in the genome of IR24; 288,764 SNPs, 59,658 InDels, and 3,226 SVs in MH63; and 259,862 SNPs, 55,500 InDels, and 3,127 SVs in SH527. Variations between samples were also determined by comparative analysis of authentic collections of SNPs, InDels, and SVs, and were functionally annotated. Furthermore, variations in several important genes were also surveyed by alignment analysis in these lines. Our results suggest that genetic variations among these lines, although far lower than those reported in the landrace population, are greater than expected, indicating a complicated genetic basis for the phenotypic diversity of the restorer lines. Identification of genome-wide variation and pattern analysis among the restorer lines will facilitate future genetic studies and the molecular improvement of hybrid rice.

Identification and genetic characterization of a gibberellin 2-oxidase gene that controls tree stature and reproductive growth in plum

Several dwarf plum genotypes (Prunus salicina L.), due to deficiency of unknown gibberellin (GA) signalling, were identified. A cDNA encoding GA 2-oxidase (PslGA2ox), the major gibberellin catabolic enzyme in plants, was cloned and used to screen the GA-deficient hybrids. This resulted in the identification of a dwarf plum hybrid, designated as DGO24, that exhibits a markedly elevated PslGA2ox signal. Grafting 'Early Golden' (EG), a commercial plum cultivar, on DGO24 (EG/D) enhanced PslGA2ox accumulation in the scion part and generated trees of compact stature. Assessment of active GAs in such trees revealed that DGO24 and EG/D accumulated relatively much lower quantities of main bioactive GAs (GA(1) and GA(4)) than control trees (EG/M). Moreover, the physiological function of PslGA2ox was studied by determining the molecular and developmental consequences due to ectopic expression in Arabidopsis. Among several lines, two groups of homozygous transgenics that exhibited contrasting phenotypes were identified. Group-1 displayed a dwarf growth pattern typical of mutants with a GA deficiency including smaller leaves, shorter stems, and delay in the development of reproductive events. In contrast, Group-2 exhibited a 'GA overdose' phenotype as all the plants showed elongated growth, a typical response to GA application, even under limited GA conditions, potentially due to co-suppression of closely related Arabidopsis homologous. The studies reveal the possibility of utilizing PslGA2ox as a marker for developing size-controlling rootstocks in Prunus.

Gibberellins – terpenoid plant hormones: Biological importance and chemical analysis

Gibberellins (GAs) are a large group of diterpenoid carboxylic acids, some members of which function as plant hormones controlling diverse aspects of growth and development. Biochemical, genetic, and genomic approaches have led to the identification of the majority of the genes that encode GA biosynthesis and deactivation enzymes. Recent studies have shown that both GA biosynthesis and deactivation pathways are tightly regulated by developmental, hormonal, and environmental signals, consistent with the role of GAs as key growth regulators. In this review, we summarize our current understanding of the GA biosynthesis and deactivation pathways in plants and fungi, and discuss methods for their qualitative and quantitative analysis. The challenges for their extraction and purification from plant tissues, which form complex matrices containing thousands of interfering substances, are discussed.

Cytochromes P450 CYP94C1 and CYP94B3 catalyze two successive oxidation steps of plant hormone Jasmonoyl-isoleucine for catabolic turnover

The jasmonate hormonal pathway regulates important defensive and developmental processes in plants. Jasmonoyl-isoleucine (JA-Ile) has been identified as a specific ligand binding the COI1-JAZ co-receptor to relieve repression of jasmonate responses. Two JA-Ile derivatives, 12OH-JA-Ile and 12COOH-JA-Ile, accumulate in wounded Arabidopsis leaves in a COI1- and JAR1-dependent manner and reflect catabolic turnover of the hormone. Here we report the biochemical and genetic characterization of two wound-inducible cytochromes P450, CYP94C1 and CYP94B3, that are involved in JA-Ile oxidation. Both enzymes expressed in yeast catalyze two successive oxidation steps of JA-Ile with distinct characteristics. CYP94B3 performed efficiently the initial hydroxylation of JA-Ile to 12OH-JA-Ile, with little conversion to 12COOH-JA-Ile, whereas CYP94C1 catalyzed preferentially carboxy-derivative formation. Metabolic analysis of loss- and gain-of-function plant lines were consistent with in vitro enzymatic properties. cyp94b3 mutants were largely impaired in 12OH-JA-Ile levels upon wounding and to a lesser extent in 12COOH-JA-Ile levels. In contrast, cyp94c1 plants showed wild-type 12OH-JA-Ile accumulation but lost about 60% 12COOH-JA-Ile. cyp94b3cyp94c1 double mutants hyperaccumulated JA-Ile with near abolition of 12COOH-JA-Ile. Distinct JA-Ile oxidation patterns in different plant genotypes were correlated with specific JA-responsive transcript profiles, indicating that JA-Ile oxidation status affects signaling. Interestingly, exaggerated JA-Ile levels were associated with JAZ repressor hyperinduction but did not enhance durably defense gene induction, revealing a novel negative feedback signaling loop. Finally, interfering with CYP94 gene expression affected root growth sensitivity to exogenous jasmonic acid. These results identify CYP94B3/C1-mediated oxidation as a major catabolic route for turning over the JA-Ile hormone.

A novel protein RLS1 with NB-ARM domains is involved in chloroplast degradation during leaf senescence in rice

Leaf senescence, a type of programmed cell death (PCD) characterized by chlorophyll degradation, is important to plant growth and crop productivity. It emerges that autophagy is involved in chloroplast degradation during leaf senescence. However, the molecular mechanism(s) involved in the process is not well understood. In this study, the genetic and physiological characteristics of the rice rls1 (rapid leaf senescence 1) mutant were identified. The rls1 mutant developed small, yellow-brown lesions resembling disease scattered over the whole surfaces of leaves that displayed earlier senescence than those of wild-type plants. The rapid loss of chlorophyll content during senescence was the main cause of accelerated leaf senescence in rls1. Microscopic observation indicated that PCD was misregulated, probably resulting in the accelerated degradation of chloroplasts in rls1 leaves. Map-based cloning of the RLS1 gene revealed that it encodes a previously uncharacterized NB (nucleotide-binding site)-containing protein with an ARM (armadillo) domain at the carboxyl terminus. Consistent with its involvement in leaf senescence, RLS1 was up-regulated during dark-induced leaf senescence and down-regulated by cytokinin. Intriguingly, constitutive expression of RLS1 also slightly accelerated leaf senescence with decreased chlorophyll content in transgenic rice plants. Our study identified a previously uncharacterized NB-ARM protein involved in PCD during plant growth and development, providing a unique tool for dissecting possible autophagy-mediated PCD during senescence in plants.

Identification and characterization of SHORTENED UPPERMOST INTERNODE 1, a gene negatively regulating uppermost internode elongation in rice

In rice, the elongated internodes are derived from the vegetative shoot apical meristem (SAM), and the transition of the SAM from the vegetative to the reproductive stage induces internode elongation. In this study, we characterize two shortened uppermost internode mutants (sui1-1 and sui1-2). During the seedling and tillering stages, sui1 plants are morphologically similar to wild-type plants. However, at the heading stage, the sui1-1 mutant exhibits a shortened uppermost internode and a partly sheathed panicle, and the sui1-2 mutant shows an extremely shortened uppermost internode and a fully sheathed panicle. Gibberellin treatment results in elongation of every internode, but the shortened uppermost internode phenotype remains unaltered. Microscopic analysis indicates that cell length of sui1-1 uppermost internode exhibits decreased. Map-based cloning revealed that SUI1 is located on Chromosome 1, and encodes a putative phosphatidyl serine synthase (PSS) family protein. Searches for matches in protein databases showed that OsSUI1 contains the InterPro domain IPR004277, which is conserved in both animal and plant kingdoms. Introduction of a wild-type SUI1 gene fully rescued the mutant phenotype of sui1-1 and sui1-2, confirming the identity of the cloned gene. Consistent with these results, the SUI1-RNAi transgenic plants displayed decreased elongation of the uppermost internode. Our results suggest that SUI1 plays an important role in regulating uppermost internode length by decreasing longitudinal cell length in rice.

Tissue-specific expression of Populus C19 GA 2-oxidases differentially regulate above- and below-ground biomass growth through control of bioactive GA concentrations

• Here, we studied the poplar C(19) gibberellin 2-oxidase (GA2ox) gene subfamily. We show that a set of paralogous gene pairs differentially regulate shoot and root development. • PtGA2ox4 and its paralogous gene PtGA2ox5 are primarily expressed in aerial organs, and overexpression of PtGA2ox5 produced a strong dwarfing phenotype characteristic of GA deficiency. Suppression of PtGA2ox4 and PtGA2ox5 led to increased biomass growth, but had no effect on root development. By contrast, the PtGA2ox2 and PtGA2ox7 paralogous pair was predominantly expressed in roots, and when these two genes were RNAi-suppressed it led to a decrease of root biomass. • The morphological changes in the transgenic plants were underpinned by tissue-specific increases in bioactive GAs that corresponded to the predominant native expression of the targeted paralogous gene pair. Although RNAi suppression of both paralogous pairs led to changes in wood development, they were much greater in the transgenics with suppressed PtGA2ox4 and PtGA2ox5. The degree of gene suppression in independent events was strongly associated with phenotypes, demonstrating dose-dependent control of growth by GA2ox RNA concentrations. • The expression and transgenic modifications reported here show that shoot- and leaf-expressed PtGA2ox4 and PtGA2ox5 specifically restrain aerial shoot growth, while root-expressed PtGA2ox2 and PtGA2ox7 promote root development.

The multiple contributions of phytochromes to the control of internode elongation in rice

Although phyAphyBphyC phytochrome-null mutants in rice (Oryza sativa) have morphological changes and exhibit internode elongation, even as seedlings, it is unknown how phytochromes contribute to the control of internode elongation. A gene for 1-aminocyclopropane-1-carboxylate oxidase (ACO1), which is an ethylene biosynthesis gene contributing to internode elongation, was up-regulated in phyAphyBphyC seedlings. ACO1 expression was controlled mainly by phyA and phyB, and a histochemical analysis showed that ACO1 expression was localized to the basal parts of leaf sheaths of phyAphyBphyC seedlings, similar to mature wild-type plants at the heading stage, when internode elongation was greatly promoted. In addition, the transcription levels of several ethylene- or gibberellin (GA)-related genes were changed in phyAphyBphyC mutants, and measurement of the plant hormone levels indicated low ethylene production and bioactive GA levels in the phyAphyBphyC mutants. We demonstrate that ethylene induced internode elongation and ACO1 expression in phyAphyBphyC seedlings but not in the wild type and that the presence of bioactive GAs was necessary for these effects. These findings indicate that phytochromes contribute to multiple steps in the control of internode elongation, such as the expression of the GA biosynthesis gene OsGA3ox2, ACO1 expression, and the onset of internode elongation.

Cytochromes p450

There are 244 cytochrome P450 genes (and 28 pseudogenes) in the Arabidopsis genome. P450s thus form one of the largest gene families in plants. Contrary to what was initially thought, this family diversification results in very limited functional redundancy and seems to mirror the complexity of plant metabolism. P450s sometimes share less than 20% identity and catalyze extremely diverse reactions leading to the precursors of structural macromolecules such as lignin, cutin, suberin and sporopollenin, or are involved in biosynthesis or catabolism of all hormone and signaling molecules, of pigments, odorants, flavors, antioxidants, allelochemicals and defense compounds, and in the metabolism of xenobiotics. The mechanisms of gene duplication and diversification are getting better understood and together with co-expression data provide leads to functional characterization.

Proper gibberellin localization in vascular tissue is required to control auxin-dependent leaf development and bud outgrowth in hybrid aspen

Bioactive gibberellins (GAs) are involved in many developmental aspects in the life cycle of plants, acting either directly or through interaction with other hormones. One way to study the role of GA in specific mechanisms is to modify the levels of bioactive GA in specific tissues. We increased GA catabolism in different parts of the vascular tissue by overexpressing two different GA 2-oxidase genes that encode oxidases with affinity for C₂₀- or C₁₉-GA. We show that, irrespective of their localization in the vascular tissue, the expression of different members of this gene family leads to similar modifications in the primary and secondary growth of the stem of hybrid aspen. We also show that the precise localization of bioactive GA downregulation is important for the proper control of other developmental aspects, namely leaf shape and bud dormancy. Expression under the control of one of the studied promoters significantly affected both the shape of the leaves and the number of sylleptic branches. These phenotypic defects were correlated with alterations in the levels and repartitioning of auxins. We conclude that a precise localization of bioactive GA in the vasculature of the apex is necessary for the normal development of the plant through the effect of GAs on auxin transport.

The genes for gibberellin biosynthesis in wheat

The gibberellin biosynthesis pathway is well defined in Arabidopsis and features seven key enzymes including ent-copalyl diphosphate synthase (CPS), ent-kaurene synthase (KS), ent-kaurene oxidase (KO), ent-kaurenoic acid oxidase (KAO), GA 20-oxidase, GA 3-oxidase, and GA 2-oxidase. The Arabidopsis genes were used to identify their counterparts in wheat and the TaCPS, TaKS, TaKO, and TaKAO genes were cloned from Chinese Spring wheat. In order to determine their chromosome locations, expression patterns and feedback regulations, three TaCPS genes, three TaKS genes, three TaKO genes, and three TaKAO genes were cloned from Chinese Spring wheat. They are mainly located on chromosomes 7A, 7B, 7D and 2A, 2B and 2D. The expression patterns of TaCPS, TaKS, TaKO, and TaKAO genes in wheat leaves, young spikes, peduncles, the third and forth internodes were investigated using quantitative PCR. The results showed that all the genes were constitutively expressed in wheat, but their relative expression levels varied in different tissues. They were mainly transcribed in stems, secondly in leaves and spikes, and the least in peduncles. Feedback regulation of the TaCPS, TaKS, TaKO, and TaKAO genes was not evident. These results indicate that all the genes and their homologs may play important roles in the developmental processes of wheat, but each of the homologs may function differently in different tissues or during different developmental stages.

Heterosis in rice seedlings: its relationship to gibberellin content and expression of gibberellin metabolism and signaling genes

Despite the accumulation of data on the genetic and molecular understanding of heterosis, there is little information on the regulation of heterosis at the physiological level. In this study, we performed a quantitative analysis of endogenous gibberellin (GA) content and expression profiling of the GA metabolism and signaling genes to investigate the possible relationship between GA signaling and heterosis for seedling development in rice (Oryza sativa). The materials used were an incomplete diallele set of 3 × 3 crosses and the six parents. In the growing shoots of the seedlings at 20 d after sowing, significant positive correlations between the contents of some GA species and performance and heterosis based on shoot dry mass were detected. Expression analyses of GA-related genes by real-time reverse transcription-polymerase chain reaction revealed that 13 out of the 16 GA-related genes examined exhibited significant differential expression among the F1 hybrid and its parents, acting predominantly in the modes of overdominance and positive dominance. Expression levels of nine genes in the hybrids displayed significant positive correlations with the heterosis of shoot dry mass. These results imply that GAs play a positive role in the regulation of heterosis for rice seedling development. In shoots plus root axes of 4-d-old germinating seeds that had undergone the deetiolation, mimicking normal germination in soil, the axis dry mass was positively correlated with the content of GA₂₉ but negatively correlated with that of GA₁₉. Our findings provide supporting evidence for GAs playing an important regulatory role in heterosis for rice seedling development.

Two Arabidopsis cytochrome P450 monooxygenases, CYP714A1 and CYP714A2, function redundantly in plant development through gibberellin deactivation

The rice gene ELONGATED UPPERMOST INTERNODE1 (EUI1) encodes a P450 monooxygenase that epoxidizes gibberellins (GAs) in a deactivation reaction. The Arabidopsis genome contains a tandemly duplicated gene pair ELA1 (CYP714A1) and ELA2 (CYP714A2) that encode EUI homologs. In this work, we dissected the functions of the two proteins. ELA1 and ELA2 exhibited overlapping yet distinct gene expression patterns. We showed that while single mutants of ELA1 or ELA2 exhibited no obvious morphological phenotype, simultaneous elimination of ELA1 and ELA2 expression in ELA1-RNAi/ela2 resulted in increased biomass and enlarged organs. By contrast, transgenic plants constitutively expressing either ELA1 or ELA2 were dwarfed, similar to those overexpressing the rice EUI gene. We also discovered that overexpression of ELA1 resulted in a severe dwarf phenotype, while overexpression of ELA2 gave rise to a breeding-favored semi-dwarf phenotype in rice. Consistent with the phenotypes, we found that the ELA1-RNAi/ela2 plants increased amounts of biologically active GAs that were decreased in the internodes of transgenic rice with ELA1 and ELA2 overexpression. In contrast, the precursor GA(12) slightly accumulated in the transgenic rice, and GA(19) highly accumulated in the ELA2 overexpression rice. Taken together, our study strongly suggests that the two Arabidopsis EUI homologs subtly regulate plant growth most likely through catalyzing deactivation of bioactive GAs similar to rice EUI. The two P450s may also function in early stages of the GA biosynthetic pathway. Our results also suggest that ELA2 could be an excellent tool for molecular breeding for high yield potential in cereal crops.

OsDOG, a gibberellin-induced A20/AN1 zinc-finger protein, negatively regulates gibberellin-mediated cell elongation in rice

The A20/AN1 zinc-finger proteins (ZFPs) play pivotal roles in animal immune responses and plant stress responses. From previous gibberellin (GA) microarray data and A20/AN1 ZFP family member association, we chose Oryza sativa dwarf rice with overexpression of gibberellin-induced gene (OsDOG) to examine its function in the GA pathway. OsDOG was induced by gibberellic acid (GA(3)) and repressed by the GA-synthesis inhibitor paclobutrazol. Different transgenic lines with constitutive expression of OsDOG showed dwarf phenotypes due to deficiency of cell elongation. Additional GA(1) and real-time PCR quantitative assay analyses confirmed that the decrease of GA(1) in the overexpression lines resulted from reduced expression of GA3ox2 and enhanced expression of GA2ox1 and GA2ox3. Adding exogenous GA rescued the constitutive expression phenotypes of the transgenic lines. OsDOG has a novel function in regulating GA homeostasis and in negative maintenance of plant cell elongation in rice.

A P450-centric view of plant evolution

Being by far the largest family of enzymes to support plant metabolism, the cytochrome P450s (CYPs) constitute an excellent reporter of metabolism architecture and evolution. The huge superfamily of CYPs found in angiosperms is built on the successful evolution of 11 ancestral genes, with very different fates and progenies. Essential functions in the production of structural components (membrane sterols), light harvesting (carotenoids) or hormone biosynthesis kept some of them under purifying selection, limiting duplication and sub/neofunctionalization. One group (the CYP71 clan) after an early trigger to diversification, has kept growing, producing bursts of gene duplications at an accelerated rate. The CYP71 clan now represents more than half of all CYPs in higher plants. Such bursts of gene duplication are likely to contribute to adaptation to specific niches and to speciation. They also occur, although with lower frequency, in gene families under purifying selection. The CYP complement (CYPomes) of rice and the model grass weed Brachypodium distachyon have been compared to view evolution in a narrower time window. The results show that evolution of new functions in plant metabolism is a very long-term process. Comparative analysis of the plant CYPomes provides information on the successive steps required for the evolution of land plants, and points to several cases of convergent evolution in plant metabolism. It constitutes a very useful tool for spotting essential functions in plant metabolism and to guide investigations on gene function.

The extreme dwarf phenotype of the GA-sensitive mutant of sunflower, dwarf2, is generated by a deletion in the ent-kaurenoic acid oxidase1 (HaKAO1) gene sequence

A dwarf mutant, dw arf 2 (dw2), was isolated from sunflower (Helianthus annuus). The most obvious alterations of dw2 plants were the lack of stem growth, reduced size of leaves, petioles and flower organs, retarded flower development. Pollen and ovules were produced but the filaments failed to extrude the anthers from the corolla. The dw2 phenotype was mainly because of reduced cell size. In dw2 leaves, the dark-green color was not so much due to higher pigment content, but was correlated with a changed leaf morphology. The mutant responded to the application of bioactive gibberellins (GAs). The levels of ent-7α-hydroxykaurenoic acid, GA(19), GA(20) and GA(1) in dw2 seedlings were severely decreased relative to those in its wild type (WT). ent-Kaurenoic acid was actively converted to ent-7α-hydroxykaurenoic acid in WT plants but quite poorly in dw2 plants. All together these data suggested that the dw2 mutation severely reduced the flux through the biosynthetic pathway leading to active GAs by hampering the conversion of ent-kaurenoic acid to GA(12). Two ent-kaurenoic acid oxidase (KAO) genes were identified. HaKAO1 was expressed everywhere in sunflower organs, while HaKAO2 was mainly expressed in roots. We demonstrated that a DNA deletion in HaKAO1 of dw2 generated aberrant mRNA-splicing, causing a premature stop codon in the amino acid sequence. In dw2 calli, Agrobacterium-mediated transfer of WT HaKAO1 cDNA restored the WT endogenous levels of GAs. In segregating BC(1) progeny, the deletion co-segregated with the dwarf phenotype. The deletion was generated near to a breakpoint of a more complex chromosome rearrangement.

BENT UPPERMOST INTERNODE1 encodes the class II formin FH5 crucial for actin organization and rice development

The actin cytoskeleton is an important regulator of cell expansion and morphogenesis in plants. However, the molecular mechanisms linking the actin cytoskeleton to these processes remain largely unknown. Here, we report the functional analysis of rice (Oryza sativa) FH5/BENT UPPERMOST INTERNODE1 (BUI1), which encodes a formin-type actin nucleation factor and affects cell expansion and plant morphogenesis in rice. The bui1 mutant displayed pleiotropic phenotypes, including bent uppermost internode, dwarfism, wavy panicle rachis, and enhanced gravitropic response. Cytological observation indicated that the growth defects of bui1 were caused mainly by inhibition of cell expansion. Map-based cloning revealed that BUI1 encodes the class II formin FH5. FH5 contains a phosphatase tensin-like domain at its amino terminus and two highly conserved formin-homology domains, FH1 and FH2. In vitro biochemical analyses indicated that FH5 is capable of nucleating actin assembly from free or profilin-bound monomeric actin. FH5 also interacts with the barbed end of actin filaments and prevents the addition and loss of actin subunits from the same end. Interestingly, the FH2 domain of FH5 could bundle actin filaments directly and stabilize actin filaments in vitro. Consistent with these in vitro biochemical activities of FH5/BUI1, the amount of filamentous actin decreased, and the longitudinal actin cables almost disappeared in bui1 cells. The FH2 or FH1FH2 domains of FH5 could also bind to and bundle microtubules in vitro. Thus, our study identified a rice formin protein that regulates de novo actin nucleation and spatial organization of the actin filaments, which are important for proper cell expansion and rice morphogenesis.

Mutation of rice BC12/GDD1, which encodes a kinesin-like protein that binds to a GA biosynthesis gene promoter, leads to dwarfism with impaired cell elongation

The kinesins are a family of microtubule-based motor proteins that move directionally along microtubules and are involved in many crucial cellular processes, including cell elongation in plants. Less is known about kinesins directly regulating gene transcription to affect cellular physiological processes. Here, we describe a rice (Oryza sativa) mutant, gibberellin-deficient dwarf1 (gdd1), that has a phenotype of greatly reduced length of root, stems, spikes, and seeds. This reduced length is due to decreased cell elongation and can be rescued by exogenous gibberellic acid (GA₃) treatment. GDD1 was cloned by a map-based approach, was expressed constitutively, and was found to encode the kinesin-like protein BRITTLE CULM12 (BC12). Microtubule cosedimentation assays revealed that BC12/GDD1 bound to microtubules in an ATP-dependent manner. Whole-genome microarray analysis revealed the expression of ent-kaurene oxidase (KO2), which encodes an enzyme involved in GA biosynthesis, was downregulated in gdd1. Electrophoretic mobility shift and chromatin immunoprecipitation assays revealed that GDD1 bound to the element ACCAACTTGAA in the KO2 promoter. In addition, GDD1 was shown to have transactivation activity. The level of endogenous GAs was reduced in gdd1, and the reorganization of cortical microtubules was altered. Therefore, BC12/GDD1, a kinesin-like protein with transcription regulation activity, mediates cell elongation by regulating the GA biosynthesis pathway in rice.

Activation of gibberellin 2-oxidase 6 decreases active gibberellin levels and creates a dominant semi-dwarf phenotype in rice (Oryza sativa L.)

Gibberellin (GA) 2-oxidase plays a key role in the GA catabolic pathway through 2beta-hydroxylation. In the present study, we isolated a CaMV 35S-enhancer activation tagged mutant, H032. This mutant exhibited a dominant dwarf and GA-deficient phenotype, with a final stature that was less than half of its wild-type counterpart. The endogenous bioactive GAs are markedly decreased in the H032 mutant, and application of bioactive GAs (GA(3) or GA(4)) can reverse the dwarf phenotype. The integrated T-DNA was detected 12.8 kb upstream of the OsGA2ox6 in the H032 genome by TAIL-PCR. An increased level of OsGA2ox6 mRNA was detected at a high level in the H032 mutant, which might be due to the enhancer role of the CaMV 35S promoter. RNAi and ectopic expression analysis of OsGA2ox6 indicated that the dwarf trait and the decreased levels of bioactive GAs in the H032 mutant were a result of the up-regulation of the OsGA2ox6 gene. BLASTP analysis revealed that OsGA2ox6 belongs to the class III of GA 2-oxidases, which is a novel type of GA2ox that uses C20-GAs (GA(12) and/or GA(53)) as the substrates. Interestingly, we found that a GA biosynthesis inhibitor, paclobutrazol, positively regulated the OsGA2ox6 gene. Unlike the over-expression of OsGA2ox1, which led to a high rate of seed abortion, the H032 mutant retained normal flowering and seed production. These results indicate that OsGA2ox6 mainly affects plant stature, and the dominant dwarf trait of the H032 mutant can be used as an efficient dwarf resource in rice breeding.

The APETALA-2-like transcription factor OsAP2-39 controls key interactions between abscisic acid and gibberellin in rice

The interaction between phytohormones is an important mechanism which controls growth and developmental processes in plants. Deciphering these interactions is a crucial step in helping to develop crops with enhanced yield and resistance to environmental stresses. Controlling the expression level of OsAP2-39 which includes an APETALA 2 (AP2) domain leads to phenotypic changes in rice. Overexpression of OsAP2-39 leads to a reduction in yield by decreasing the biomass and the number of seeds in the transgenic rice lines. Global transcriptome analysis of the OsAP2-39 overexpression transgenic rice revealed the upregulation of a key abscisic acid (ABA) biosynthetic gene OsNCED-I which codes for 9-cis-epoxycarotenoid dioxygenase and leads to an increase in the endogenous ABA level. In addition to OsNCED-1, the gene expression analysis revealed the upregulation of a gene that codes for the Elongation of Upper most Internode (EUI) protein, an enzyme that catalyzes 16α, 17-epoxidation of non-13-hydroxylated GAs, which has been shown to deactivate gibberellins (GAs) in rice. The exogenous application of GA restores the wild-type phenotype in the transgenic line and ABA application induces the expression of EUI and suppresses the expression of OsAP2-39 in the wild-type line. These observations clarify the antagonistic relationship between ABA and GA and illustrate a mechanism that leads to homeostasis of these hormones. In vivo and in vitro analysis showed that the expression of both OsNCED-1 and EUI are directly controlled by OsAP2-39. Together, these results reveal a novel mechanism for the control of the ABA/GA balance in rice which is regulated by OsAP2-39 that in turn regulates plant growth and seed production.

ACO1, a gene for aminocyclopropane-1-carboxylate oxidase: effects on internode elongation at the heading stage in rice

Although reports on a gene for 1-amino-cyclopropane-1-carboxylate (ACC) oxidase (ACO1) in rice (Oryza sativa L.) suggest that high levels of its transcript are associated with internode elongation of deep-water rice during submergence, the role of ACO1 in rice development is largely unknown. The tissue-specificity of ACO1 expression indicated that its transcript significantly accumulated in lower parts of elongating internodes at the heading stage. Histochemical analysis and in situ hybridization showed that the ACO1 expression was localized in the basal parts of leaf sheaths immediately above nodes or the lower parts of elongating internodes. To further examine the role of ACO1, ACO1-deficient (aco1) and overexpressing (ACO1-OX) mutants were characterized. The total length of the elongated internodes of aco1 mutants was slightly shorter than that of wild-type plants and that of ACO1-OX mutants was longer. Interestingly, expression of the ACC synthase gene ACS1 and ethylene signalling gene OsEIN2 was up-regulated in the aco1 mutants. This study suggests that the ACO1 has a little effect on internode elongation at the heading stage, and that up-regulation of the ACS1 and OsEIN2 expression may attenuate inhibition of internode elongation.

The submergence tolerance regulator Sub1A mediates stress-responsive expression of AP2/ERF transcription factors

We previously characterized the rice (Oryza sativa) Submergence1 (Sub1) locus encoding three ethylene-responsive factor (ERF) transcriptional regulators. Genotypes carrying the Sub1A-1 allele are tolerant of prolonged submergence. To elucidate the mechanism of Sub1A-1-mediated tolerance, we performed transcriptome analyses comparing the temporal submergence response of Sub1A-1-containing tolerant M202(Sub1) with the intolerant isoline M202 lacking this gene. We identified 898 genes displaying Sub1A-1-dependent regulation. Integration of the expression data with publicly available metabolic pathway data identified submergence tolerance-associated pathways governing anaerobic respiration, hormone responses, and antioxidant systems. Of particular interest were a set of APETALA2 (AP2)/ERF family transcriptional regulators that are associated with the Sub1A-1-mediated response upon submergence. Visualization of expression patterns of the AP2/ERF superfamily members in a phylogenetic context resolved 12 submergence-regulated AP2/ERFs into three putative functional groups: (1) anaerobic respiration and cytokinin-mediated delay in senescence via ethylene accumulation during submergence (three ERFs); (2) negative regulation of ethylene-dependent gene expression (five ERFs); and (3) negative regulation of gibberellin-mediated shoot elongation (four ERFs). These results confirm that the presence of Sub1A-1 impacts multiple pathways of response to submergence.

Diversification of P450 genes during land plant evolution

Plant cytochromes P450 (P450s) catalyze a wide variety of monooxygenation/hydroxylation reactions in primary and secondary metabolism. The number of P450 genes in plant genomes is estimated to be up to 1% of total gene annotations of each plant species. This implies that diversification within P450 gene superfamilies has led to the emergence of new metabolic pathways throughout land plant evolution. The conserved P450 families contribute to chemical defense mechanisms under terrestrial conditions and several are involved in hormone biosynthesis and catabolism. Species-specific P450 families are essential for the biosynthetic pathways of species-specialized metabolites. Future genome-wide analyses of P450 gene clusters and coexpression networks should help both in identifying the functions of many orphan P450s and in understanding the evolution of this versatile group of enzymes.

SAUR39, a small auxin-up RNA gene, acts as a negative regulator of auxin synthesis and transport in rice

The phytohormone auxin plays a critical role for plant growth by regulating the expression of a set of genes. One large auxin-responsive gene family of this type is the small auxin-up RNA (SAUR) genes, although their function is largely unknown. The expression of the rice (Oryza sativa) SAUR39 gene showed rapid induction by transient change in different environmental factors, including auxin, nitrogen, salinity, cytokinin, and anoxia. Transgenic rice plants overexpressing the SAUR39 gene resulted in lower shoot and root growth, altered shoot morphology, smaller vascular tissue, and lower yield compared with wild-type plants. The SAUR39 gene was expressed at higher levels in older leaves, unlike auxin biosynthesis, which occurs largely in the meristematic region. The transgenic plants had a lower auxin level and a reduced polar auxin transport as well as the down-regulation of some putative auxin biosynthesis and transporter genes. Biochemical analysis also revealed that transgenic plants had lower chlorophyll content, higher levels of anthocyanin, abscisic acid, sugar, and starch, and faster leaf senescence compared with wild-type plants at the vegetative stage. Most of these phenomena have been shown to be negatively correlated with auxin level and transport. Transcript profiling revealed that metabolic perturbations in overexpresser plants were largely due to transcriptional changes of genes involved in photosynthesis, senescence, chlorophyll production, anthocyanin accumulation, sugar synthesis, and transport. The lower growth and yield of overexpresser plants was largely recovered by exogenous auxin application. Taken together, the results suggest that SAUR39 acts as a negative regulator for auxin synthesis and transport.

Diversity, regulation, and evolution of the gibberellin biosynthetic pathway in fungi compared to plants and bacteria

Bioactive gibberellins (GAs) are diterpene plant hormones that are biosynthesized through complex pathways and control diverse aspects of growth and development. GAs were first isolated as metabolites of a fungal rice pathogen, Gibberella fujikuroi, since renamed Fusarium fujikuroi. Although higher plants and the fungus produce structurally identical GAs, significant differences in their GA pathways, enzymes involved and gene regulation became apparent with the identification of GA biosynthetic genes in Arabidopsis thaliana and F. fujikuroi. Recent identifications of GA biosynthetic gene clusters in two other fungi, Phaeosphaeria spp. and Sphaceloma manihoticola, and the high conservation of GA cluster organization in these distantly related fungal species indicate that fungi evolved GA and other diterpene biosynthetic pathways independently from plants. Furthermore, the occurrence of GAs and recent identification of the first GA biosynthetic genes in the bacterium Bradyrhizobium japonicum make it possible to study evolution of GA pathways in general. In this review, we summarize our current understanding of the GA biosynthesis pathway, specifically the genes and enzymes involved as well as gene regulation and localization in the genomes of different fungi and compare it with that in higher and lower plants and bacteria.

From dwarves to giants? Plant height manipulation for biomass yield

The increasing demand for lignocellulosic biomass for the production of biofuels provides value to vegetative plant tissue and leads to a paradigm shift for optimizing plant architecture in bioenergy crops. Plant height (PHT) is among the most important biomass yield components and is the focus of this review, with emphasis on the energy grasses maize (Zea mays) and sorghum (Sorghum bicolor). We discuss the scientific advances in the identification of PHT quantitative trait loci (QTLs) and the understanding of pathways and genes controlling PHT, especially gibberellins and brassinosteroids. We consider pleiotropic effects of QTLs or genes affecting PHT on other agronomically important traits and, finally, we discuss strategies for applying this knowledge to the improvement of dual-purpose or dedicated bioenergy crops.

[Substitution mapping of QTL for panicle exertion using CSSL in rice (Oryza sativa L.)]

Panicle exertion, the distance between the leaf cushion of flag and the neck-panicle node, is an important morphological trait, which has significant impact on hybrid seed production of rice. In this study, 94 chromosome segment substitution lines (CSSL), derived from 9311/Nipponbare with 9311 as the recurrent parent, were used to analyze quantitative trait loci (QTL) controlling the panicle exertion. The results showed that 17 CSSL contained QTL for the panicle exertion. Using substitution mapping, 8 QTLs were mapped on rice chromosomes 2, 3, 7, 8, 9, and 11, respectively. The QTL additive effect ranged from 0.10 to 3.20, and the additive effects of qPE-9 and qPE-11 were 3.15 and 2.95, respectively, showing the feature of a major gene. In addition, qPE-2-2, qPE-3-1, qPE-3-2, qPE-7, and qPE-8 were mapped in a marker interval less than 10.0 cM. Mapping of the QTL has laid a foundation for improving the panicle exertion with marker-assisted selection and cloning of the QTL in rice.