Marjorie Reyes-Díaz

Molecular and physiological strategies to increase aluminum resistance in plants

Aluminum (Al) toxicity is a primary limitation to plant growth on acid soils. Root meristems are the first site for toxic Al accumulation, and therefore inhibition of root elongation is the most evident physiological manifestation of Al toxicity. Plants may resist Al toxicity by avoidance (Al exclusion) and/or tolerance mechanisms (detoxification of Al inside the cells). The Al exclusion involves the exudation of organic acid anions from the root apices, whereas tolerance mechanisms comprise internal Al detoxification by organic acid anions and enhanced scavenging of free oxygen radicals. One of the most important advances in understanding the molecular events associated with the Al exclusion mechanism was the identification of the ALMT1 gene (Al-activated malate transporter) in Triticum aestivum root cells, which codes for a plasma membrane anion channel that allows efflux of organic acid anions, such as malate, citrate or oxalate. On the other hand, the scavenging of free radicals is dependent on the expression of genes involved in antioxidant defenses, such as peroxidases (e.g. in Arabidopsisthaliana and Nicotiana tabacum), catalases (e.g. in Capsicum annuum), and the gene WMnSOD1 from T. aestivum. However, other recent findings show that reactive oxygen species (ROS) induced stress may be due to acidic (low pH) conditions rather than to Al stress. In this review, we summarize recent findings regarding molecular and physiological mechanisms of Al toxicity and resistance in higher plants. Advances have been made in understanding some of the underlying strategies that plants use to cope with Al toxicity. Furthermore, we discuss the physiological and molecular responses to Al toxicity, including genes involved in Al resistance that have been identified and characterized in several plant species. The better understanding of these strategies and mechanisms is essential for improving plant performance in acidic, Al-toxic soils.

Excess manganese differentially inhibits photosystem I versus II in Arabidopsis thaliana

The effects of exposure to increasing manganese concentrations (50–1500 µM) from the start of the experiment on the functional performance of photosystem II (PSII) and photosystem I (PSI) and photosynthetic apparatus composition of Arabidopsis thaliana were compared. In agreement with earlier studies, excess Mn caused minimal changes in the PSII photochemical efficiency measured as Fv/Fm, although the characteristic peak temperature of the S2/3QB – charge recombinations was shifted to lower temperatures at the highest Mn concentration. SDS-PAGE and immunoblot analyses also did not exhibit any significant change in the relative abundance of PSII-associated polypeptides: PSII reaction centre protein D1, Lhcb1 (major light-harvesting protein of LHCII complex), and PsbO (OEC33, a 33kDa protein of the oxygen-evolving complex). In addition, the abundance of Rubisco also did not change with Mn treatments. However, plants grown under excess Mn exhibited increased susceptibility to PSII photoinhibition. In contrast, in vivo measurements of the redox transients of PSI reaction centre (P700) showed a considerable gradual decrease in the extent of P700 photooxidation (P700+) under increased Mn concentrations compared to control. This was accompanied by a slower rate of P700+ re-reduction indicating a downregulation of the PSI-dependent cyclic electron flow. The abundance of PSI reaction centre polypeptides (PsaA and PsaB) in plants under the highest Mn concentration was also significantly lower compared to the control. The results demonstrate for the first time that PSI is the major target of Mn toxicity within the photosynthetic apparatus of Arabidopsis plants. The possible involvement mechanisms of Mn toxicity targeting specifically PSI are discussed.

ANTIOXIDANT COMPOUNDS IN SKIN AND PULP OF FRUITS CHANGE AMONG GENOTYPES AND MATURITY STAGES IN HIGHBUSH BLUEBERRY (Vaccinium corymbosum L.) GROWN IN SOUTHERN CHILE

We evaluated the genotype and maturity effects on antioxidant activity and phenolic compounds of whole, skin and pulp fruits from three highbush blueberry cultivare (cv. Brigitta, cv. Bluegold and cv. Legacy) grown in southern Chile. Total antioxidant activity (TAA) in ripe fruits varied among the cultivare in the order Legacy > Brigitta > Bluegold. We found that TAA in unripe green and fully ripe fruits was high and similar between them, whereas the lowest levels were found in intermediate ripe fruits. The same trend was observed for fruit total phenolic content. This could be attributed to the higher concentrations of phenolic acids (mainly chlorogenic acid) and flavonols (mainly rutin) at immature fruit stages; whereas the high TAA in mature fruits could be explained by the elevated amounts of anthocyanin. All antioxidant compounds were mostly located in the skin. High amounts of delphinidin aglycone were found. HPLC-DAD/MS revealed that the main contents of skin anthocyanins are petunidin-3-glucoside and petunidin-3-arabinnoside followed by malvidin-3-galactoside. It is noticeable that highbush blueberry fruits grown in southern Chile have exceptionally higher antioxidant activity and anthocyanins contents compared with those cultivated in the northern hemisphere.

Ecotypic Differentiation in Morphology and Cold Resistance in Populations of Colobanthus quitensis (Caryophyllaceae) from the Andes of Central Chile and the Maritime Antarctic

Abstract Colobanthus quitensis is one of only two vascular plant species that occur in the Antarctic islands. We evaluated morphological and physiological traits and survival after freezing in individuals of C. quitensis collected in two populations: one from the Andes of central Chile and the other from the maritime Antarctic. We addressed whether these populations are ecotypes in terms of shoot morphology and physiology, and cold resistance. We also evaluated genetic differentiation at the internal transcribed spacer (ITS) regions. Under controlled growing conditions, Andean plants had longer and narrower leaves and longer peduncles. However, shoot biomass was similar in both populations. There was a 1.17% sequence divergence of ITS regions between populations. Survival to freezing at temperatures from 0° to −16°C was not different in nonacclimated plants. After cold-acclimation at 4°C, freezing tolerance was greater in Antarctic plants. Both Andean and Antarctic plants increased soluble sugars with increasing acclimation time, but it did not differ between populations. Shoot water content decreased with acclimation time in both populations, but Antarctic plants maintained slightly higher water contents after cold exposure. Shoot growth did not differ during acclimation. The studied populations of C. quitensis represent different morphological and cold resistant ecotypes despite their genetic similarity. However, the mechanistic basis of the higher survival to freezing of Antarctic plants was not elucidated.

Carbohydrate storage, survival, and growth of two evergreen Nothofagus species in two contrasting light environments

A number of traits have been attributed important roles in tolerance of shade by plants. Some explanations emphasize traits enhancing net carbon gain; others emphasize energy conservation traits such as storage of non-structural carbohydrates (NSC). To date, cross-species studies have provided mixed support for the role of NSC storage in low-light survival. We examined NSC status, survival, biomass, and growth of large seedlings of two evergreen species of differing shade tolerance (Nothofagus nitida and N. dombeyi) grown in deep shade and 50% light for two growing seasons. We expected to find higher NSC concentration in the more shade-tolerant N. nitida and since allocation to storage involves sacrificing growth, higher growth rate in the shade-intolerant N. dombeyi. NSC concentration of both species was >twofold higher in 50% light than in deep shade, and in roots and stems did not differ significantly between species in either environment. NSC contents per plant were also similar between dead and living plants in deep shade. N. dombeyi outgrew N. nitida in 50% light, while this pattern was reversed in deep shade. Survival in deep shade was not correlated with NSC concentration. Leaf mass fraction was similar between species in 50% light, but lower in N. dombeyi in deep shade. Results provide little evidence of a link between carbohydrate storage and low-light survival in Nothofagus species, and support the view that understorey survival is primarily a function of net carbon gain. Patterns of variation in NSC concentration of the temperate species we studied are likely dominated by more important influences than adaptation to shade, such as limitation of growth or adaptation to cold stress.

Calcium sulfate ameliorates the effect of aluminum toxicity differentially in genotypes of highbush blueberry (Vaccinium corymbosum L.)

Abstract The effect of gypsum (CaSO 4 ) amendment in the reduction of Al phytotoxicity of blueberry cultivars differing in Al resistance (Legacy and Brigitta, Al-resistant and Bluegold, Al-sensitive) was studied in a Hoagland’s nutrient solution under acidic conditions for 2 weeks. Treatments were: Control (Hoagland solution), 2.5 mM CaSO 4 , 5 mM CaSO 4 , 100 µM Al (AlCl 3 ), 100 µM Al + 2.5 mM CaSO 4 , 100 µM Al + 5 mM CaSO 4 . Physiological, biochemical and chemical features of leaves and roots were determined to establish the amendment efficiency in the reduction of Al toxicity in these cultivars. Results showed that under Al toxicity the three investi-gated cultivars accumulated high Al concentrations in leaves and roots. These con-centrations decreased with CaSO 4 application. Statistically significant interactions among Al in leaves but not in roots ( p =0.719) and cultivars ( p <0.001), were found. The lowest Ca concentration was found in the most Al-sensitive cultivar (Bluegold) and the highest in the more Al-resistant cultivars (Legacy and Brigitta). Among the underlying processes affected by Al stress in these blueberry cultivars the most evident changes were exhibited by the Al-sensitive cultivar Bluegold, where the photosynthetic performance decreased showing a slight recovery in presence of gyp -sum amendment at the end of experiment. Instead, the more Al-resistant cultivar (Legacy) did not change its photosynthetic parameters in presence of the gypsum amendments during the treatment, whereas in Brigitta, only a slight recovery at the end of treatment was evidenced by the gypsum application. Thus, in relation to these parameters the gypsum amendment was efficient in complete recovery from the toxic Al effect in the Al-resistant cultivar Brigitta and a slight recovery of the toxic Al effect in the Al-sensitive cultivar Bluegold. Nonetheless, this amendment is a good alternative to ameliorate Al toxicity in Al-sensitive cultivars and additionally provides a good source of Ca and S.

Seasonal changes in the photosynthetic performance of two evergreen Nothofagus species in south central Chile

Las siempreverdes, Nothofagus dombeyi y Nothofagus nitida, representantes importantes de los bosques lluviosos templados de Chile, raramente crecen juntos en forma natural. Nothofagus nitida es mas sensible al exceso de luz y deficit de agua que N. dombeyi. Se postula que diferentes propiedades del aparato fotosintetico bajo condiciones de jardin comun de estas especies podrian explicar sus contrastantes preferencias de habitat. Se estudian algunas de estas propiedades en estas dos especies creciendo en un jardin comun en el centro sur de Chile. La eficiencia fotoquimica maxima (Fv/Fm) de ambas especies permanece en el rango de los valores normales (» 0,8) con excepcion de un decrecimiento al mediodia y en verano en N. dombeyi, que sugiere una reduccion reversible de la eficiencia fotoquimica maxima del fotosistema II (PSII). Durante el verano, la eficiencia fotoquimica efectiva del PSII (FPSII), el apagamiento fotoquimico (qP), la fotosintesis (Amax), conductancia estomatica (gs) y las tasas de transpiracion (E) en N. dombeyi fueron mayores que en N. nitida. Los mayores incrementos en los contenidos de pigmentos fotoprotectores (zeaxantina + anteraxantina y luteina) entre prealba y mediodia se obtuvieron en verano en N. dombeyi. En N. nitida se presento en invierno una retencion nocturna de zeaxantina sin decrecimiento de Fv/Fm. Los resultados sugieren que bajo condiciones de mayor intensidad luminica, mayor temperatura y sequia, N. dombeyi mostro un mejor desempeno fotosintetico que N. nitida. Los datos presentados son consistentes con la capacidad pionera de N. dombeyi y las propiedades de semitolerancia a la sombra y distribucion mas restringida de N. nitida. Estas caracteristicas fotosinteticas, junto con las diferencias de la resistencia al congelamiento, sequia y anegamiento pueden resultar de su separacion de habitat

Manganese toxicity and UV-B radiation differentially influence the physiology and biochemistry of highbush blueberry (Vaccinium corymbosum) cultivars

Manganese (Mn2+) toxicity or UV-B radiation and their individual effects on plants have been documented previously. However, no study about the combined effect of these stresses is available. We evaluated the individual and combined effects of excess Mn2+ and UV-B radiation on physiological and biochemical parameters in two highbush blueberry (Vaccinium corymbosum L.) cultivars differing in resistance to Mn toxicity (Brigitta (resistant) and Bluegold (sensitive)). Plants grown in Hoagland nutrient solution were subjected to the following treatments: 2µM MnCl2 (control), 500µM MnCl2 (toxic Mn2+), UV-B radiation (a daily dose of 94.4kJm-2), and the combined treatment (toxic Mn2++UV-B) for 30 days. In both cultivars, the Mn2++UV-B treatment caused a more negative effect on net photosynthesis (Pn), stomatal conductance (gs), the photochemical parameters of PSII and the chl a/b ratio than the treatments with toxic Mn2+ or UV-B alone. However, Brigitta showed also a better acclimation response in Pn and gs than Bluegold at the end of the experiment. The Mn2++UV-B treatment inhibited growth, enhanced radical scavenging activity and superoxide dismutase activity, and increased the concentration of total UV-absorbing compounds, phenols and anthocyanins, mainly in Bluegold. In conclusion, Mn-resistant Brigitta showed a better acclimation response and greater resistance to the combined stress of Mn2+ toxicity and UV-B exposure than the Mn-sensitive Bluegold. An increased concentration of photoprotective compounds and enhanced resistance to oxidative stress in Brigitta could underpin increased resistance to the combined stress.

Tree size and light availability increase photochemical instead of non-photochemical capacities of Nothofagus nitida trees growing in an evergreen temperate rain forest

Nothofagus nitida (Phil.) Krasser (Nothofagaceae) regenerates under the canopy in microsites protected from high light. Nonetheless, it is common to find older saplings in clear areas and adults as emergent trees of the Chilean evergreen forest. We hypothesized that this shade to sun transition in N. nitida is supported by an increase in photochemical and non-photochemical energy dissipation capacities of both photosystems in parallel with the increase in plant size and light availability. To dissect the relative contribution of light environment and plant developmental stage to these physiological responses, the photosynthetic performance of both photosystems was studied from the morpho-anatomical to the biochemical level in current-year leaves of N. nitida plants of different heights (ranging from 0.1 to 7 m) growing under contrasting light environments (integrated quantum flux (IQF) 5-40 mol m(-2). Tree height (TH) and light environment (IQF) independently increased the saturated electron transport rates of both photosystems, as well as leaf and palisade thickness, but non-photochemical energy flux, photoinhibition susceptibility, state transition capacity, and the contents of D1 and PsbS proteins were not affected by IQF and TH. Spongy mesophyll thickness and palisade cell diameter decreased with IQF and TH. A(max), light compensation and saturation points, Rubisco and nitrogen content (area basis) only increased with light environment (IQF), whereas dark respiration (R(d)) decreased slightly and relative chlorophyll content was higher in taller trees. Overall, the independent effects of more illuminated environment and tree height mainly increased the photochemical instead of the non-photochemical energy flux. Regardless of the photochemical increase with TH, carbon assimilation only significantly improved with higher IQF. Therefore it seems that mainly acclimation to the light environment supports the phenotypic transition of N. nitida from shade to sun.

Photosynthetic impairment caused by manganese toxicity and associated antioxidative responses in perennial ryegrass

Abstract. Manganese (Mn) toxicity can induce oxidative stress and impair photosynthesis in plants. The activity of antioxidant enzymes such as superoxide dismutase (SOD) is increased in Lolium perenne (perennial ryegrass) in response to Mn toxicity (mainly in tolerant cultivars), but it remains unclear whether non-enzymatic antioxidant compounds may have a role in Mn tolerance. Seedlings of perennial ryegrass cv. Nui (Mn-sensitive) and cv. Kingston (Mn-tolerant) were grown in a greenhouse in nutrient solution at increasing Mn doses over 21 days. Even though both cultivars showed similar Mn uptake, dry weight decreases and lipid peroxidation caused by excess Mn were higher in cv. Nui than in Mn-tolerant Kingston. Maximum quantum yield of photosystem II (PSII) (Fv/Fm) declined only in cv. Nui at the highest Mn dose. Effective quantum yield (Ф PSII), electron transport rate, CO2 assimilation, and total chlorophyll concentration in leaves decreased under excess Mn, particularly in the sensitive cultivar. Interestingly, chlorophyll a/b ratio increased (indicating relatively lower concentration of light-harvesting chlorophyll proteins as an adaptive defence mechanism) with an increase in Mn supply only in cv. Kingston, which partially explained its greater Mn tolerance compared with Nui. Concentration of carotenoids was not directly associated with non-photochemical quenching values, suggesting that ryegrass did not dissipate an excess of absorbed energy under Mn toxicity by this mechanism. At increasing excess Mn, both enzymatic (SOD activity) and non-enzymatic antioxidant responses (radical scavenging ability and phenolic concentration) were enhanced, mainly in Kingston. The enhanced antioxidant response in this cultivar suggests the hypothesis of increased capacity to control Mn-triggered oxidative stress as reflected in the reduced lipid peroxidation.