Gibberellin and auxin signals control scape cell elongation and proliferation in <Emphasis Type="Italic">Agapanthus praecox</Emphasis> ssp. <Emphasis Type="Italic">orientalis</Emphasis>

Plant height is determined by the processes of cell proliferation and elongation. Plant hormones play key roles in a species-dependent manner in these processes. We used paclobutrazol (PAC) at 400 mg·L^-1 in this study to spray Agapanthus praecox ssp. orientalis plants in order to induce dwarf scape (inflorescence stem). Morphological examination showed that PAC reduced scape height by inhibiting the cell elongation by 54.56% and reducing cell proliferation by 10.45% compared to the control. Quantification and immunolocalization of endogenous gibberellins (GAs) and indole-3-acetic acid (IAA) showed that the GA₁, GA₃, and GA₄ levels and the IAA gradient were reduced. Among these hormones, GA4 was the key component of GAs, which decreased 59.51-92.01% compared to the control in scape. The expression of cell wall synthesis related genes cellulose synthase (CESA) and UDP-glucuronic acid decarboxylase (UXS) were upregulated significantly, whereas cell wall loosening gene xyloglucan endotransglucosylase 2 (XET2) was downregulated by 99.99% surprisingly. Correlation analysis suggested GA regulated cell elongation and auxin modulated cell proliferation in Agapanthus scape. Additionally, the accumulation of sugars played roles in cell wall synthesis and cell expansion. These results indicated GA and IAA signals triggered a downstream signaling cascade, controlled cell expansion and proliferation during scape elongation.     

Activation of gibberellin 20-oxidase 2 undermines auxin-dependent root and root hair growth in NaCl-stressed <Emphasis Type="Italic">Arabidopsis</Emphasis> seedlings

Although salt stress mainly disturbs plant root growth by affecting the biosynthesis and signaling of phytohormones, such as gibberellin (GA) and auxin, the exact mechanisms of the crosstalk between these two hormones remain to be clarified. Indole-3-acetic acid (IAA) is a biologically active auxin molecule. In this study, we investigated the role of Arabidopsis GA20-oxidase 2 (GA20ox2), a final rate-limiting enzyme of active GA biosynthesis, in IAA-directed root growth under NaCl stress. Under the NaCl treatment, seedlings of a loss-of-function ga20ox2-1 mutant exhibited primary root and root hair elongation, altered GA₄ accumulation, and decreased root Na⁺ contents compared with the wild-type, transgenic GA20ox2-complementing, and GA20ox2-overexpression plant lines. Concurrently, ga20ox2-1 alleviated the tissue-specific inhibition of NaCl on IAA generation by YUCCAs, IAA transport by PIN1 and PIN2, and IAA accumulation in roots, thereby explaining how NaCl increased GA20ox2 expression in shoots but disrupted primary root and root hair growth in wild-type seedlings. In addition, a loss-of-function pin2 mutant impeded GA20ox2 expression, indicating that GA20ox2 function requires PIN2 activity. Thus, the activation of GA20ox2 retards IAA-directed primary root and root hair growth in response to NaCl stress.     

Photoperiodic flower induction in <Emphasis Type="Italic">Ipomoea nil</Emphasis> is accompanied by decreasing content of gibberellins

The involvement of gibberellins (GAs) in the control of flower induction in the short-day plant Ipomoea nil has been investigated. To clarify the molecular basis of this process, we identified the full-length cDNAs of the InGA20ox3 and InGA2ox1 genes, which encode enzymes responsible for GA biosynthesis and catabolism, respectively. We studied the expression patterns of both genes and determined the tissue and cellular immunolocalisation of gibberellic acid (GA₃) in the cotyledons of 5-day-old seedlings growing under inductive and non-inductive photoperiodic conditions. In the second half of the inductive night, which is crucial for flower induction in I. nil, InGA20ox3 expression decreased, whereas InGA2ox1 mRNA accumulated, which indicates that photoperiod regulates the activity of both genes. Furthermore, these changes are correlated with GA₃ level. Thus, our results support the thesis that the proper balance between the expression of the InGA20ox3 and InGA2ox1 genes and low GA₃ content correlate with photoperiodic flower induction in I. nil.     

Jasmonic acid modulates xylem development by controlling polar auxin transport in vascular tissues

Development of xylem cells is affected by environmental stresses such as drought and oxidative stress, and recent findings suggested that jasmonic acid (JA) mediates this process through interaction with other phytohormones such as cytokinin. In this study, we showed that polar auxin transport regulated by PIN3 and PIN7 is involved in the JA-mediated xylem development in vascular tissues. The mutant plants that lack the activity of PIN3 and PIN7 responsible for the auxin transport developed extra xylems in vascular tissues such as the JA-treated wild-type plants. Visualization of auxin response and xylem development in the roots treated with NPA, an inhibitor of polar auxin transport, suggested that disruption of polar auxin transport is involved in the xylem phenotype of pin3 pin7 double mutants. We also found that cytokinin increases expressions of PIN3 and PIN7 responsible for the auxin transport while JA decreases only PIN7. These suggested that PIN7-mediated polar auxin transport system modulates xylem development in response to JA. The finding that JA affects auxin distribution in root vascular tissues further supported this. Collectively, these suggest that JA promotes xylem development by disrupting auxin transport in vascular tissues, and the auxin efflux genes, more especially PIN7 whose expression is suppressed by JA mediates this process.     

Engineering gibberellin metabolism in <Emphasis Type="Italic">Solanum nigrum</Emphasis> L. by ectopic expression of gibberellin oxidase genes

Gibberellins (GAs) control many aspects of plant development, including seed germination, shoot growth, flower induction and growth and fruit expansion. Leaf explants of Solanum nigrum (Black Nightshade; Solanaceae) were used for Agrobacterium-mediated delivery of GA-biosynthetic genes to determine the influence of their encoded enzymes on the production of bioactive GAs and plant stature in this species. Constructs were prepared containing the neomycin phosphotransferase (nptII) gene for kanamycin resistance as a selectable marker, and the GA-biosynthetic genes, their expression under the control of the CaMV 35S promoter. The GA-biosynthetic genes comprised AtGA20ox1, isolated from Arabidopsis thaliana, the product from which catalyses the formation of C₁₉-GAs, and MmGA3ox1 and MmGA3ox2, isolated from Marah macrocarpus, which encode functionally different GA 3-oxidases that convert C₁₉-GAs to biologically active forms. Increase in stature was observed in plants transformed with AtGA20ox1, MmGA3ox2 and MmGA3ox1 + MmGA3ox2, their presence and expression being confirmed by PCR and RT-PCR, respectively, accompanied by an increase in GA₁ content. Interestingly, MmGA3ox1 alone did not induce a sustained increase in plant height, probably because of only a marginal increase in bioactive GA₁ content in the transformed plants. The results are discussed in the context of regulating plant stature, since this strategy would decrease the use of chemicals to promote plant growth.     

The biosynthesis of gibberellic acids by the transformants of orchid-associated <Emphasis Type="Italic">Fusarium oxysporum</Emphasis>

The genus Fusarium, including multiple strains in the Gibberella fujikuroi species complex (GFC), is well known for its production of diverse secondary metabolites. F. fujikuroi, associated with the “bakanae” disease of rice, is an active producer of gibberellins (GAs), a wide class of plant hormones. In addition to some members of the GFC, the GA biosynthetic gene cluster, or parts of it, occurs also in some isolates of the closely related species of F. oxysporum, which does not belong to the GFC. However, production of GAs has never been observed in any F. oxysporum strain. In this study, we report on the GA biosynthetic activity in an orchid-associated F. oxysporum strain by transforming a cosmid with the entire F. fujikuroi GA gene cluster. Southern and Northern blot analyses confirmed not only the integration of the entire gene cluster into the genome but also the active expression of the seven GA biosynthetic genes under nitrogen-limiting conditions. The transformants produced GAs at levels similar to those of F. fujikuroi. These data show that the regulatory network for expression of GA genes is fully active in the F. oxysporum background.     

Revealing physiological and genetic properties of a dominant maize dwarf <Emphasis Type="Italic">Dwarf11</Emphasis> (<Emphasis Type="Italic">D11</Emphasis>) by integrative analysis

Plant height is an important agronomic trait involved in lodging resistance and harvest index. The identification and characterization of mutants that are defective in plant height have implications for trait improvement in breeding programs. Two dominant maize dwarf mutants D8 and D9 have been well-characterized. Here, we report the characterization of a dominant maize dwarf mutant Dwarf11 (D11). Dwarf stature of D11 was mainly attributed to the inhibition of longitudinal cell elongation. The levels of bioactive GA₃ were significantly lower in D11. Contrarily, D8 mutant accumulates markedly higher levels of GA₃. The expression of GA biosynthetic and catabolic genes was dramatically decreased in D11. Expression variations of d8 and d9 genes were not observed in D11 mutant. Moreover, genetic suppressors of D11 were identified in inbred line Chang 7-2. Integrated omics data indicated that D11 is a novel dominant maize dwarf. The ultimate D11 gene cloning and its regulatory network elucidation may strengthen our understanding of the genetic basis of plant architecture and provide cues for breeding of crops with plant height ideotypes.     

Identification of the dwarf gene <Emphasis Type="Italic">GmDW1</Emphasis> in soybean (<Emphasis Type="Italic">Glycine max</Emphasis> L.) by combining mapping-by-sequencing and linkage analysis

Key message

GmDW1 encodes an ent-kaurene synthase (KS) acting at the early step of the biosynthesis pathway for gibberellins (GAs) and regulates the development of plant height in soybean.

Abstract

Plant height is an important component of plant architecture, and significantly affects crop breeding practices and yield. Here, we report the characterization of an EMS-induced dwarf mutant (dw) of the soybean cultivar Zhongpin 661 (ZDD23893). The dw mutant displayed reduced plant height and shortened internodes, both of which were mainly attributed to the longitudinally decreased cell length. The bioactive GA₁ (gibberellin A₁) and GA₄ (gibberellin A₄) were not detectable in the stem of dw, and the dwarf phenotype could be rescued by treatment with exogenous GA₃. Genetic analysis showed that the dwarf trait of dw was controlled by a recessive nuclear gene. By combining linkage analysis and mapping-by-sequencing, we mapped the GmDW1 gene to an approximately 460-kb region on chromosome (Chr.) 8, containing 36 annotated genes in the reference Willliams 82 genome. Of these genes, we identified two nonsynonymous single nucleotide polymorphisms (SNPs) that are present in the encoding regions of Gmdw1 and Glyma.08G165100 in dw, respectively. However, only the SNP mutation (T>A) at nucleotide 1224 in Gmdw1 cosegregated with the dwarf phenotype. GmDW1 encodes an ent-kaurene synthase, and was expressed in various tissues including root, stem, and leaf. Further phenotypic analysis of the allelic variations in soybean accessions strongly indicated that GmDW1 is responsible for the dwarf phenotype in dw. Our results provide important information for improving our understanding of the genetics of soybean plant height and crop breeding.

     

Analysis the role of arabidopsis <Emphasis Type="Italic">CKRC6/ASA1</Emphasis> in auxin and cytokinin biosynthesis

The crosstalk between auxin and cytokinin (CK) is important for plant growth and development, although the underlying molecular mechanisms remain unclear. Here, we describe the isolation and characterization of a mutant of Arabidopsis Cytokinin-induced Root Curling 6 (CKRC6), an allele of ANTHRANILATE SYNTHASE ALPHA SUBUNIT 1 (ASA1) that encodes the á-subunit of AS in tryptophan (Trp) biosynthesis. The ckrc6 mutant exhibits root gravitropic defects and insensitivity to both CK and the ethylene precursor 1-aminocyclopropane-1-carboxylicacid (ACC) in primary root growth. These defects can be rescued by exogenous indole-3-acetic acid (IAA) or tryptophan (Trp) supplementation. Furthermore, our results suggest that the ckrc6 mutant has decreased IAA content, differential expression patterns of auxin biosynthesis genes and CK biosynthesis isopentenyl transferase (IPT) genes in comparison to wild type. Collectively, our study shows that auxin controls CK biosynthesis based on that CK sensitivity is altered in most auxin-resistant mutants and that CKs promote auxin biosynthesis but inhibit auxin transport and response. Our results also suggest that CKRC6/ASA1 may be located at an intersection of auxin, CK and ethylene metabolism and/or signaling.     

Overexpression of <Emphasis Type="Italic">PSP1</Emphasis> enhances growth of transgenic Arabidopsis plants under ambient air conditions

Cellulose biosynthesis is mediated by cellulose synthases (CesAs), which constitute into rosette-like cellulose synthase complexe (CSC) on the plasma membrane. Two types of CSCs in Arabidopsis are believed to be involved in cellulose synthesis in the primary cell wall and secondary cell walls, respectively. In this work, we found that the two type CSCs participated cellulose biosynthesis in differentiating xylem cells undergoing secondary cell wall thickening in Populus. During the cell wall thickening process, expression of one type CSC genes increased while expression of the other type CSC genes decreased. Suppression of different type CSC genes both affected the wall-thickening and disrupted the multilaminar structure of the secondary cell walls. When CesA7A was suppressed, crystalline cellulose content was reduced, which, however, showed an increase when CesA3D was suppressed. The CesA suppression also affected cellulose digestibility of the wood cell walls. The results suggest that two type CSCs are involved in coordinating the cellulose biosynthesis in formation of the multilaminar structure in Populus wood secondary cell walls.     

Both epiphytic and endophytic strains of <Emphasis Type="Italic">Rhodococcus fascians</Emphasis> influence transporter gene expression and cytokinins in infected <Emphasis Type="Italic">Pisum sativum</Emphasis> L. seedlings

Some strains of the soil bacterium Rhodococcus fascians maintain an epiphytic life style while others become endophytic. Virulent, endophytic strains cause multiple shoot growth and inhibit root growth of seed-inoculated Pisum sativum L. We were interested in assessing, at the molecular level, the impact of strains of contrasting niche on the emerging shoots and roots of inoculated seeds. The presence of R. fascians was monitored microscopically, endogenous cytokinin and chlorophyll levels were measured, and the expression of genes monitored by RT-qPCR. The expression of the pea sugar transporter genes (SWEET and SUT), amino acid (AAP) transporters and cell wall invertase gene family members, as well as expression of plant and bacterial cytokinin biosynthesis (IPT), activation (LOG) and degradation (CKX) genes were monitored. Both the virulent strain and the epiphytic strain affected the expression of the transporter genes, with less obvious differences between the strains on the shoot compared with the effect on the root. Strong expression of the R. fascians genes, RfIPT, RfLOG and RfCKX, in pea seedlings at 15 days post inoculation was mirrored by increased expression of transporter gene family members in the plant. However, the elevated levels of isopentenyl adenine-type and zeatin-type cytokinins were not consistently associated with the virulent strain. In conclusion, while both the virulent strain and the epiphytic strain impacted the expression of transporter genes in the shoots and roots, only the virulent strain affected morphology. The inhibited root growth, the greening of the roots, and the expression of the pea response regulators in the infected roots are indicative of a response to cytokinin, but a role for the ‘classical’ cytokinins as virulence determinants was not established.     

Role of ABA and Gibberellin A<Subscript>3</Subscript> on gene expression pattern of sugar transporters and invertases in <Emphasis Type="Italic">Vitis vinifera</Emphasis> cv. Malbec during berry ripening

Sugars are key constituents that affect quality of grape berries, and consequently the grape metabolic profile relevant to wine’s industry. However, enzymes and transporter genes expression involved in sugar transport at different phenological stages are scarcely studied. In addition, little is known about the role of the plant hormones ABA and Gibberellin (GA₃) as endogenous regulators, over the expression pattern of the sugars transporters genes in grapevine. The aim of this study was to analyze the expression pattern of the most relevant sugar transporters and invertases in leaves and berries of grapevine plants cv. Malbec during berry ripening stages and its shift after ABA and GA₃ sprays. In leaves, VvHT1 was the sugar transporter highly expressed, whereas VvHT6 was the most abundant in berries throughout berry ripening. Moreover, VvSUC12 and VvSUC27 were expressed at veraison greater in leaves than in berries, suggesting an active phloem loading at the onset of ripening. Applications of ABA and GA₃ enhanced the expression of VvSUC12 and VvSUC27 in pre-veraison leaves. Furthermore, hormones increased the expression of VvHT2, VvHT3 and VvHT6 in berries at different stages of ripening favoring sugar unloading from phloem. In conclusion, ABA and GA₃ are involved in the long-distance sugar transport from leaves to berries in Vitis vinifera L. cv. Malbec, and their exogenous application could be a suitable strategy to improve the process.