Nitric oxide (NO) is a plant signal contributing to plant stress responses and development. We here review some of the key advances in this field but also highlight certain key aspects of plant NO biology that require further attention.

Summary• The cpr5-1 Arabidopsis thaliana mutant exhibits constitutive activation of salicylic acid (SA), jasmonic acid (JA) and ethylene (ET) signalling pathways and displays enhanced tolerance of heat stress (HS).• cpr5-1 crossed with jar1-1 (a JA-amino acid synthetase) was compromised in basal thermotolerance, as were the mutants opr3 (mutated in OPDA reductase3) and coi1-1 (affected in an E3 ubiquitin ligase F-box; a key JA-signalling component). In addition, heating wild-type Arabidopsis led to the accumulation of a range of jasmonates: JA, 12-oxophytodienoic acid (OPDA) and a JA-isoleucine (JA-Ile) conjugate. Exogenous application of methyl jasmonate protected wild-type Arabidopsis from HS.• Ethylene was rapidly produced during HS, with levels being modulated by both JA and SA. By contrast, the ethylene mutant ein2-1 conferred greater thermotolerance.• These data suggest that JA acts with SA, conferring basal thermotolerance while ET may act to promote cell death.

Summary• Unravelling mechanisms that control plant growth as a function of nutrient availability presents a major challenge in plant biology. This study reports the first transcriptome response to long-term (1 wk) magnesium (Mg) depletion and restoration in Arabidopsis thaliana.• Before the outbreak of visual symptoms, genes responding to Mg starvation and restoration were monitored in the roots and young mature leaves and compared with the Mg fully supplied as control.• After 1 wk Mg starvation in roots and leaves, 114 and 2991 genes were identified to be differentially regulated, respectively, which confirmed the later observation that the shoot development was more affected than the root in Arabidopsis. After 24 h of Mg resupply, restoration was effective for the expression of half of the genes altered. We emphasized differences in the expression amplitude of genes associated with the circadian clock predominantly in leaves, a higher expression of genes in the ethylene biosynthetic pathway, in the reactive oxygen species detoxification and in the photoprotection of the photosynthetic apparatus. Some of these observations at the molecular level were verified by metabolite analysis.• The results obtained here will help us to better understand how changes in Mg availability are translated into adaptive responses in the plant.

Bacteria emit a wealth of volatiles. The combination of coupled gas chromatography/mass spectrometry (GC/MS) and proton-transfer-reaction mass spectrometry (PTR-MS) analyses provided a most comprehensive profile of volatiles of the rhizobacterium Serratia odorifera 4Rx13. An array of compounds, highly dominated by sodorifen (approximately 50%), a bicyclic oligomethyl octadiene, could be detected. Other volatiles included components of the biogeochemical sulfur cycle such as dimethyl disulfide (DMDS), dimethyl trisulfide and methanethiol, terpenoids, 2-phenylethanol, and other aromatic compounds. The composition of the bouquet of S. odorifera did not change significantly during the different growth intervals. At the beginning of the stationary phase, 60 μg of volatiles per 24 h and 60 easily detectable components were released. Ammonia was also released by S. odorifera, while ethylene, nitric oxide (NO) and hydrogen cyanide (HCN) could not be detected. Dual culture assays proved that 20 μmol DMDS and 2.5 μmol ammonia, individually applied, represent the IC(50) concentrations that cause negative effects on Arabidopsis thaliana.

Ethylene controls multiple physiological processes in plants, including cell elongation. Consequently, ethylene synthesis is regulated by internal and external signals. We show that a light-entrained circadian clock regulates ethylene release from unstressed, wild-type Arabidopsis (Arabidopsis thaliana) seedlings, with a peak in the mid-subjective day. The circadian clock drives the expression of multiple ACC SYNTHASE genes, resulting in peak RNA levels at the phase of maximal ethylene synthesis. Ethylene production levels are tightly correlated with ACC SYNTHASE 8 steady-state transcript levels. The expression of this gene is controlled by light, by the circadian clock, and by negative feedback regulation through ethylene signaling. In addition, ethylene production is controlled by the TIMING OF CAB EXPRESSION 1 and CIRCADIAN CLOCK ASSOCIATED 1 genes, which are critical for all circadian rhythms yet tested in Arabidopsis. Mutation of ethylene signaling pathways did not alter the phase or period of circadian rhythms. Mutants with altered ethylene production or signaling also retained normal rhythmicity of leaf movement. We conclude that circadian rhythms of ethylene production are not critical for rhythmic growth.Since the discovery of ethylene production in plants in the 1930s, researchers have tried to elucidate mechanisms governing ethylene formation. A major breakthrough was the completion of the enzymatic pathway for ethylene biosynthesis 50 years later (for review, see Yang and Hoffman, 1984). Shortly thereafter, the first genes encoding ethylene biosynthetic enzymes were cloned (Sato and Theologis, 1989;Van Der Straeten et al., 1990;Hamilton et al., 1991;Spanu et al., 1991). With the use of tomato (Lycopersicon esculentum) and especially Arabidopsis (Arabidopsis thaliana) as model plants, molecular biological and genetic analysis has shed light on many physiological processes involving ethylene (Abeles et al., 1992;Somerville and Meyerowitz, 2002). In higher plants, the enzymes for ethylene biosynthesis are encoded by gene families. The members of these families are differentially responsive to various ethylene-inducing factors, including wounding, fruit ripening, pathogen infections, auxins, and cytokinins (for review, see Fluhr and Mattoo, 1996).In Arabidopsis, there are 12 genes in the family of enzymes that produces the ethylene precursor 1-amino-cyclopropane-1-carboxylic acid (ACC), one of which, ACC SYNTHASE 3 (ACS3), is a pseudogene (Yamagami et al., 2003;Tsuchisaka and Theologis, 2004). ACS1 is not functional as an ACS (Liang et al., 1992). ACS10 and ACS12 do not function as ACSs either, but as aminotransferases (Yamagami et al., 2003). Many of the ACS genes are regulated on the transcriptional level. ACS2 transcription in leaves is switched off when tissues mature (Rodrigues-Pousada et al., 1993; in this paper the gene was designated ACS1). ACS4 can be induced by auxins (Abel et al., 1995). ACS5 is regulated by cytokinins that cause stabilization of the protein (Chae et al., 2003). ACS6 is induce...