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Plants are sessile organisms and are, consequently, exposed to a wide variety of environmental stresses, both abiotic and biotic, exerted by their surroundings. The most common of these is temperature. Within the range of temperatures tolerable to plants, the response to low temperature, particularly near-freezing temperature, is well understood. Plants have evolved a number of adaptive mechanisms to meet the challenge of low temperature. In Arabidopsis, flowering is accelerated by prolonged exposure to cold, a process called vernalization. The epigenetic silencing of the FLOWERING LOCUS C (FLC) (Michaels and Amasino 1999;Sheldon et al. 1999) is central to the vernalization process (Sung and Amasino 2005), and this silencing has been attributed to the activities of the VERNALIZATION1 (VRN1), VERNAL-IZATION2 (VRN2), and VERNALIZATION INSENSI-TIVE3 (VIN3) genes (Gendall et al. 2001;Levy et al. 2002;Sung and Amasino 2004). Cold acclimation is another well-characterized response to low temperature (Guy 1990). Plants become tolerant to freezing temperatures by being previously exposed to short periods of low but nonfreezing temperatures. Analyses of mutant plants have identified C-Repeat-binding factor (CBF)-dependent and CBF-independent signaling pathways in cold acclimation (Sharma et al. 2005), suggesting that plants use distinct mechanisms to respond to low temperature.There is increasing concern about the potential impact of global temperature changes, which significantly affect ambient temperature, on plant development. Several lines of evidence suggest that the recently observed alterations in the flowering times of many plant species and the increase in plant respiration rates are closely associated with these changes in ambient temperature (Fitter and Fitter 2002;Atkin and Tjoelker 2003). Although a great deal of progress has been made in our understanding of the regulation of plant development by low temperature, less is currently known about the molecular mechanisms underlying the responses of plants to changes in ambient temperature (Coupland and Prat Monguio 2005;Samach and Wigge 2005). Here, we show that the SHORT VEGETATIVE PHASE (SVP) gene mediates ambient temperature signaling in Arabidopsis and that the SVP-mediated control of FLOWERING LOCUS T (FT) expression is one of the molecular mechanisms evolved by plants to modulate the timing of the developmental transition to flowering phase in response to changes in the ambient temperature. Results and DiscussionAs a first step to determining the mechanism underlying the perception and transduction of ambient temperature signaling in plants, we assessed mutants in known flowering time genes for their insensitivity to changes in ambient growth temperature. Of the flowering time mutants tested, one with a lesion in svp was indeed insensitive to such changes. The flowering of the majority of these flowering time mutants was noticeably delayed at 16°C, with flowering time ratios (16°C/23°C) ranging from 1.1 to 2.0 (Fig. 1A), the exception being ld-1. However,...

Methyl jasmonate is a plant volatile that acts as an important cellular regulator mediating diverse developmental processes and defense responses. We have cloned the novel gene JMT encoding an S-adenosyl-L-methionine:jasmonic acid carboxyl methyltransferase (JMT) from Arabidopsis thaliana. Recombinant JMT protein expressed in Escherichia coli catalyzed the formation of methyl jasmonate from jasmonic acid with K m value of 38.5 M. JMT RNA was not detected in young seedlings but was detected in rosettes, cauline leaves, and developing flowers. In addition, expression of the gene was induced both locally and systemically by wounding or methyl jasmonate treatment. This result suggests that JMT can perceive and respond to local and systemic signals generated by external stimuli, and that the signals may include methyl jasmonate itself. Transgenic Arabidopsis overexpressing JMT had a 3-fold elevated level of endogenous methyl jasmonate without altering jasmonic acid content. The transgenic plants exhibited constitutive expression of jasmonate-responsive genes, including VSP and PDF1.2. Furthermore, the transgenic plants showed enhanced level of resistance against the virulent fungus Botrytis cinerea. Thus, our data suggest that the jasmonic acid carboxyl methyltransferase is a key enzyme for jasmonate-regulated plant responses. Activation of JMT expression leads to production of methyl jasmonate that could act as an intracellular regulator, a diffusible intercellular signal transducer, and an airborne signal mediating intra-and interplant communications.

CONSTANS (CO) regulates flowering time by positively regulating expression of two floral integrators, FLOWERING LOCUS T (FT) and SUPPRESSOR OF OVEREXPRESSION OF CO 1 (SOC1), in Arabidopsis (Arabidopsis thaliana). FT and SOC1 have been proposed to act in parallel pathways downstream of CO based on genetic analysis using weak ft alleles, since ft soc1 double mutants showed an additive effect in suppressing the early flowering of CO overexpressor plants. However, this genetic analysis was inconsistent with the sequential induction pattern of FT and SOC1 found in inducible CO overexpressor plants. Hence, to identify genetic interactions of CO, FT, and SOC1, we carried out genetic and expression analyses with a newly isolated T-DNA allele of FT, ft-10. We found that ft-10 almost completely suppressed the early flowering phenotype of CO overexpressor plants, whereas soc1-2 partially suppressed the phenotype, suggesting that FT is the major output of CO. Expression of SOC1 was altered in gain-or loss-of-function mutants of FT, whereas expression of FT remained unchanged in gain-or loss-of-function mutants of SOC1, suggesting that FT positively regulates SOC1 to promote flowering. In addition, inactivation of FTcaused down-regulation of SOC1 even in plants overexpressing CO, indicating that FT is required for SOC1 induction by CO. Taken together, these data suggest that CO activates SOC1 through FT to promote flowering in Arabidopsis.The phase transition to flowering in plants is precisely controlled by environmental conditions and endogenous developmental cues so that plants produce their progeny under favorable conditions. The response to multiple factors suggests the existence of a complex network regulating this phase transition in plants ( Koornneef et al., 1998). To identify genes that control the transition, mutants that showed accelerated or delayed flowering under different conditions, commonly known as flowering-time mutants, have been isolated (Redei, 1975). These mutants were grouped according to their responses to various physiological conditions and then integrated into genetic pathways to explain the control of flowering time. Four floral promotion pathways have been genetically identified in Arabidopsis (Arabidopsis thaliana): the photoperiod, autonomous, vernalization, and GA pathways . Among these pathways, genes within the photoperiod pathway, or the long-day pathway, play an important role in controlling flowering time (Komeda, 2004), since Arabidopsis is a facultative long-day plant.One of the central regulators in the photoperiod pathway is CONSTANS (CO), which encodes a nuclear protein that contains a CCT motif and two B-box-type zinc-finger domains (Putterill et al., 1995). Loss of CO function delays the phase transition, whereas gain of function of CO accelerates it, suggesting that CO positively regulates flowering time in Arabidopsis. Furthermore, CO mRNA levels show a circadian rhythm under continuous light, such that CO mRNA levels peak at night and are reduced during the day (SuarezLopez et...