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SummaryIsoform 3 of sucrose synthase (SUS3) is highly expressed in guard cells; however, the precise function of SUS3 in this cell type remains to be elucidated.Here, we characterized transgenic Nicotiana tabacum plants overexpressing SUS3 under the control of the stomatal-specific KST1 promoter, and investigated the changes in guard cell metabolism during the dark to light transition.Guard cell-specific SUS3 overexpression led to increased SUS activity, stomatal aperture, stomatal conductance, transpiration rate, net photosynthetic rate and growth. Although only minor changes were observed in the metabolite profile in whole leaves, an increased fructose level and decreased organic acid levels and sucrose to fructose ratio were observed in guard cells of transgenic lines. Furthermore, guard cell sucrose content was lower during lightinduced stomatal opening. In a complementary approach, we incubated guard cell-enriched epidermal fragments in 13 C-NaHCO 3 and followed the redistribution of label during dark to light transitions; this revealed increased labeling in metabolites of, or associated with, the tricarboxylic acid cycle. The results suggest that sucrose breakdown is a mechanism to provide substrate for the provision of organic acids for respiration, and imply that manipulation of guard cell metabolism may represent an effective strategy for plant growth improvement.

Such a synergistic effect of noncovalent PEGylation of poly(amino methacrylate)/DNA complexes allows a new and versatile approach to tune up transfection efficiency.

Background and aims Rice can be cultivated in highlands, which can expose it to iron deficiency, or under irrigation, which can lead to iron toxicity and lower productivity. This study aimed to investigate the strategies used by rice plants under different divalent and trivalent sources of iron excess. Methods Rice plants from a lowland and upland cultivar were grown in nutrient solution with toxic concentrations of ferrous or ferric iron. A mineral nutrient quantification and anatomical analysis were performed on leaves and roots. Physiological damage was assessed by leaf photochemical parameters and lipid peroxidation. Expression levels of genes related to iron homeostasis were analyzed. Results More pronounced nutritional deficiencies, oxidative stress and physiological damage were observed in plants exposed to toxic levels of ferrous iron. Ferritin expression increased in leaves of both cultivars under ferrous or ferric iron excess. Conclusions We showed that sulfate iron was more toxic to the two rice cultivars even though this iron source was less translocated in the plant. Trivalent iron complexed to citrate is easily translocated through rice plants, but it is less toxic than the divalent iron. Rice plants are able to cope with this iron overload by keeping photosynthetic apparatus working properly.

Wood properties influence the leaf life span (LL) of tree crowns. As lignin is an important component of wood and the water transport system, we investigated its relationship with embolism resistance and the LL of several tree species in a seasonally dry tropical ecosystem. We determined total lignin and the monomer contents of guaiacyl (G) and syringyl (S) and related them to wood traits and xylem vulnerability to embolism (Ψ ) for the most common species of the Brazilian semiarid, locally known as Caatinga. Leaf life span was negatively related to Ψ and positively related to S : G, which was negatively related to Ψ . This means that greater S : G increases LL by reducing Ψ . Lignin content was not correlated with any variable. We found two apparently unrelated axes of drought resistance. One axis, associated with lignin monomeric composition, increases LL in the dry season as a result of lower xylem embolism vulnerability. The other, associated with wood density and stem water content, helps leafless trees to withstand drought and allows them to resprout at the end of the dry season. The monomeric composition of lignin (S : G) is therefore an important functional wood attribute affecting several key functional aspects of tropical tree species in a semiarid climate.

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Abbreviations -A, net photosynthetic assimilation rate; APX, ascorbate peroxidase; CAT, catalase; Chl a, chlorophyll a; Chl b, chlorophyll b; Chl a/Chl b, ratio between Chl a and Chl b; E, transpiration rate; ETR, electron transport rate; ETR/A, electron transport to oxygen molecules; F0, initial fluorescence; Fv/fm, potential quantum yield of photosystem II; g s , stomatal conductance; H 2 O 2 , hydrogen peroxide; HS, hydrogen sulphide; LDM, leaf dry matter; MDA, malondialdehyde; MWHC, maximum water holding capacity; NPQ, non-photochemical quenching coefficient; POX, peroxidase activity; PP, potassium phosphite; RDW, root dry weight; REL, rate of electrolyte leakage; RWC, relative water content; SDM, shoot dry matter; SOD, superoxide dismutase; TDM, total dry matter; WD, water deficit; WUEg, intrinsic water use efficiency; WW, well-watered; ZS, zinc sulphate; Ψ s , osmotic potential; Ψ w , water potential; Φ PSII , effective quantum yield of photosystem II.