Edmundo L. Ploschuk

Phytochrome B enhances photosynthesis at the expense of water-use efficiency in Arabidopsis.

In open places, plants are exposed to higher fluence rates of photosynthetically active radiation and to higher red to far-red ratios than under the shade of neighbor plants. High fluence rates are known to increase stomata density. Here we show that high, compared to low, red to far-red ratios also increase stomata density in Arabidopsis (Arabidopsis thaliana). High red to far-red ratios increase the proportion of phytochrome B (phyB) in its active form and the phyB mutant exhibited a constitutively low stomata density. phyB increased the stomata index (the ratio between stomata and epidermal cells number) and the level of anphistomy (by increasing stomata density more intensively in the adaxial than in the abaxial face). phyB promoted the expression of FAMA and TOO MANY MOUTHS genes involved in the regulation of stomata development in young leaves. Increased stomata density resulted in increased transpiration per unit leaf area. However, phyB promoted photosynthesis rates only at high fluence rates of photosynthetically active radiation. In accordance to these observations, phyB reduced long-term water-use efficiency estimated by the analysis of isotopic discrimination against 13CO2. We propose a model where active phyB promotes stomata differentiation in open places, allowing plants to take advantage of the higher irradiances at the expense of a reduction of water-use efficiency, which is compensated by a reduced leaf area.

Increased Phytochrome B Alleviates Density Effects on Tuber Yield of Field Potato Crops

The possibility that reduced photomorphogenic responses could increase field crop yield has been suggested often, but experimental support is still lacking. Here, we report that ectopic expression of the Arabidopsis PHYB (phytochrome B) gene, a photoreceptor involved in detecting red to far-red light ratio associated with plant density, can increase tuber yield in field-grown transgenic potato (Solanum tuberosum) crops. Surprisingly, this effect was larger at very high densities, despite the intense reduction in the red to far-red light ratios and the concomitant narrowed differences in active phytochrome B levels between wild type and transgenics at these densities. Increased PHYB expression not only altered the ability of plants to respond to light signals, but they also modified the light environment itself. This combination resulted in larger effects of enhanced PHYB expression on tuber number and crop photosynthesis at high planting densities. The PHYB transgenics showed higher maximum photosynthesis in leaves of all strata of the canopy, and this effect was largely due to increased leaf stomatal conductance. We propose that enhanced PHYB expression could be used in breeding programs to shift optimum planting densities to higher levels.

Capitulum position in sunflower affects grain temperature and duration of grain filling

Effects of sunflower capitulum position on the thermal regime of grains and receptacle, and on the duration and rate of grain filling, were explored using a sunflower cultivar that normally hides its head within the upper leaves of the canopy shortly after anthesis. The thermal regime was manipulated by restraining the head in a vertical position (V heads), and by using black (Vw heads) or reflective (Vc heads) envelopes around vertically restrained heads. Capitulum temperature dynamics and grain growth were followed in heads in the natural position (H heads) and in the vertically restrained ones. Once the H heads had become hidden in the canopy, large differences in daytime temperature regimes developed (38°C maximum temperature in V grains, 10°C higher than H grains and air temperature at the same time on sunny days). Vw heads were exposed to higher temperatures than V heads, and Vc heads were slightly warmer than H heads. Duration of grain filling in V heads was reduced by 2 to 6 days for grains at the periphery of the capitulum (P < 0.05). Individual grain weight was reduced up to 21% (P < 0.05), and final grain yield per head by 10%. Most of this effect was attributable to shortening of the grain-filling period, as the rate of grain growth was not affected. Cultivars with horizontal heads would have an advantage in yield potential with respect to cultivars that maintain the capitulum in a vertical position during grain filling.

Phototropins But Not Cryptochromes Mediate the Blue Light-Specific Promotion of Stomatal Conductance, While Both Enhance Photosynthesis and Transpiration under Full Sunlight

Leaf epidermal peels of Arabidopsis (Arabidopsis thaliana) mutants lacking either phototropins 1 and 2 (phot1 and phot2) or cryptochromes 1 and 2 (cry1 and cry2) exposed to a background of red light show severely impaired stomatal opening responses to blue light. Since phot and cry are UV-A/blue light photoreceptors, they may be involved in the perception of the blue light-specific signal that induces the aperture of the stomatal pores. In leaf epidermal peels, the blue light-specific effect saturates at low irradiances; therefore, it is considered to operate mainly under the low irradiance of dawn, dusk, or deep canopies. Conversely, we show that both phot1 phot2 and cry1 cry2 have reduced stomatal conductance, transpiration, and photosynthesis, particularly under the high irradiance of full sunlight at midday. These mutants show compromised responses of stomatal conductance to irradiance. However, the effects of phot and cry on photosynthesis were largely nonstomatic. While the stomatal conductance phenotype of phot1 phot2 was blue light specific, cry1 cry2 showed reduced stomatal conductance not only in response to blue light, but also in response to red light. The levels of abscisic acid were elevated in cry1 cry2. We conclude that considering their effects at high irradiances cry and phot are critical for the control of transpiration and photosynthesis rates in the field. The effects of cry on stomatal conductance are largely indirect and involve the control of abscisic acid levels.

Stem Transcriptome Reveals Mechanisms to Reduce the Energetic Cost of Shade-Avoidance Responses in Tomato

While the most conspicuous response to low red/far-red ratios (R:FR) of shade light perceived by phytochrome is the promotion of stem growth, additional, less obvious effects may be discovered by studying changes in the stem transcriptome. Here, we report rapid and reversible stem transcriptome responses to R:FR in tomato (Solanum lycopersicum). As expected, low R:FR promoted the expression of growth-related genes, including those involved in the metabolism of cell wall carbohydrates and in auxin responses. In addition, genes involved in flavonoid synthesis, isoprenoid metabolism, and photosynthesis (dark reactions) were overrepresented in clusters showing reduced expression in the stem of low R:FR-treated plants. Consistent with these responses, low R:FR decreased the levels of flavonoids (anthocyanin, quercetin, kaempferol) and selected isoprenoid derivatives (chlorophyll, carotenoids) in the stem and severely reduced the photosynthetic capacity of this organ. However, lignin contents were unaffected. Low R:FR reduced the stem levels of jasmonate, which is a known inducer of flavonoid synthesis. The rate of stem respiration was also reduced in low R:FR-treated plants, indicating that by downsizing the stem photosynthetic apparatus and the levels of photoprotective pigments under low R:FR, tomato plants reduce the energetic cost of shade-avoidance responses.

Development of alternative Lesquerella species in Patagonia (Argentina): potential of Lesquerella angustifolia

The genus Lesquerella is a promising source of hydroxy fatty acid (HFA). Within this genus, Lesquerella fendleri is a good candidate for domestication in the arid southwestern United States because it has high seed and oil yields, low seed dormancy, and low fruit dehiscence. Other species of Lesquerella, however, could be developed and expanded for other regions of the world with different climatic and environmental conditions. The objective of this study was to test the yield and oil content of several Lesquerella species in the Chubut River Valley, Patagonia, Argentina. The area is characterized by low temperatures during winter and high temperatures and radiation during summer. Two experiments were conducted for two consecutive years. Seed yield and oil content were determined for seven species of Lesquerella. In the first year, both fall and spring sowings were measured for all tested species. During the second year, Lesquerella angustifolia performance was evaluated together with three different accessions of L. fendleri. Spring sowings reduced seed and oil yield about 80% in L. fendleri and L. gordonii (average of the two species, P<0.05) compared with the fall sowings. These results were presumably caused by the longer daylength period during the summer at these latitudes compared with other sites. Yield was higher in L. angustifolia than other species and three times higher than L. fendleri, which was sown at the same time. The higher yield obtained with L. angustifolia was the result of the combination of both high seed weight and seed number per plant. When L. angustifolia was compared with L. fendleri germplasm originated from arid environments and with two accessions of the same species collected from high elevations, the better performance of the former was confirmed. These results clearly demonstrate that L. angustifolia performs better than L. fendleri in the irrigated valleys of Patagonia, and also that L. fendleri yields would be low under environments with very cold winters and high latitudes. Thus, fall plantings of L. angustifolia could be considered as an alternative source of HFA at this site and other similar high-latitude environments.

Maintenance respiration coefficient for sunflower grains is less than that for the entire capitulum

Abstract Differences between grain and entire head maintenance respiration ( R m ), effects of capitulum position on the partitioning between respiration related to growth ( R g ) and R m , and ontogenic changes on growth ( g ) and maintenance ( m ) coefficients during grain filling were examined in sunflower. The head of cultivar used is normally hidden within the upper leaves of the canopy shortly after anthesis, and heads with the natural position (H) were compared with heads restrained in vertical position (V), simulating a cultivar that maintains the head in a vertical position during grain filling. These treatments generated a higher thermal regime in the V heads, causing a reduction of 4 days in grain-filling duration and 10% in grain yield respect to H heads. The dynamics of grain and receptacle total respiration ( R t ), R g and R m were followed during the whole of the grain-filling period. R m was estimated (1) as R t − R g , (2) by extrapolating the relationship between respiration and grain relative growth rate to zero, and (3) by measuring respiration of detached seeds until rates stabilized. The relative contribution of R m to R t was not affected by the position of the head, and was estimated as 40% and 4% of R t for the entire head and grains, respectively. Estimates of m differed between methods. Nevertheless, m for grains (between 0.001 and 0.013 mmol CO 2 g −1 h −1 ) was less than for the entire head (0.016 mmol CO 2 g −1 h −1 ), consistent with the notion that much of the proteins in storage organs are inactive and their maintenance costs are probably close to zero. g and m changed markedly during grain filling, but were not affected by changes generated by treatment in the thermal regime of the entire head. The yield of respiratory processes ( Y , 0.38 g head biomass/g glucose) was not modified by changes in thermal regime induced by capitulum position. Thus, the principal effect of the latter factor on yield, mediated by thermal regime, is due to the reduction in grain-filling duration.

Differential growth of Spartina densiflora populations under saline flooding is related to adventitious root formation and innate root ion regulation

Global change anticipates scenarios of sea level rise that would provoke long lasting floods, especially in lowland areas of salt marshes. Our aim was to evaluate the morpho-physiological adjustment ability to deal with continuous saline flooding of Spartina densiflora Brogn. plants from lowlands and uplands along a subtle topographical gradient (0.2m differential altitude). Plants from both origins were subjected to continuous saline flooding (300mM NaCl) for 35 days. Responses associated to adventitious rooting, aerenchyma formation, concentration of Na+, K+ and Cl- in roots and shoots tissues, tillering and growth were assessed. Root responses differentiated populations given that lowland plants showed higher ability for adventitious root formation and innate superior root ion regulation than upland plants. High constitutive K+ concentration plus high Na+ exclusion in root tissues led to significant low values of Na+:K+ ratios in lowland plants. Better root functioning was, in turn, related with more consistent shoot performance as lowland plants maintained plant tiller number and shoot relative growth rate unaltered while upland plants decreased both parameters by 35 and 18%, respectively, when in saline flooding. The superior performance of lowland plants indicates that locally adapted populations can be promoted in salt marsh habitats with subtle differences at topographic level.

Heterologous Expression of AtBBX21 Enhances the Rate of Photosynthesis and Alleviates Photoinhibition in Solanum tuberosum

Expression of AtBBX21 in potato causes morphological and physiological changes that improve photosynthetic rates in high-irradiance conditions without negatively affecting water use efficiency. B-box (BBX) proteins are zinc-finger transcription factors containing one or two B-box motifs. BBX proteins act as key factors in the networks regulating growth and development. The relevance of BBX21 to light and abscisic acid signaling in seedling development is well established; however, its importance in adult plant development and agronomic species is poorly understood. Therefore, we studied the effect of heterologous expression of Arabidopsis (Arabidopsis thaliana) BBX21 in potato (Solanum tuberosum) var Spunta. Three independent AtBBX21-expressing lines and the wild-type control were cultivated under sunlight and at controlled temperatures in a greenhouse. By anatomical, physiological, biochemical, and gene expression analysis, we demonstrated that AtBBX21-expressing plants were more robust and produced more tubers than wild-type plants. Interestingly, AtBBX21-expressing plants had higher rates of photosynthesis, with a significant increase in photosynthetic gene expression, and higher stomatal conductance, with increased size of the stomatal opening, without any associated decline in water use efficiency. Furthermore, AtBBX21-expressing potato plants had reduced photoinhibition associated with higher production of anthocyanins and phenolic compounds, and higher expression of genes in the phenylpropanoid biosynthesis pathway. To gain insights into the mechanism of BBX21, we evaluated the molecular, morphological, metabolic, and photosynthetic behavior in adult BBX21-overexpressing Arabidopsis. We conclude that BBX21 overexpression improved morphological and physiological attributes, and photosynthetic rates in nonoptimal, high-irradiance conditions, without associated impairment of water use efficiency. These characteristics of BBX21 may be useful for increasing production of potatoes, and potentially of other crops.

Growth during recovery evidences the waterlogging tolerance of forage grasses

Abstract. Waterlogging is a stress of increasing importance for pastures as a consequence of global climate change. We evaluated the impact of waterlogging on four forage grasses with alleged differential tolerance, emphasising not only responses during the stress but also their reported ability to recover from it. To do this, 42-day plants of Dactylis glomerata, Bromus catharticus, Festuca arundinacea and Phalaris aquatica were subjected to 15-day waterlogging, followed by a subsequent 15-day recovery period. Shoot and root growth (i.e. RGR) during both periods, in addition to net photosynthesis and stomatal conductance rates during waterlogging were assessed. Sensitivity exhibited by D. glomerata and B. catharticus during waterlogging was related to growth arrest of roots – but not of shoots – along with a progressive fall in stomatal conductance and net photosynthesis. The injury during waterlogging preceded a negligible growth of shoots and roots, only evident during recovery in both species. By contrast, P. aquatica exhibited unaltered root RGR and promoted shoot RGR with no impact on leaf gas exchange during waterlogging; whereas F. arundinacea showed intermediate tolerance as root RGR was reduced during waterlogging, with stomatal conductance, net photosynthesis and shoot RGR remaining unaffected. These latter two species fully regained shoot and root RGR during recovery. So, P. aquatica and F. arundinacea seem more suitable for prone-to-flood lowlands, whereas to be conclusive about waterlogging tolerance, it is necessary to examine plant recovery as shown in D. glomerata and B. catharticus.