SummaryCONSTANS promotes¯owering of Arabidopsis in response to long-day conditions. We show that CONSTANS is a member of an Arabidopsis gene family that comprises 16 other members. The CO-Like proteins encoded by these genes contain two segments of homology: a zinc ®nger containing region near their amino terminus and a CCT (CO, CO-Like, TOC1) domain near their carboxy terminus. Analysis of seven classical co mutant alleles demonstrated that the mutations all occur within either the zinc ®nger region or the CCT domain, con®rming that the two regions of homology are important for CO function. The zinc ®ngers are most similar to those of B-boxes, which act as protein±protein interaction domains in several transcription factors described in animals. Segments of CO protein containing the CCT domain localize GFP to the nucleus, but one mutation that affects the CCT domain delays¯owering without affecting the nuclear localization function, suggesting that this domain has additional functions. All eight co alleles, including one recovered by pollen irradiation in which DNA encoding both B-boxes is deleted, are shown to be semidominant. This dominance appears to be largely due to a reduction in CO dosage in the heterozygous plants. However, some alleles may also actively delay¯owering, because overexpression from the CaMV 35S promoter of the co-3 allele, that has a mutation in the second B-box, delayed¯owering of wild-type plants. The signi®cance of these observations for the role of CO in the control of¯owering time is discussed.

For self-pollinating plants to reproduce, male and female organ development must be coordinated as flowers mature. The Arabidopsis transcription factors AUXIN RESPONSE FACTOR 6 (ARF6) and ARF8 regulate this complex process by promoting petal expansion, stamen filament elongation, anther dehiscence, and gynoecium maturation, thereby ensuring that pollen released from the anthers is deposited on the stigma of a receptive gynoecium. ARF6 and ARF8 induce jasmonate production, which in turn triggers expression of MYB21 and MYB24, encoding R2R3 MYB transcription factors that promote petal and stamen growth. To understand the dynamics of this flower maturation regulatory network, we have characterized morphological, chemical, and global gene expression phenotypes of arf, myb, and jasmonate pathway mutant flowers. We found that MYB21 and MYB24 promoted not only petal and stamen development but also gynoecium growth. As well as regulating reproductive competence, both the ARF and MYB factors promoted nectary development or function and volatile sesquiterpene production, which may attract insect pollinators and/or repel pathogens. Mutants lacking jasmonate synthesis or response had decreased MYB21 expression and stamen and petal growth at the stage when flowers normally open, but had increased MYB21 expression in petals of older flowers, resulting in renewed and persistent petal expansion at later stages. Both auxin response and jasmonate synthesis promoted positive feedbacks that may ensure rapid petal and stamen growth as flowers open. MYB21 also fed back negatively on expression of jasmonate biosynthesis pathway genes to decrease flower jasmonate level, which correlated with termination of growth after flowers have opened. These dynamic feedbacks may promote timely, coordinated, and transient growth of flower organs.

The Arabidopsis mutant early in short days4 ( esd4 ) shows extreme early flowering and alterations in shoot development. We have identified ESD4 and demonstrate that it encodes a nuclear protein located predominantly at the periphery of the nucleus. ESD4 contains a segment of Ͼ 200 amino acids with strong similarity to yeast and animal proteases that are specific for the protein modifier SMALL UBIQUITIN-RELATED MODIFIER (SUMO). ESD4 shows a similar function to these proteases in vitro and processes the precursor of Arabidopsis SUMO (AtSUMO) to generate the mature form. This activity of ESD4 is prevented by mutations that affect the predicted active site of the protease or the cleavage site of the AtSUMO precursor. In yeast, these proteases also recycle SUMO from conjugates, and this appears to be the major role of ESD4 in vivo. This is suggested because esd4 mutants contain less free AtSUMO and more SUMO conjugates than wild-type plants, and a transgene expressing mature SUMO at high levels enhanced aspects of the esd4 phenotype. ESD4 defines an important role for protein modification by AtSUMO in the regulation of flowering.

SUMMARYAuxin regulates plant growth and development in part by activating gene expression. Arabidopsis thaliana SMALL AUXIN UP RNAs (SAURs) are a family of early auxin-responsive genes with unknown functionality. Here, we show that transgenic plant lines expressing artificial microRNA constructs (aMIR-SAUR-A or -B) that target a SAUR subfamily (SAUR61-SAUR68 and SAUR75) had slightly reduced hypocotyl and stamen filament elongation. In contrast, transgenic plants expressing SAUR63:GFP or SAUR63:GUS fusions had long hypocotyls, petals and stamen filaments, suggesting that these protein fusions caused a gain of function. SAUR63:GFP and SAUR63:GUS seedlings also accumulated a higher level of basipetally transported auxin in the hypocotyl than did wild-type seedlings, and had wavy hypocotyls and twisted inflorescence stems. Mutations in auxin efflux carriers could partially suppress some SAUR63:GUS phenotypes. In contrast, SAUR63:HA plants had wild-type elongation and auxin transport. SAUR63:GFP protein had a longer half-life than SAUR63:HA. Fluorescence imaging and microsomal fractionation studies revealed that SAUR63:GFP was localized mainly in the plasma membrane, whereas SAUR63:HA was present in both soluble and membrane fractions. Low light conditions increased SAUR63:HA protein turnover rate. These results indicate that membrane-associated Arabidopsis SAUR63 promotes auxin-stimulated organ elongation.

SUMMARYearly in short days 4, a mutation in Arabidopsis that causes early flowering and reduces the mRNA abundance of the floral repressor FLC

Three genetic pathways promote flowering of Arabidopsis under long photoperiods. These pathways are represented by the genes CO, FCA, and GA1, which act in the long-day, autonomous, and gibberellin pathways, respectively. To test whether these are the only pathways that promote flowering under long photoperiods, the co-2 fca-1 ga1-3 triple mutant was constructed. These plants never flowered under long-or short-day conditions, indicating that the three pathways impaired by these mutations are absolutely required for flowering under these conditions. The triple mutant background represents a "vegetative ground state" enabling the roles of single pathways to be described in the corresponding double mutants. The phenotypes of plants carrying all eight combinations of wild-type and mutant alleles at the three loci were compared under long-and short-day conditions. This analysis demonstrated that under long photoperiods the long-day pathway promoted flowering most effectively, whereas under short photoperiods the gibberellin pathway had the strongest effect. The autonomous pathway had a weak effect when acting alone under either photoperiod but appeared to play an important role in facilitating the promotion of flowering by the other two pathways. The vegetative phenotype of the triple mutant could be overcome by vernalization, suggesting that a fourth pathway promoted flowering under these conditions. These observations are discussed in light of current models describing the regulation of flowering time in Arabidopsis.

No abstract

Plant organs grow to their final size by cell expansion. Several plant hormones, including auxin, brassinosteroids, ethylene, and GAs, regulate expansion growth, and environmental signals such as temperature and light can affect abundance of or responsiveness to these hormones (Wolters and Jürgens, 2009; Depuydt and Hardtke, 2011; Leivar and Monte, 2014). Integration of these signals with metabolic signals determines the degree of cell expansion and, hence, the final size of stems, leaves, and other organs. Hypocotyl elongation after seed germination is sensitive to multiple hormonal and environmental signals, and occurs largely by the expansion of cells produced in the embryo. In Arabidopsis (Arabidopsis thaliana), mutations in genes regulating light perception, hormone response, the circadian rhythm, or transcription can each affect hypocotyl elongation. Many such mutations also affect growth in other organs, indicating that common mechanisms regulate expansion growth in multiple tissues. Thus, the hypocotyl is a useful model tissue in which to explore general control mechanisms of cell expansion. High temperature and far-red-enriched light stimulate hypocotyl elongation, and the circadian rhythm gates hypocotyl elongation, in part by regulating auxin response. Integrated signals from these and other inputs affect the levels and activity of PHYTOCHROME INTERACTING FACTOR (PIF) transcription factors (