Carlos L. Ballaré

Rapid synthesis of auxin via a new tryptophan-dependent pathway is required for shade avoidance in plants

Plants grown at high densities perceive a decrease in the red to far-red (R:FR) ratio of incoming light, resulting from absorption of red light by canopy leaves and reflection of far-red light from neighboring plants. These changes in light quality trigger a series of responses known collectively as the shade avoidance syndrome. During shade avoidance, stems elongate at the expense of leaf and storage organ expansion, branching is inhibited, and flowering is accelerated. We identified several loci in Arabidopsis, mutations in which lead to plants defective in multiple shade avoidance responses. Here we describe TAA1, an aminotransferase, and show that TAA1 catalyzes the formation of indole-3-pyruvic acid (IPA) from L-tryptophan (L-Trp), the first step in a previously proposed, but uncharacterized, auxin biosynthetic pathway. This pathway is rapidly deployed to synthesize auxin at the high levels required to initiate the multiple changes in body plan associated with shade avoidance.

Effects of solar ultraviolet radiation on terrestrial ecosystems. Patterns, mechanisms, and interactions with climate change

Ultraviolet radiation (UV) is a minor fraction of the solar spectrum reaching the ground surface. In this assessment we summarize the results of previous work on the effects of the UV-B component (280-315 nm) on terrestrial ecosystems, and draw attention to important knowledge gaps in our understanding of the interactive effects of UV radiation and climate change. We highlight the following points: (i) The effects of UV-B on the growth of terrestrial plants are relatively small and, because the Montreal Protocol has been successful in limiting ozone depletion, the reduction in plant growth caused by increased UV-B radiation in areas affected by ozone decline since 1980 is unlikely to have exceeded 6%. (ii) Solar UV-B radiation has large direct and indirect (plant-mediated) effects on canopy arthropods and microorganisms. Therefore, trophic interactions (herbivory, decomposition) in terrestrial ecosystems appear to be sensitive to variations in UV-B irradiance. (iii) Future variations in UV radiation resulting from changes in climate and land-use may have more important consequences on terrestrial ecosystems than the changes in UV caused by ozone depletion. This is because the resulting changes in UV radiation may affect a greater range of ecosystems, and will not be restricted solely to the UV-B component. (iv) Several ecosystem processes that are not particularly sensitive to UV-B radiation can be strongly affected by UV-A (315-400 nm) radiation. One example is the physical degradation of plant litter. Increased photodegradation (in response to reduced cloudiness or canopy cover) will lead to increased carbon release to the atmosphere via direct and indirect mechanisms.

Dual role of lignin in plant litter decomposition in terrestrial ecosystems

Plant litter decomposition is a critical step in the formation of soil organic matter, the mineralization of organic nutrients, and the carbon balance in terrestrial ecosystems. Biotic decomposition in mesic ecosystems is generally negatively correlated with the concentration of lignin, a group of complex aromatic polymers present in plant cell walls that is recalcitrant to enzymatic degradation and serves as a structural barrier impeding microbial access to labile carbon compounds. Although photochemical mineralization of carbon has recently been shown to be important in semiarid ecosystems, litter chemistry controls on photodegradative losses are not understood. We evaluated the importance of litter chemistry on photodegradation of grass litter and cellulose substrates with varying levels of lignin [cellulose-lignin (CL) substrates] under field conditions. Using wavelength-specific light attenuation filters, we found that light-driven mass loss was promoted by both UV and visible radiation. The spectral dependence of photodegradation correlated with the absorption spectrum of lignin but not of cellulose. Field incubations demonstrated that increasing lignin concentration reduced biotic decomposition, as expected, but linearly increased photodegradation. In addition, lignin content in CL substrates consistently decreased in photodegradative incubations. We conclude that lignin has a dual role affecting litter decomposition, depending on the dominant driver (biotic or abiotic) controlling carbon turnover. Under photodegradative conditions, lignin is preferentially degraded because it acts as an effective light-absorbing compound over a wide range of wavelengths. This mechanistic understanding of the role of lignin in plant litter decomposition will allow for more accurate predictions of carbon dynamics in terrestrial ecosystems.

Light signals perceived by crop and weed plants

Abstract Light signals perceived by specific plant photoreceptors such as phytochromes, cryptochromes and phototropin play a central role controlling the physiology and development of weed and crop plants. Knowledge about these controls has been gained in the last decade thanks to the combination of eco-physiological experiments under conditions of natural radiation with classical photobiological techniques and genetic and biotechnological tools. This progress has important ramifications for our understanding of the physiology of crop growth and development, as well as the mechanisms of crop–weed competition. In this paper we discuss some of the recent advances in the field of environmental photomorphogenesis and highlight their agricultural implications.

Ecological modulation of plant defense via phytochrome control of jasmonate sensitivity

For plants, the tradeoff between resource investment in defense and increased growth to out-compete neighbors creates an allocation dilemma. How plants resolve this dilemma, at the mechanistic level, is unclear. We found that Arabidopsis plants produced an attenuated defense phenotype under conditions of crowding and when exposed to far-red (FR) radiation, a light signal that plants use to detect the proximity of neighbors via the photoreceptor phytochrome. This phenotype was detectable through standard bioassays that measured the growth of Spodoptera frugiperda caterpillars. Two possible explanations for the effect of FR are: (i) a simple by-product of the diversion of resources to competition, and (ii) a specific effect of phytochrome on defense signaling. The first possibility was ruled out by the fact that the auxin-deficient sav3 mutant, which fails to induce growth responses to FR, still responded to FR with an attenuated defense phenotype. In support of the second hypothesis, we found that phytochrome inactivation by FR caused a strong reduction of plant sensitivity to jasmonates, which are key regulators of plant immunity. The effects of FR on jasmonate sensitivity were restricted to certain elements of the pathway. Supporting the idea that the FR effects on jasmonate signaling are functionally significant, we found that FR failed to increase tissue quality in jar1, a mutant impaired in jasmonate response. We conclude that the plant modulates its investment in defense as a function of the perceived risk of competition, and that this modulation is effected by phytochrome via selective desensitization to jasmonates.

Jasmonate-induced defenses: a tale of intelligence, collaborators and rascals

Plants have sophisticated defense systems to protect their tissues against the attack of herbivorous organisms. Many of these defenses are orchestrated by the oxylipin jasmonate. A growing body of evidence indicates that the expression of jasmonate-induced responses is tightly regulated by the ecological context of the plant. Ecological information is provided by molecular signals that indicate the nature of the attacker, the value of the attacked organs, phytochrome status and thereby proximity of competing plants, association with beneficial organisms and history of plant interactions with pathogens and herbivores. This review discusses recent advances in this field and highlights the need to map the activities of informational modulators to specific control points within our emerging model of jasmonate signaling.

Convergent responses to stress. Solar ultraviolet-B radiation and Manduca sexta herbivory elicit overlapping transcriptional responses in field-grown plants of Nicotiana longiflora.

The effects of solar ultraviolet (UV)-B (280–315 nm) on plants have been studied intensively over the last 2 decades in connection with research on the biological impacts of stratospheric ozone depletion. However, the molecular mechanisms that mediate plant responses to solar (ambient) UV-B and their interactions with response mechanisms activated by other stressors remain for the most part unclear. Using a microarray enriched in wound- and insect-responsive sequences, we examined expression responses of 241 genes to ambient UV-B in field-grown plants of Nicotiana longiflora Cav. Approximately 20% of the sequences represented on the array showed differential expression in response to solar UV-B. The expression responses to UV-B had parallels with those elicited by simulated Manduca sexta herbivory. The most obvious similarities were: (a) down-regulation of several photosynthesis-related genes, and (b) up-regulation of genes involved in fatty acid metabolism and oxylipin biosynthesis such as HPL (hydroperoxide lyase), α-DIOX (alpha-dioxygenase), LOX (13-lipoxygenase), and AOS (allene oxide synthase). Genes encoding a WRKY transcription factor, a ferredoxin-dependent glutamate-synthase, and several other insect-responsive genes of unknown function were also similarly regulated by UV-B and insect herbivory treatments. Our results suggest that UV-B and caterpillar herbivory activate common regulatory elements and provide a platform for understanding the mechanisms of UV-B impacts on insect herbivory that have been documented in recent field studies.

Solar Ultraviolet-B Radiation Affects Seedling Emergence, DNA Integrity, Plant Morphology, Growth Rate, and Attractiveness to Herbivore Insects in Datura ferox.

To study functional relationships between the effects of solar ultraviolet-B radiation (UV-B) on different aspects of the physiology of a wild plant, we carried out exclusion experiments in the field with the summer annual Datura ferox L. Solar UV-B incident over Buenos Aires reduced daytime seedling emergence, inhibited stem elongation and leaf expansion, and tended to reduce biomass accumulation during early growth. However, UV-B had no effect on calculated net assimilation rate. Using a monoclonal antibody specific to the cyclobutane-pyrimidine dimer (CPD), we found that plants receiving full sunlight had more CPDs per unit of DNA than plants shielded from solar UV-B, but the positive correlation between UV-B and CPD burden tended to level off at high (near solar) UV-B levels. At our field site, Datura plants were consumed by leaf beetles (Coleoptera), and the proportion of plants attacked by insects declined with the amount of UV-B received during growth. Field experiments showed that plant exposure to solar UV-B reduced the likelihood of leaf beetle attack by one-half. Our results highlight the complexities associated with scaling plant responses to solar UV-B, because they show: (a) a lack of correspondence between UV-B effects on net assimilation rate and whole-plant growth rate, (b) nonlinear UV-B dose-response curves, and (c) UV-B effects of plant attractiveness to natural herbivores.

Light regulation of plant defense.

Precise allocation of limited resources between growth and defense is critical for plant survival. In shade-intolerant species, perception of competition signals by informational photoreceptors activates shade-avoidance responses and reduces the expression of defenses against pathogens and insects. The main mechanism underlying defense suppression is the simultaneous downregulation of jasmonate and salicylic acid signaling by low ratios of red:far-red radiation. Inactivation of phytochrome B by low red:far-red ratios appears to suppress jasmonate responses by altering the balance between DELLA and JASMONATE ZIM DOMAIN (JAZ) proteins in favor of the latter. Solar UVB radiation is a positive modulator of plant defense, signaling through jasmonate-dependent and jasmonate-independent pathways. Light, perceived by phytochrome B and presumably other photoreceptors, helps plants concentrate their defensive arsenals in photosynthetically valuable leaves. The discovery of connections between photoreceptors and defense signaling is revealing novel mechanisms that control key resource allocation decisions in plant canopies.

Environmental effects of ozone depletion and its interactions with climate change: progress report, 2011

The Environmental Effects Assessment Panel (EEAP) is one of three Panels that regularly informs the Parties (countries) to the Montreal Protocol on the effects of ozone depletion and the consequences of climate change interactions with respect to human health, animals, plants, biogeochemistry, air quality, and materials. The Panels provide a detailed assessment report every four years. The most recent 2014 Quadrennial Assessment by the EEAP was published as a special issue of seven papers in 2015 (Photochem. Photobiol. Sci., 2015, 14, 1-184). The next Quadrennial Assessment will be published in 2018/2019. In the interim, the EEAP generally produces an annual update or progress report of the relevant scientific findings. The present progress report for 2015 assesses some of the highlights and new insights with regard to the interactive nature of the effects of UV radiation, atmospheric processes, and climate change.