Brassinosteroid regulates carotenoid content through BZR1-mediated suppression of 4-HYDROXYPHENYLPYRUVATE DIOXYGENASEin Arabidopsis seedlings.

The crosstalk mechanism regulating content and signal transduction between brassinosteroids (BRs) and salicylic acid (SA) for plant defense was investigated in Arabidopsis. Compared to the wild type, an increased bacterial resistance was observed in bzr1-1D, a dominant mutant of the BR transcription factor BZR1. In bzr1-1D, SA biosynthetic gene ICS1 expression and endogenous SA content greatly increased upon Pst DC3000 infection, and the direct binding of BZR1 to the ICS1 promoter was confirmed through EMSA and ChIP. In bzr1-1D where NPR1 expression was almost absent, expression of PR genes was increased, and both BZR1 and PR5 expressions increased after SA treatment. EMSA and ChIP verified that BZR1 binds directly to the cis-element present in the PR5 promoter and a pull-down assay showed that TGAs, SA transcription factors upstream of PR genes, interact with BZR1 at the protein level. Crude enzyme assays demonstrated that BR C-6 oxidase activity, a CYP85A1 function, greatly increased during Pst DC3000 infection. In the tga1 tga4 double mutant lacking SA transcription factors TGA1 and TGA4, BR biosynthetic gene CYP85A1 expression was significantly reduced. EMSA and ChIP confirmed that both TGA1 and TGA4 bind to the cis-element present in the CYP85A1 promoter, and castasterone (CS), a bio-active BR, was significantly reduced in tga1 tga4. Taken together, the upregulation of ICS1 expression by BZR1 and CYP85A1 expression by TGA1/4 mutually enhanced endogenous level of BR and SA in Arabidopsis. Furthermore, TGAs and BZR1 interaction at the protein level induces SA-induced immunity through the upregulation of PR5 expression, increasing bacterial resistance in the plant. These results explain the mutual control mechanisms of the synergistic effects BR and SA have on plant defense and confirm BR's effect on plant defense and growth promotion in A. thaliana.

This study generated the transgenic Arabidopsis lines pAt5g54000-AtBZR1::Col-0 and pAt5g54000-AtBES1::Col-0, in which Arabidopsis BZR1 and BES1 (AtBZR1 and AtBES1) were seed-specifically expressed with pAt5g54000, a seed-specific promoter. Semi-quantitative RT-PCR and GUS-staining analysis demonstrated that the inserted AtBZR1 and AtBES1 were concentrated in seeds in siliques of transgenic plants. Seed number, length, width, and mass increased in the pAt5g54000-AtBZR1::Col-0 and pAt5g54000-AtBES1::Col-0 mutants compared to untransformed Arabidopsis. The endogenous levels of primary metabolites, such as carbohydrates, proteins, and lipids, in transgenic seeds were also higher than those in wild-type seeds, indicating that both seed size and quality are improved by seed-specific expression of AtBZR1 and AtBES1 in Arabidopsis. In both transgenic Arabidopsis seeds, relative to wild-type seeds, the expression of positive regulatory genes involved in determining seed size, such as SHORT HYPOCOTYL UNDER BLUE1 (AtSHB1), MINISEED3 (AtMINI3), and HAIKU2 (AtIKU2), was increased by up-regulation of AtBZR1 and AtBES1 as well as down-regulation of ABA Deficient 2 (AtABA2) and ABA Insensitive 5 (AtABI5). This result suggests that AtBZR1- and AtBES1-mediated signaling pathways such as AtBZR1/AtBES1 -> AtSHB1 -> AtMINI3 -> AtIKU2 and AtBZR1/AtBES1 -> AtABA2, and/or AtABI5 -> AtSHB1 -> AtMINI3 -> AtIKU2 increase the yield and quality of seeds in transgenic Arabidopsis. Taken together, our findings demonstrated the usefulness and applicability of seed-specific introduction of AtBZR1 and AtBES1 encoding key transcription factors in brassinosteroid signaling to improve seed yield and quality in Arabidopsis.

Expression of ABA-deficient 2 (ABA2) gene involved in ABA biosynthesis was downregulated in bes1-D, but upregulated in bes1-KO. Hence, the expression of ABA2 is negatively controlled by the BES1 transcription factor of brassinosteroid signaling in Arabidopsis thaliana. BES1 is directly bound to the E-box sequences in the promoter of ABA2, which reduced the endogenous levels of ABA in the plant. The seeds of aba2-1 and bes1-KO were larger and smaller, respectively, than those of the wild type. In bes1-KO x aba2-1, the reduced seed size in bes1-KO was partly restored to the seed size in aba2-1. Hence, BES1-mediated regulation of seed size is an upstream process for the homeostasis of endogenous ABA to control the seed size. BES1 suppressed the expression of ABA Insensitive 5 (ABI5), a major transcription factor gene in ABA signaling that determines the seed size. It is directly bound to the promoter of ABI5 by BES1-induced downregulation of ABI5. The expression of SHORT HYPOCOTYL UNDER BLUE1, MINISEED3, and HAIKU2 as positively regulatory genes determining the seed size was activated, increasing the seed size in bes1-D. Conclusively, brassinosteroid signaling through BES1 downregulated both the biosynthesis and signaling of ABA, which increased the seed size in Arabidopsis.

Dust grains absorb half of the radiation emitted by stars throughout the history of the universe, re-emitting this energy at infrared wavelengths(1-3). Polycyclic aromatic hydrocarbons (PAHs) are large organic molecules that trace millimetre-size dust grains and regulate the cooling of interstellar gas within galaxies(4,5). Observations of PAH features in very distant galaxies have been difficult owing to the limited sensitivity and wavelength coverage of previous infrared telescopes(6,7). Here we present James Webb Space Telescope observations that detect the 3.3 mu m PAH feature in a galaxy observed less than 1.5 billion years after the Big Bang. The high equivalent width of the PAH feature indicates that star formation, rather than black hole accretion, dominates infrared emission throughout the galaxy. The light from PAH molecules, hot dust and large dust grains and stars are spatially distinct from one another, leading to order-of-magnitude variations in PAH equivalent width and ratio of PAH to total infrared luminosity across the galaxy. The spatial variations we observe suggest either a physical offset between PAHs and large dust grains or wide variations in the local ultraviolet radiation field. Our observations demonstrate that differences in emission from PAH molecules and large dust grains are a complex result of localized processes within early galaxies.