SWP73 Subunits of Arabidopsis SWI/SNF Chromatin Remodeling Complexes Play Distinct Roles in Leaf and Flower Development

Arabidopsis thaliana SWP73A and SWP73B are homologs of mammalian BRAHMA-associated factors (BAF60s) that tether SWITCH/SUCROSE NONFERMENTING chromatin remodeling complexes to transcription factors of genes regulating various cell differentiation pathways. Here, we show that Arabidopsis thaliana SWP73s modulate several important developmental pathways. While undergoing normal vegetative development, swp73a mutants display reduced expression of FLOWERING LOCUS C and early flowering in short days. By contrast, swp73b mutants are characterized by retarded growth, severe defects in leaf and flower development, delayed flowering, and male sterility. MNase-Seq, transcript profiling, and ChIP-Seq studies demonstrate that SWP73B binds the promoters of ASYMMETRIC LEAVES1 and 2, KANADI1 and 3, and YABBY2, 3, and 5 genes, which regulate leaf development and show coordinately altered transcription in swp73b plants. Lack of SWP73B alters the expression patterns of APETALA1, APETALA3, and the MADS box gene AGL24, whereas other floral organ identity genes show reduced expression correlating with defects in flower development. Consistently, SWP73B binds to the promoter regions of APETALA1 and 3, SEPALLATA3, LEAFY, UNUSUAL FLORAL ORGANS, TERMINAL FLOWER1, AGAMOUS-LIKE24, and SUPPRESSOR OF CONSTANS OVEREXPRESSION1 genes, and the swp73b mutation alters nucleosome occupancy on most of these loci. In conclusion, SWP73B acts as important modulator of major developmental pathways, while SWP73A functions in flowering time control

Genetic Analyses of Interactions among Gibberellin, Abscisic Acid, and Brassinosteroids in the Control of Flowering Time in Arabidopsis thaliana

Genetic interactions between phytohormones in the control of flowering time in Arabidopsis thaliana have not been extensively studied. Three phytohormones have been individually connected to the floral-timing program. The inductive function of gibberellins (GAs) is the most documented. Abscisic acid (ABA) has been demonstrated to delay flowering. Finally, the promotive role of brassinosteroids (BRs) has been established. It has been reported that for many physiological processes, hormone pathways interact to ensure an appropriate biological response.We tested possible genetic interactions between GA-, ABA-, and BR-dependent pathways in the control of the transition to flowering. For this, single and double mutants deficient in the biosynthesis of GAs, ABA, and BRs were used to assess the effect of hormone deficiency on the timing of floral transition. Also, plants that over-express genes encoding rate-limiting enzymes in each biosynthetic pathway were generated and the flowering time of these lines was investigated.Loss-of-function studies revealed a complex relationship between GAs and ABA, and between ABA and BRs, and suggested a cross-regulatory relation between GAs to BRs. Gain-of-function studies revealed that GAs were clearly limiting in their sufficiency of action, whereas increases in BRs and ABA led to a more modest phenotypic effect on floral timing. We conclude from our genetic tests that the effects of GA, ABA, and BR on timing of floral induction are only in partially coordinated action

Overexpression lines for rate-limiting enzymes in various phytohormone pathways.

<p>Transgenic lines harboring <i>35S::DWF4</i>, <i>35S::GA5</i> and <i>35S::NCED3</i> constructs. <b><i>A.</i></b> Over-expression was confirmed by RT-PCR with primers specific for <i>DWF4</i>, <i>GA5</i> and <i>NCED3</i>. Primers specific for the elongation factor 1-alpha gene were used as a control. Representative lines are shown. All lines tested showed over-expression of the gene of interest >3 fold. <b><i>B.</i></b> Images of 3-weeks-old plants grown under long days (16 h light/8 h darkness) in the greenhouse. The white bar indicates 1 cm.</p

Student's t-test for flowering-time differences between mutant genotypes.

<p>Listed are pairs of compared genotypes. P values for each pair are provided.</p><p>ø No significant difference P>0.05;</p><p>statistically significant differences:</p><p>***P<0.0001,</p><p>**P<0.001,</p><p>*P<0.05.</p

Simplified hormone biosynthetic pathways.

<p>The hormone biosynthetic pathways of Arabidopsis for gibberellins <b><i>A.</i></b>, ABA <b><i>B.</i></b>, and, brassinolide <b><i>C.</i></b>. The biosynthesis mutants used in this study and sites of their lesions are shown. Also, the biosynthetic genes over-expressed to increase the levels of respective hormones are indicated. <b><i>A.</i></b> The <i>ga1</i> mutant is impaired in the first stage of GA-biosynthesis: the cyclization of geranylgeranyl diphosphate (GGPP) to copalyl diphosphate (CPP). <b><i>B.</i></b> The <i>aba2</i> mutant is blocked at the cis-xanthoxin to ABA-aldehyde conversion. <b><i>C.</i></b> The conversion of 6-Deoxocathasterone/Cathasterone to 6- Deoxoteasterone/teasterone does not occur in the <i>cpd</i> mutant. <b><i>A.</i></b> The <i>GA5</i> gene encodes a GA 20-oxidase that catalyzes the formation of the GA20 and GA9, the final precursors of the bioactive GAs. <b><i>B.</i></b> The <i>NCED3</i> encodes 9-<i>cis</i>-epoxycarotenoid dioxygenase that catalyzes the oxidative cleavage of a 9-<i>cis</i> isomer of epoxycarotenoid (9-<i>cis</i>-violaxanthin or 9’-<i>cis</i>-neoxanthin) to form xanthoxin. <b><i>C.</i></b> The <i>DWF4</i> gene encodes a 22-a hydroxylase (CYP90B1) that catalyzes the conversion of 6- Oxocampestanol/Campestanol to 6-Deoxocathasterone/Cathasterone. IPP, Isopentenyl pyrophosphate. ABA, abscisic acid. Adapted from <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0014012#pone.0014012-Srivastava1" target="_blank">[49]</a>.</p