Identification and expression analysis of twelve members of the nucleobase-ascorbate transporter (NAT) gene family in Arabidopsis thaliana.

By screening genome databases, 12 genes encoding membrane proteins homologous to nucleobase-ascorbate transporters (NATs) were identified in Arabidopsis thaliana. A similar number of genes was found in the rice genome. The plant NAT proteins split into five clades (I-V) based on protein multisequence alignments. This classification nicely correlates with the patterns of organ- and tissue-specific expression during the whole life cycle of A. thaliana. Interestingly, expression of two members of clade III, AtNAT7 and AtNAT8, was found to be up-regulated in undifferentiated tissues such as callus or tumors produced by Agrobacterium tumefaciens. Clade V comprises AtNAT12 possessing a hydrophilic N-terminal extension. Transient expression of green fluorescent protein (GFP) fusions in different systems showed that AtNAT12 along with AtNAT7 and -8 are located in the plasma membrane. Mutations in any of the AtNAT genes do not induce phenotypic alterations. The absence of obvious mutant phenotypes in single but also in double and triple mutants suggests a high degree of functional redundancy between AtNAT genes, but might also point to redundant functions provided by genes or pathways unrelated to the AtNATs.

Do mature shade leaves of tropical tree seedlings acclimate to high sunlight and UV radiation?

Seedlings of neotropical forest trees grown in low light were exposed to 0.5-9 h d-1 direct sunlight, for up to 3 months, to test the capability of mature shade leaves to acclimate to full solar visible and UV radiation. Photosynthetic pigments and the antioxidant, ascorbate, were analysed in leaves of two pioneer and two late-succession species. Seedlings of one or two of these species were used to assess further acclimative responses. Sun-exposure for 0.5 or 1 h d-1 resulted in strongly decreased α-carotene and increased ß-carotene and lutein levels. The pool size of xanthophyll-cycle pigments (sum of viola-, anthera- and zeaxanthin) was increased and their turnover was enhanced. These changes were associated with an increase in the capacity of non-photochemical fluorescence quenching and its 'energy-dependent' component, qE, and with reduced susceptibility to photoinhibition of PSII. Prolonged exposure to full direct sunlight (approximately 4 or 9 h d-1) resulted in a marked decrease of chlorophyll a + b content and increase in chlorophyll a / b ratios and the pool of xanthophyll-cycle pigments (based on chlorophyll), leading to extremely high zeaxanthin levels during high-light periods. Contents of ascorbate and UV-B-absorbing substances were substantially increased. PSI activity exhibited a response to full sunlight that is characteristic of sun leaves. Rates of net photosynthetic CO2 assimilation under saturating light were increased. The data show that mature shade leaves of seedlings of both early- and late-succession tree species can substantially acclimate to full-sunlight conditions by employing similar physiological mechanisms.

Sudden exposure to solar UV-B radiation reduces net CO(2) uptake and photosystem I efficiency in shade-acclimated tropical tree seedlings.

Tree seedlings developing in the understory of the tropical forest have to endure short periods of high-light stress when tree-fall gaps are formed, and direct solar radiation, including substantial UV light, reaches the leaves. In experiments simulating the opening of a tree-fall gap, the response of photosynthesis in leaves of shade-acclimated seedlings (Anacardium excelsum, Virola surinamensis, and Calophyllum longifolium) to exposure to direct sunlight (for 20-50 min) was investigated in Panama (9 degrees N). To assess the effects of solar UV-B radiation (280-320 nm), the sunlight was filtered through plastic films that selectively absorbed UV-B or transmitted the complete spectrum. The results document a strong inhibition of CO(2) assimilation by sun exposure. Light-limited and light-saturated rates of photosynthetic CO(2) uptake by the leaves were affected, which apparently occurred independently of a simultaneous inhibition of potential photosystem (PS) II efficiency. The ambient UV-B light substantially contributed to these effects. The photochemical capacity of PSI, measured as absorbance change at 810 nm in saturating far-red light, was not significantly affected by sun exposure of the seedlings. However, a decrease in the efficiency of P700 photooxidation by far-red light was observed, which was strongly promoted by solar UV-B radiation. The decrease in PSI efficiency may result from enhanced charge recombination in the reaction center, which might represent an incipient inactivation of PSI, but contributes to thermal dissipation of excessive light energy and thereby to photoprotection.