Ethylene synthesis regulated by biphasic induction of 1-aminocyclopropane-1-carboxylic acid synthase and 1-aminocyclopropane-1-carboxylic acid oxidase genes is required for hydrogen peroxide accumulation and cell death in ozone-exposed tomato

We show that above a certain threshold concentration, ozone leads to leaf injury in tomato (Lycopersicon esculentum). Ozone-induced leaf damage was preceded by a rapid increase in 1-aminocyclopropane-1-carboxylic acid (ACC) synthase activity, ACC content, and ethylene emission. Changes in mRNA levels of specific ACC synthase, ACC oxidase, and ethylene receptor genes occurred within 1 to 5 h. Expression of the genes encoding components of ethylene biosynthesis and perception, and biochemistry of ethylene synthesis suggested that ozone-induced ethylene synthesis in tomato is under biphasic control. In transgenic plants containing an LE-ACO1 promoter-beta-glucuronidase fusion construct, beta-glucuronidase activity increased rapidly at the beginning of the O(3) exposure and had a spatial distribution resembling the pattern of extracellular H(2)O(2) production at 7 h, which coincided with the cell death pattern after 24 h. Ethylene synthesis and perception were required for active H(2)O(2) production and cell death resulting in visible tissue damage. The results demonstrate a selective ozone response of ethylene biosynthetic genes and suggest a role for ethylene, in combination with the burst of H(2)O(2) production, in regulating the spread of cell death.     

Knock-down of protein phosphatase 2A subunit B’γ promotes phosphorylation of CALRETICULIN 1 in Arabidopsis thaliana

Different types of plant pathogens may cause enormous losses in agriculture and also have an ecological impact in the nature. On molecular level, disease resistance is acquired through the action of tightly interconnected signaling pathways that may induce highly specific immune reactions in plant cells. Controlled protein dephosphorylation through protein phosphatase 2A activity is emerging as a crucial mechanism that regulates diverse signaling events in plants. PP2A is predominantly trimeric, and consists of a catalytic subunit, a scaffold subunit A, and a variable regulatory subunit B, which determines the target specificity of the PP2A holoenzyme.¹ Recently, we uncovered a specific role for a regulatory subunit B’γ of PP2A as a negative regulator of immune reactions in Arabidopsis thaliana (hereafter Arabidopsis).² Knock-down pp2a-b’γ mutants show constitutive activation of defense related genes, imbalanced antioxidant metabolism and premature disintegration of chloroplasts upon ageing. Proteomic analysis of soluble leaf extracts further revealed that the constitutive defense response in pp2a-b’γ leaves associates with increased levels of Cu/Zn superoxide dismutase, aconitase as well as components of the methionine-salvage pathway, suggesting PP2A-B’γ modulates methionine metabolism in leaves.     

PROTON GRADIENT REGULATION5 Is Essential for Proper Acclimation of Arabidopsis Photosystem I to Naturally and Artificially Fluctuating Light Conditions

In nature, plants are challenged by constantly changing light conditions. To reveal the molecular mechanisms behind acclimation to sometimes drastic and frequent changes in light intensity, we grew Arabidopsis thaliana under fluctuating light conditions, in which the low light periods were repeatedly interrupted with high light peaks. Such conditions had only marginal effect on photosystem II but induced damage to photosystem I (PSI), the damage being most severe during the early developmental stages. We showed that PROTON GRADIENT REGULATION5 (PGR5)-dependent regulation of electron transfer and proton motive force is crucial for protection of PSI against photodamage, which occurred particularly during the high light phases of fluctuating light cycles. Contrary to PGR5, the NAD(P)H dehydrogenase complex, which mediates cyclic electron flow around PSI, did not contribute to acclimation of the photosynthetic apparatus, particularly PSI, to rapidly changing light intensities. Likewise, the Arabidopsis pgr5 mutant exhibited a significantly higher mortality rate compared with the wild type under outdoor field conditions. This shows not only that regulation of PSI under natural growth conditions is crucial but also the importance of PGR5 in PSI protection.     

Complex phenotypic profiles leading to ozone sensitivity in Arabidopsis thaliana mutants

Genetically tractable model plants offer the possibility of defining the plant O₃ response at the molecular level. To this end, we have isolated a collection of ozone (O₃)-sensitive mutants of Arabidopsis thaliana . Mutant phenotypes and genetics were characterized. Additionally, parameters associated with O₃ sensitivity were analysed, including stomatal conductance, sensitivity to and accumulation of reactive oxygen species, antioxidants, stress gene-expression and the accumulation of stress hormones. Each mutant has a unique phenotypic profile, with O₃ sensitivity caused by a unique set of alterations in these systems. O₃ sensitivity in these mutants is not caused by gross deficiencies in the antioxidant pathways tested here. The rcd3 mutant exhibits misregulated stomata. All mutants exhibited changes in stress hormones consistent with the known hormonal roles in defence and cell death regulation. One mutant, dubbed re-8 , is an allele of the classic leaf development mutant reticulata and exhibits phenotypes dependent on light conditions. This study shows that O₃ sensitivity can be determined by deficiencies in multiple interacting plant systems and provides genetic evidence linking these systems.     

Reactive oxygen species signaling and stomatal movement in plant responses to drought stress and pathogen attack

Stomata, the pores formed by a pair of guard cells, are the main gateways for water transpiration and photosynthetic CO_2 exchange, as well as pathogen invasion in land plants. Guard cell movement is regulated by a combination of environmental factors, including water status, light, CO_2 levels and pathogen attack, as well as endogenous signals, such as abscisic acid and apoplastic reactive oxygen species(ROS). Under abiotic and bioticstress conditions, extracellular ROS are mainly produced by plasma membrane-localized NADPH oxidases, whereas intracellular ROS are produced in multiple organelles. These ROS form a sophisticated cellular signaling network, with the accumulation of apoplastic ROS an early hallmark of stomatal movement. Here, we review recent progress in understanding the molecular mechanisms of the ROS signaling network,primarily during drought stress and pathogen attack. We summarize the roles of apoplastic ROS in regulating stomatal movement, ABA and CO_2 signaling, and immunity responses.Finally, we discuss ROS accumulation and communication between organelles and cells. This information provides a conceptual framework for understanding how ROS signaling is integrated with various signaling pathways during plant responses to abiotic and biotic stress stimuli.