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Phenolic glycosides are effective reactive oxygen scavengers and peroxidase substrates, suggesting that compounds in addition to ascorbate may have functional importance in defence responses against ozone (O(3)), especially in the leaf apoplast. The apoplastic concentrations of ascorbic acid (AA) and phenolic glycosides in Arabidopsis thaliana L. Col-0 wild-type plants were determined following exposure to a range of O(3) concentrations (5, 125 or 175 nL L(-1)) in controlled environment chambers. AA in leaf apoplast extracts was almost entirely oxidized in all treatments, suggesting that O(3) scavenging by direct reactions with reduced AA was very limited. In regard to phenolics, O(3) stimulated transcription of numerous phenylpropanoid pathway genes and increased the apoplastic concentration of sinapoyl malate. However, modelling of O(3) scavenging in the apoplast indicated that sinapoyl malate concentrations were too low to be effective protectants. Furthermore, null mutants for sinapoyl esters (fah1-7), kaempferol glycosides (tt4-1) and the double mutant (tt4-1/fah1-7) were equally sensitive to chronic O(3) as Ler-0 wild-type plants. These results indicate that current understanding of O(3) defence schemes deserves reassessment as mechanisms other than direct scavenging of O(3) by extracellular AA and antioxidant activity of some phenolics may predominate in some plant species., (Published 2012. This article is a U.S. Government work and is in the public domain in the USA.)

The heterotrimeric G-protein complex provides signal amplification and target specificity. The Arabidopsis (Arabidopsis thaliana) Gβ-subunit of this complex (AGB1) interacts with and modulates the activity of target cytoplasmic proteins. This specificity resides in the structure of the interface between AGB1 and its targets. Important surface residues of AGB1, which were deduced from a comparative evolutionary approach, were mutated to dissect AGB1-dependent physiological functions. Analysis of the capacity of these mutants to complement well-established phenotypes of Gβ-null mutants revealed AGB1 residues critical for specific AGB1-mediated biological processes, including growth architecture, pathogen resistance, stomata-mediated leaf-air gas exchange, and possibly photosynthesis. These findings provide promising new avenues to direct the finely tuned engineering of crop yield and traits.

While exposure of C3 plants to elevated [CO2] would be expected to reduce production of reactive oxygen species (ROS) in leaves because of reduced photorespiratory metabolism, results obtained in the present study suggest that exposure of plants to elevated [CO2] can result in increased oxidative stress. First, in Arabidopsis and soybean, leaf protein carbonylation, a marker of oxidative stress, was often increased when plants were exposed to elevated [CO2]. In soybean, increased carbonyl content was often associated with loss of leaf chlorophyll and reduced enhancement of leaf photosynthetic rate (Pn) by elevated [CO2]. Second, two-dimensional (2-DE) difference gel electrophoresis (DIGE) analysis of proteins extracted from leaves of soybean plants grown at elevated [CO2] or [O3] revealed that both treatments altered the abundance of a similar subset of proteins, consistent with the idea that both conditions may involve an oxidative stress. The 2-DE analysis of leaf proteins was facilitated by a novel and simple procedure to remove ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) from soluble soybean leaf extracts. Collectively, these findings add a new dimension to our understanding of global change biology and raise the possibility that oxidative signals can be an unexpected component of plant response to elevated [CO2].

Growth, seed yield and accumulation of ultraviolet (UV)-absorbing compounds were studied in chalcone isomerase-defective tt-5 mutant of Arabidopsis thaliana and its Landsberg erecta (Ler) progenitor under full-spectrum solar radiation and a series of filters which attenuated progressively larger portions of the UV-B and UV-A radiation bands. The purpose was to determine: (1) whether or not the tt-5 mutant could be induced to grow more or less normally, given adequate protection from damaging UV in the presence of high levels of photosynthetic active radiation (PAR) so that it could be used as a surrogate for mechanistic high UV studies; (2) whether the generalized plant action spectrum or the alfalfa DNA damage action spectrum would best describe the observed responses; and (3) if the traditional Mylar (polyester) filter provides an adequate control for UV damage studies. Maximum rosette diameter (MRD), plant height and fresh weight at harvest and seed yield were measured, along with absorbance of leaf extracts at 300 nm and accumulation of total phenolics before and after exposure to UV. Three types of UV filters were used: cellulose diacetate (CD), which non-selectively transmits all the UV reaching the earth''s surface; Mylar, which cuts off UV below about 320 nm; and polyvinyl chloride (PVC) which cuts off UV below about 340 nm. Generally, Ler showed no significant growth effects under any of the treatments except for plant height which was reduced in Mylar and CD when compared to PVC. Conversely, tt-5 generally exhibited progressive decreases in all the measures of plant growth with PVC resulting in the best growth, Mylar treatments showing significant reductions and CD treatments even greater reductions. It was clear that even under these circumstances: the disruption to secondary metabolism in tt-5 makes it unsuitable for mechanistic studies of high UV-B damage; the alfalfa DNA action spectrum seemed the best correlated with observed responses and suggests a significant damaging radiation band which is not affected by stratospheric ozone; and since the damaging radiation extends beyond the Mylar cut-in, this material will not provide an adequate control for UV damage studies. [Copyright &y& Elsevier]