Over-expression of the <Emphasis Type="Italic">IGI1</Emphasis> leading to altered shoot-branching development related to MAX pathway in <Emphasis Type="Italic">Arabidopsis</Emphasis>

Shoot branching and growth are controlled by phytohormones such as auxin and other components in Arabidopsis. We identified a mutant (igi1) showing decreased height and bunchy branching patterns. The phenotypes reverted to the wild type in response to RNA interference with the IGI1 gene. Histochemical analysis by GUS assay revealed tissue-specific gene expression in the anther and showed that the expression levels of the IGI1 gene in apical parts, including flowers, were higher than in other parts of the plants. The auxin biosynthesis component gene, CYP79B2, was up-regulated in igi1 mutants and the IGI1 gene was down-regulated by IAA treatment. These results indicated that there is an interplay regulation between IGI1 and phytohormone auxin. Moreover, the expression of the auxin-related shoot branching regulation genes, MAX3 and MAX4, was down-regulated in igi1 mutants. Taken together, these results indicate that the overexpression of the IGI1 influenced MAX pathway in the shoot branching regulation.     

<Emphasis Type="Italic">SnRK2</Emphasis> Homologs in <Emphasis Type="Italic">Gossypium</Emphasis> and <Emphasis Type="Italic">GhSnRK2.6</Emphasis> Improved Salt Tolerance in Transgenic Upland Cotton and <Emphasis Type="Italic">Arabidopsis</Emphasis>

SnRK2s are a large family of plant-specific protein kinases, which play important roles in multiple abiotic stress responses in various plant species. But the family in Gossypium has not been well studied. Here, we identified 13, 10, and 13 members of the SnRK2 family from Gossypium raimondii, Gossypium arboreum, and Gossypium hirsutum, respectively, and analyzed the locations of SnRK2 homologs in chromosomes based on genome data of cotton species. Phylogenetic tree analysis of SnRK2 proteins showed that these families were classified into three groups. All SnRK2 genes were comprised of nine exons and eight introns, and the exon distributions and the intron phase of homolog genes among different cotton species were analogous. Moreover, GhSnRK2.6 was overexpressed in Arabidopsis and upland cotton, respectively. Under salt treatment, overexpressed Arabidopsis could maintain higher biomass accumulation than wild-type plants, and GhSnRK2.6 overexpression in cotton exhibited higher germination rate than the control. So, the gene GhSnRK2.6 could be utilized in cotton breeding for salt tolerance.     

A <Emphasis Type="Italic">Vitis vinifera</Emphasis> xanthine dehydrogenase gene, <Emphasis Type="Italic">VvXDH</Emphasis>, enhances salinity tolerance in transgenic <Emphasis Type="Italic">Arabidopsis</Emphasis>

Xanthine dehydrogenase (EC1.1.1.204; XDH) plays an important role in purine catabolism that catalyzes the oxidative hydroxylation of hypoxanthine to xanthine and of xanthine to uric acid. Long attributed to its role in recycling and remobilization of nitrogen, recently, XDH is implicated in plant stress responses and acclimation, such research efforts, however, have thus far been restricted to Arabidopsis XDH-knockdown/knockout studies. This study, using an ectopic overexpression approach, is expected to provide novel findings. In this study, a XDH gene from Vitis vinifera, named VvXDH, was synthesized and overexpressed in Arabidopsis, the transgenic Arabidopsis showed enhanced salt tolerance. The VvXDH gene was investigated and the results demonstrated the explicit role of VvXDH in conferring salt stress by increasing allantoin accumulation and activating ABA signaling pathway, enhancing ROS scavenging in transgenic Arabidopsis. In addition, the water loss and chlorophyll content loss were reduced in transgenic plants; the transgenic plants showed higher proline level and lower MDA content than that of wild-type Arabidopsis, respectively. In conclusion, the VvXDH gene has the potential to be applied in increasing allantoin accumulation and enhancing the tolerance to abiotic stresses in Arabidopsis and other plants.     

Increased tolerance to salt stress in the phosphate-accumulating <Emphasis Type="Italic">Arabidopsis</Emphasis> mutants <Emphasis Type="Italic">siz1</Emphasis> and <Emphasis Type="Italic">pho2</Emphasis>

High salinity is an environmental factor that inhibits plant growth and development, leading to large losses in crop yields. We report here that mutations in SIZ1 or PHO2, which cause more accumulation of phosphate compared with the wild type, enhance tolerance to salt stress. The siz1 and pho2 mutations reduce the uptake and accumulation of Na⁺. These mutations are also able to suppress the Na⁺ hypersensitivity of the sos3-1 mutant, and genetic analyses suggest that SIZ1 and SOS3 or PHO2 and SOS3 have an additive effect on the response to salt stress. Furthermore, the siz1 mutation cannot suppress the Li⁺ hypersensitivity of the sos3-1 mutant. These results indicate that the phosphate-accumulating mutants siz1 and pho2 reduce the uptake and accumulation of Na⁺, leading to enhanced salt tolerance, and that, genetically, SIZ1 and PHO2 are likely independent of SOS3-dependent salt signaling.     

The responsiveness of the <Emphasis Type="Italic">IAA2</Emphasis> promoter to IAA and IBA is differentially affected in <Emphasis Type="Italic">Arabidopsis</Emphasis> roots and shoots by flavonoids

The structural features of flavonoids which are involved in the modulation of auxin distribution in Arabidopsis thaliana were evaluated. An auxin-inducible promoter IAA2 fused to a reporter gene (GUS) was used to monitor the tissue responsiveness to auxins. The following aspects were investigated: 1) the influence of flavonoids (quercetin, naringenin, kaempferol, myricetin and isorhamnetin) on the distribution of indole-3-acetic acid (IAA) and indole-3-butyric acid (IBA) in roots and leaves, 2) differences in flavonoid uptake into roots and shoots depending on flavonoid concentration in the medium, and 3) influence of structurally different flavonoids on the gravitropic response and growth of roots. The same flavonoids differently affected IAA and IBA distribution in leaves and roots. There were several structural requirements for the flavonoids which resulted in the changes of auxin response/distribution. Great differences between the ability of shoots and roots to take up quercetin were showed. Also, flavonoids influenced gravitropism and root growth of Arabidopsis seedlings in a structure-dependent manner.     

Activation of a flavin monooxygenase gene <Emphasis Type="Italic">YUCCA7</Emphasis> enhances drought resistance in <Emphasis Type="Italic">Arabidopsis</Emphasis>

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.     

Sugar-induced adventitious roots in<Emphasis Type="Italic"> Arabidopsis</Emphasis> seedlings

The effects of sugars on root growth and on development of adventitious roots were analyzed in Arabidopsis thaliana. Seeds were sown on agar plates containing 0.0–5.0% sugars and placed vertically in darkness (DD) or under long day (LD, 16 h:8 h) conditions, so that the seedlings were constantly attached to the agar medium. In the sucrose-supplemented medium, seedlings showed sustained growth in both DD and LD. However, only dark-grown seedlings developed adventitious roots from the elongated hypocotyl. The adventitious roots began to develop 5 days after imbibition and increased in number until day 11. They could, however, be initiated at any position along the hypocotyl, near the cotyledon or the primary root. They were initiated in the pericycle in the same manner as ordinary lateral roots. Sucrose, glucose and fructose greatly stimulated the induction of adventitious roots, but mannose or sorbitol did not. Sucrose at concentrations of 0.5–2.0% was most effective in inducing adventitious roots, although 5.0% sucrose suppressed induction. Direct contact of the hypocotyl with the sugar-supplemented agar medium was indispensable for the induction of adventitious roots. Electronic Publication     

Optimization of conditions for transient <Emphasis Type="Italic">Agrobacterium</Emphasis>-mediated gene expression assays in <Emphasis Type="Italic">Arabidopsis</Emphasis>

Transient genetic transformation of plant organs is an indispensable way of studying gene function in plants. This study was aimed to develop an optimized system for transient Agrobacterium-mediated transformation of the Arabidopsis leaves. The β-glucuronidase (GUS) reporter gene was employed to evaluate growth and biochemical parameters that influence the levels of transient expression. The effects of plant culture conditions, Agrobacterial genetic backgrounds, densities of Agrobacterial cell suspensions, and of several detergents were analyzed. We found that optimization of plant culture conditions is the most critical factor among the parameters analyzed. Higher levels of transient expression were observed in plants grown under short day conditions (SDs) than in plants grown under long day conditions (LDs). Furthermore, incubation of the plants under SDs at high relative humidity (85–90%) for 24 h after infiltration greatly improved the levels of transient expression. Under the optimized culture conditions, expression of the reporter gene reached the peak 3 days after infiltration and was rapidly decreased after the peak. Among the five Agrobacterial strains examined, LAB4404 produced the highest levels of expression. We also examined the effects of detergents, including Triton X-100, Tween-20, and Silwet L-77. Supplementation of the infiltration media either with 0.01% Triton X-100 or 0.01% Tween-20 improved the levels of expression by approximately 1.6-fold. Our observations indicate that transient transformation of the Arabidopsis leaves in the infiltration media supplemented with 0.01% Triton X-100 and incubation of the infiltrated plants under SDs at high relative humidity are necessary for maximal levels of expression.     

<Emphasis Type="Italic">Ty</Emphasis>1<Emphasis Type="Italic">/copia</Emphasis>- and <Emphasis Type="Italic">Ty</Emphasis>3<Emphasis Type="Italic">/gypsy</Emphasis>-like DNA sequences in <Emphasis Type="Italic">Helianthus </Emphasis>species

Two repeated DNA sequences isolated from a partial genomic DNA library of Helianthus annuus, p HaS13 and p HaS211, were shown to represent portions of the int gene of a Ty3 /gypsy retroelement and of the RNase-Hgene of a Ty1 /copia retroelement, respectively. Southern blotting patterns obtained by hybridizing the two probes to BglII- or DraI-digested genomic DNA from different Helianthus species showed p HaS13 and p HaS211 were parts of dispersed repeats at least 8 and 7 kb in length, respectively, that were conserved in all species studied. Comparable hybridization patterns were obtained in all species with p HaS13. By contrast, the patterns obtained by hybridizing p HaS211 clearly differentiated annual species from perennials. The frequencies of p HaS13- and p HaS211-related sequences in different species were 4.3x10(4)-1.3x10(5) copies and 9.9x10(2)-8.1x10(3) copies per picogram of DNA, respectively. The frequency of p HaS13-related sequences varied widely within annual species, while no significant difference was observed among perennial species. Conversely, the frequency variation of p HaS211-related sequences was as large within annual species as within perennials. Sequences of both families were found to be dispersed along the length of all chromosomes in all species studied. However, Ty3 /gypsy-like sequences were localized preferentially at the centromeric regions, whereas Ty1/ copia-like sequences were less represented or absent around the centromeres and plentiful at the chromosome ends. These findings suggest that the two sequence families played a role in Helianthusgenome evolution and species divergence, evolved independently in the same genomic backgrounds and in annual or perennial species, and acquired different possible functions in the host genomes.