Lights, location, action: Shade avoidance signalling over spatial scales

Plants growing in dense vegetation stands need to flexibly position their photosynthetic organs to ensure optimal light capture in a competitive environment. They do so through a suite of developmental responses referred to as the shade avoidance syndrome. Belowground, root development is also adjusted in response to aboveground neighbour proximity. Canopies are dynamic and complex environments with heterogenous light cues in the far-red, red, blue and UV spectrum, which can be perceived with photoreceptors by spatially separated plant tissues. Molecular regulation of plant architecture adjustment via PHYTOCHROME-INTERACTING FACTOR (PIF) transcription factors and growth-related hormones such as auxin, gibberellic acid, brassinosteroids and abscisic acid were historically studied without much attention to spatial or tissue-specific context. Recent developments and technologies have, however, sparked strong interest in spatially explicit understanding of shade avoidance regulation. Other environmental factors such as temperature and nutrient availability interact with the molecular shade avoidance regulation network, often depending on the spatial location of the signals, and the responding organs. Here, we aim to review recent advances in how plants respond to heterogenous light cues and integrate these with other environmental signals.

A low-cost open-source imaging platform reveals spatiotemporal insight into leaf elongation and movement

Plant organs move throughout the diurnal cycle, changing leaf and petiole positions to balance light capture, leaf temperature, and water loss under dynamic environmental conditions. Upward movement of the petiole, called hyponasty, is one of several traits of the shade avoidance syndrome (SAS). SAS traits are elicited upon perception of vegetation shade signals such as far-red light (FR) and improve light capture in dense vegetation. Monitoring plant movement at a high temporal resolution allows studying functionality and molecular regulation of hyponasty. However, high temporal resolution imaging solutions are often very expensive, making this unavailable to many researchers. Here, we present a modular and low-cost imaging set-up, based on small Raspberry Pi computers, that can track leaf movements and elongation growth with high temporal resolution. We also developed an open-source, semi-automated image analysis pipeline. Using this setup we followed responses to FR enrichment, light intensity, and their interactions. Tracking both elongation and the angle of the petiole, lamina, and entire leaf in Arabidopsis (Arabidopsis thaliana) revealed insight into R:FR sensitivities of leaf growth and movement dynamics and the interactions of R:FR with background light intensity. The detailed imaging options of this system allowed us to identify spatially separate bending points for petiole and lamina positioning of the leaf.

Mechanodetection of neighbor plants elicits adaptive leaf movements through calcium dynamics

Plants detect their neighbors via various cues, including reflected light and touching of leaf tips, which elicit upward leaf movement (hyponasty). It is currently unknown how touch is sensed and how the signal is transferred from the leaf tip to the petiole base that drives hyponasty. Here, we show that touch-induced hyponasty involves a signal transduction pathway that is distinct from light-mediated hyponasty. We found that mechanostimulation of the leaf tip upon touching causes cytosolic calcium ([Ca2+]cyt induction in leaf tip trichomes that spreads towards the petiole. Both perturbation of the calcium response and the absence of trichomes reduce touch-induced hyponasty. Finally, using plant competition assays, we show that touch-induced hyponasty is adaptive in dense stands of Arabidopsis. We thus establish a novel, adaptive mechanism regulating hyponastic leaf movement in response to mechanostimulation by neighbors in dense vegetation.

Plant signaling: The sugar-coated story of root growth

A new study draws attention to photosynthetically produced sucrose as a major shoot-derived and auxin-dependent regulator of root growth and development in plants.

Architecture and plasticity: optimizing plant performance in dynamic environments

Plasticity in plant architecture drives plant performance through dedicated molecular networks.

Mechanisms of far-red light-mediated dampening of defense against Botrytis cinerea in tomato leaves

Plants detect neighboring competitors through a decrease in the ratio between red and far-red light (R:FR). This decreased R:FR is perceived by phytochrome photoreceptors and triggers shade avoidance responses such as shoot elongation and upward leaf movement (hyponasty). In addition to promoting elongation growth, low R:FR perception enhances plant susceptibility to pathogens: the growth-defense tradeoff. Although increased susceptibility in low R:FR has been studied for over a decade, the associated timing of molecular events is still unknown. Here, we studied the chronology of FR-induced susceptibility events in tomato (Solanum lycopersicum) plants pre-exposed to either white light (WL) or WL supplemented with FR light (WL+FR) prior to inoculation with the necrotrophic fungus Botrytis cinerea (B.c.). We monitored the leaf transcriptional changes over a 30-h time course upon infection and followed up with functional studies to identify mechanisms. We found that FR-induced susceptibility in tomato is linked to a general dampening of B.c.-responsive gene expression, and a delay in both pathogen recognition and jasmonic acid-mediated defense gene expression. In addition, we found that the supplemental FR-induced ethylene emissions affected plant immune responses under the WL+FR condition. This study improves our understanding of the growth-immunity tradeoff, while simultaneously providing leads to improve tomato resistance against pathogens in dense cropping systems.

The role of seasonality in reproduction of multiannual delayed gametophytes of Saccharina latissima

Delayed gametophytes are able to grow vegetatively for prolonged periods of time. As such, they are potentially very valuable for kelp aquaculture given their great promise in opening up novel opportunities for kelp breeding and farming. However, large-scale application would require more in-depth understanding of how to control reproduction in delayed gametophytes. For newly formed gametophytes, many environmental factors for reproduction have been identified, with key drivers being light intensity, temperature, and the initial gametophyte density. However, the question of whether delayed gametophytes react similarly to these life cycle controls remains open for exploration. In this study, we performed a full factorial experiment on the influences of light intensity, temperature, and density on the reproduction of multiannual delayed gametophytes of Saccharina latissima, during which the number of sporophytes formed was counted. We demonstrate that delayed gametophytes of S. latissima can reliably reproduce sexually after more than a year of vegetative growth, depending on the effects between light intensity and temperature. Under higher light intensities (≥29 µmol photons · m-2 · s-1 ), optimal reproduction was observed at lower temperatures (10.2°C), while at lower light intensities (≤15 µmol photons · m-2 · s-1 ), optimal reproduction was observed at higher temperatures (≥12.6°C). Given the seasonal lag between solar radiation and sea surface temperature in natural systems, these conditions resemble those found during spring (i.e., increasing light intensity with low temperatures) and autumn (i.e., decreasing light intensity with higher temperatures). Seasonality can be used as an aquaculture tool to better control the reproduction of delayed gametophytes.

Light signalling shapes plant-plant interactions in dense canopies

Plants growing at high densities interact via a multitude of pathways. Here, we provide an overview of mechanisms and functional consequences of plant architectural responses initiated by light cues that occur in dense vegetation. We will review the current state of knowledge about shade avoidance, as well as its possible applications. On an individual level, plants perceive neighbour-associated changes in light quality and quantity mainly with phytochromes for red and far-red light and cryptochromes and phototropins for blue light. Downstream of these photoreceptors, elaborate signalling and integration takes place with the PHYTOCHROME INTERACTING FACTORS, several hormones and other regulators. This signalling leads to the shade avoidance responses, consisting of hyponasty, stem and petiole elongation, apical dominance and life cycle adjustments. Architectural changes of the individual plant have consequences for the plant community, affecting canopy structure, species composition and population fitness. In this context, we highlight the ecological, evolutionary and agricultural importance of shade avoidance.

Reducing shade avoidance can improve Arabidopsis canopy performance against competitors

Plants that grow in high density communities activate shade avoidance responses to consolidate light capture by individuals. Although this is an evolutionary successful strategy, it may not enhance performance of the community as a whole. Resources are invested in shade responses at the expense of other organs and light penetration through the canopy is increased, allowing invading competitors to grow better. Here we investigate if suppression of shade avoidance responses would enhance group performance of a monoculture community that is invaded by a competitor. Using different Arabidopsis genotypes, we show that suppression of shade-induced upward leaf movement in the pif7 mutant increases the pif7 communal performance against invaders as compared to a wild-type canopy. The invaders were more severely suppressed and the community grew larger as compared to wild type. Using computational modelling, we show that leaf angle variations indeed strongly affect light penetration and growth of competitors that invade the canopy. Our data thus show that modifying specific shade avoidance aspects can improve plant community performance. These insights may help to suppress weeds in crop stands.

Regulation of Lateral Root Development by Shoot-Sensed Far-Red Light via HY5 Is Nitrate-Dependent and Involves the NRT2.1 Nitrate Transporter

Plants are very effective in responding to environmental changes during competition for light and nutrients. Low Red:Far-Red (low R:FR)-mediated neighbor detection allows plants to compete successfully with other plants for available light. This above-ground signal can also reduce lateral root growth by inhibiting lateral root emergence, a process that might help the plant invest resources in shoot growth. Nitrate is an essential nutrient for plant growth and Arabidopsis thaliana responds to low nitrate conditions by enhancing nutrient uptake and reducing lateral and main root growth. There are indications that low R:FR signaling and low nitrate signaling can affect each other. It is unknown which response is prioritized when low R:FR light- and low nitrate signaling co-occur. We investigated the effect of low nitrate conditions on the low R:FR response of the A. thaliana root system in agar plate media, combined with the application of supplemental Far-Red (FR) light to the shoot. We observed that under low nitrate conditions main and lateral root growth was reduced, but more importantly, that the response of the root system to low R:FR was not present. Consistently, a loss-of-function mutant of a nitrate transporter gene NRT2.1 lacked low R:FR-induced lateral root reduction and its root growth was hypersensitive to low nitrate. ELONGATED HYPOCOTYL5 (HY5) plays an important role in the root response to low R:FR and we found that it was less sensitive to low nitrate conditions with regards to lateral root growth. In addition, we found that low R:FR increases NRT2.1 expression and that low nitrate enhances HY5 expression. HY5 also affects NRT2.1 expression, however, it depended on the presence of ammonium in which direction this effect was. Replacing part of the nitrogen source with ammonium also removed the effect of low R:FR on the root system, showing that changes in nitrogen sources can be crucial for root plasticity. Together our results show that nitrate signaling can repress low R:FR responses and that this involves signaling via HY5 and NRT2.1.

Control of Plant Growth and Defense by Photoreceptors: From Mechanisms to Opportunities in Agriculture

Plants detect and respond to the proximity of competitors using light signals perceived by photoreceptor proteins. A low ratio of red to far-red radiation (R:FR ratio) is a key signal of competition that is sensed by the photoreceptor phytochrome B (phyB). Low R:FR ratios increase the synthesis of growth-related hormones, including auxin and gibberellins, promoting stem elongation and other shade-avoidance responses. Other photoreceptors that help plants to optimize their developmental configuration and resource allocation patterns in the canopy include blue light photoreceptors, such as cryptochromes and phototropins, and UV receptors, such as UVR8. All photoreceptors act by directly or indirectly controlling the activity of two major regulatory nodes for growth and development: the COP1/SPA ubiquitin E3 ligase complex and the PIF transcription factors. phyB is also an important modulator of hormonal pathways that regulate plant defense against herbivores and pathogens, including the jasmonic acid signaling pathway. In this Perspective, we discuss recent advances on the studies of the mechanisms that link photoreceptors with growth and defense. Understanding these mechanisms is important to provide a functional platform for breeding programs aimed at improving plant productivity, stress tolerance, and crop health in species of agronomic interest, and to manipulate the light environments in protected agriculture.

A Gas-and-Brake Mechanism of bHLH Proteins Modulates Shade Avoidance

Plants detect proximity of competitors through reduction in the ratio between red and far-red light that triggers the shade avoidance syndrome, inducing responses such as accelerated shoot elongation and early flowering. Shade avoidance is regulated by PHYTOCHROME INTERACTING FACTORs, a group of basic helix-loop-helix (bHLH) transcription factors. Another (b)HLH protein, KIDARI (KDR), which is non-DNA-binding, was identified in de-etiolation studies and proposed to interact with LONG HYPOCOTYL IN FAR-RED1 (HFR1), a (b)HLH protein that inhibits shade avoidance. Here, we established roles of KDR in regulating shade avoidance in Arabidopsis (Arabidopsis thaliana) and investigated how KDR regulates the shade avoidance network. We showed that KDR is a positive regulator of shade avoidance and interacts with several negative growth regulators. We identified KDR interactors using a combination of yeast two-hybrid screening and dedicated confirmations with bimolecular fluorescence complementation. We demonstrated that KDR is translocated primarily to the nucleus when coexpressed with these interactors. A genetic approach confirmed that several of these interactions play a functional role in shade avoidance; however, we propose that KDR does not interact with HFR1 to regulate shade avoidance. Based on these observations, we propose that shade avoidance is regulated by a three-layered gas-and-brake mechanism of bHLH protein interactions, adding a layer of complexity to what was previously known.

Far-red light promotes Botrytis cinerea disease development in tomato leaves via jasmonate-dependent modulation of soluble sugars

Plants experience a decrease in the red:far-red light ratio (R:FR) when grown at high planting density. In addition to eliciting the shade avoidance response, low R:FR also enhances plant susceptibility to pathogens via modulation of defense hormone-mediated responses. However, other mechanisms, also affected by low R:FR, have not been considered as potential components in FR-induced susceptibility. Here, we identify FR-induced accumulation of leaf soluble sugars as a novel component of FR-induced susceptibility. We observed that phytochrome inactivation by FR or phytochrome B mutation was associated with elevated leaf glucose and fructose levels and enhanced disease severity caused by Botrytis cinerea. By experimentally manipulating internal leaf sugar levels, we found that the FR-induced susceptibility in tomato was partly sugar-dependent. Further analysis revealed that the observed sugar accumulation in supplemental FR occurred in a jasmonic acid (JA)-dependent manner, and the JA biosynthesis mutant def1 also displayed elevated soluble sugar levels, which was rescued by exogenous methyl jasmonate (MeJA) application. We propose that the reduced JA responsiveness under low R:FR promotes disease symptoms not only via dampened induction of defense responses, but also via increased levels of soluble sugars that supports pathogen growth in tomato leaves.

Far-red light promotes Botrytis cinerea disease development in tomato leaves via jasmonate-dependent modulation of soluble sugars

Plants experience a decrease in the red:far-red light ratio (R:FR) when grown at high planting density. In addition to eliciting the shade avoidance response, low R:FR also enhances plant susceptibility to pathogens via modulation of defense hormone-mediated responses. However, other mechanisms, also affected by low R:FR, have not been considered as potential components in FR-induced susceptibility. Here, we identify FR-induced accumulation of leaf soluble sugars as a novel component of FR-induced susceptibility. We observed that phytochrome inactivation by FR or phytochrome B mutation was associated with elevated leaf glucose and fructose levels and enhanced disease severity caused by Botrytis cinerea. By experimentally manipulating internal leaf sugar levels, we found that the FR-induced susceptibility in tomato was partly sugar-dependent. Further analysis revealed that the observed sugar accumulation in supplemental FR occurred in a jasmonic acid (JA)-dependent manner, and the JA biosynthesis mutant def1 also displayed elevated soluble sugar levels, which was rescued by exogenous methyl jasmonate (MeJA) application. We propose that the reduced JA responsiveness under low R:FR promotes disease symptoms not only via dampened induction of defense responses, but also via increased levels of soluble sugars that supports pathogen growth in tomato leaves.

Photoreceptors Regulate Plant Developmental Plasticity through Auxin

Light absorption by plants changes the composition of light inside vegetation. Blue (B) and red (R) light are used for photosynthesis whereas far-red (FR) and green light are reflected. A combination of UV-B, blue and R:FR-responsive photoreceptors collectively measures the light and temperature environment and adjusts plant development accordingly. This developmental plasticity to photoreceptor signals is largely regulated through the phytohormone auxin. The phytochrome, cryptochrome and UV Resistance Locus 8 (UVR8) photoreceptors are inactivated in shade and/or elevated temperature, which releases their repression of Phytochrome Interacting Factor (PIF) transcription factors. Active PIFs stimulate auxin synthesis and reinforce auxin signalling responses through direct interaction with Auxin Response Factors (ARFs). It was recently discovered that shade-induced hypocotyl elongation and petiole hyponasty depend on long-distance auxin transport towards target cells from the cotyledon and leaf tip, respectively. Other responses, such as phototropic bending, are regulated by auxin transport and signalling across only a few cell layers. In addition, photoreceptors can directly interact with components in the auxin signalling pathway, such as Auxin/Indole Acetic Acids (AUX/IAAs) and ARFs. Here we will discuss the complex interactions between photoreceptor and auxin signalling, addressing both mechanisms and consequences of these highly interconnected pathways.

The bHLH network underlying plant shade-avoidance

Shade is a potential threat to many plant species. When shade-intolerant plants detect neighbours, they elongate their stems and leaves in an effort to maximise their light capture. This developmental programme, known as 'shade-avoidance' is tightly controlled by specialised photoreceptors and a suite of transcriptional regulators. The basic helix-loop-helix (bHLH) family of transcription factors are particularly important for shade-induced elongation. In recent years, it has become apparent that many members of this family heterodimerise and that together they form a complex regulatory network. This review summarises recent work into the structure of the bHLH network and how it regulates elongation growth. In addition to this, we highlight how photoreceptors modulate the function of the network via direct interaction with transcription factors. It is hoped that the information integrated in this review will provide a useful theoretical framework for future studies on the molecular basis of shade-avoidance in plants.

How light and biomass density influence the reproduction of delayed Saccharina latissima gametophytes (Phaeophyceae)

Kelp life-cycle transitions are complex and susceptible to various (a)biotic controls. Understanding the microscopic part of the kelp's lifecycle is of key importance, as gametophytes form a critical phase influencing, among others, the distributional limits of the species. Many environmental controls have been identified that affect kelp gametogenesis, whose interactive effects can be subtle and counterintuitive. Here we performed a fully factorial experiment on the (interactive) influences of light intensity, light quality, and the Initial Gametophyte Density (IGD) on Saccharina latissima reproduction and vegetative growth of delayed gametophytes. A total of 144 cultures were followed over a period of 21 d. The IGD was a key determinant for reproductive success, with increased IGDs (≥0.04 mg DW · mL-1 ) practically halting reproduction. Interestingly, the effects of IGDs were not affected by nutrient availability, suggesting a resource-independent effect of density on reproduction. The Photosynthetically Usable Radiation (PUR), overarching the quantitative contribution of both light intensity and light quality, correlated with both reproduction and vegetative growth. The PUR furthermore specifies that the contribution of light quality, as a lifecycle control, is a matter of absorbed photon flux instead of color signaling. We hypothesize that (i) the number of photons absorbed, independent of their specific wavelength, and (ii) IGD interactions, independent of nutrient availability, are major determinants of reproduction in S. latissima gametophytes. These insights help understand kelp gametophyte development and dispersal under natural conditions, while also aiding the control of in vitro gametophyte cultures.

Canopy Light Quality Modulates Stress Responses in Plants

Plants growing at high density are in constant competition for light with each other. The shade avoidance syndrome (SAS) is an effective way to escape neighboring vegetation. Even though the molecular mechanisms regulating SAS have been long studied, interactions between light and other environmental signaling pathways have only recently received attention. Under natural conditions, plants deal with multiple stresses simultaneously. It is, therefore, key to identify commonalities, distinctions, and interactions between plant responses to different environmental cues. This review outlines the current understanding of the interplay between canopy light signaling and other stresses, both biotic and abiotic. Understanding plant responses to multiple stimuli, factoring in the dominance of light for plant life, is essential to generate crops with increased resilience against climate change.

Soil Salinity Limits Plant Shade Avoidance

Global food production is set to keep increasing despite a predicted decrease in total arable land [1]. To achieve higher production, denser planting will be required on increasingly degraded soils. When grown in dense stands, crops elongate and raise their leaves in an effort to reach sunlight, a process termed shade avoidance [2]. Shade is perceived by a reduction in the ratio of red (R) to far-red (FR) light and results in the stabilization of a class of transcription factors known as PHYTOCHROME INTERACTING FACTORS (PIFs) [3, 4]. PIFs activate the expression of auxin biosynthesis genes [4, 5] and enhance auxin sensitivity [6], which promotes cell-wall loosening and drives elongation growth. Despite our molecular understanding of shade-induced growth, little is known about how this developmental program is integrated with other environmental factors. Here, we demonstrate that low levels of NaCl in soil strongly impair the ability of plants to respond to shade. This block is dependent upon abscisic acid (ABA) signaling and the canonical ABA signaling pathway. Low R:FR light enhances brassinosteroid (BR) signaling through BRASSINOSTEROID SIGNALING KINASE 5 (BSK5) and leads to the activation of BRI1 EMS SUPPRESSOR 1 (BES1). ABA inhibits BSK5 upregulation and interferes with GSK3-like kinase inactivation by the BR pathway, thus leading to a suppression of BES1:PIF function. By demonstrating a link between light, ABA-, and BR-signaling pathways, this study provides an important step forward in our understanding of how multiple environmental cues are integrated into plant development.

Three Auxin Response Factors Promote Hypocotyl Elongation

The hormone auxin regulates growth largely by affecting gene expression. By studying Arabidopsis (Arabidopsis thaliana) mutants deficient in AUXIN RESPONSE FACTORS (ARFs), we have identified three ARF proteins that are required for auxin-responsive hypocotyl elongation. Plants deficient in these factors have reduced responses to environmental conditions that increase auxin levels, including far-red-enriched light and high temperature. Despite having decreased auxin responses, the ARF-deficient plants responded to brassinosteroid and gibberellin, indicating that different hormones can act partially independently. Aux/IAA proteins, encoded by IAA genes, interact with ARF proteins to repress auxin response. Silencing expression of multiple IAA genes increased hypocotyl elongation, suggesting that Aux/IAA proteins modulate ARF activity in hypocotyls in a potential negative feedback loop.

Location Matters: Canopy Light Responses over Spatial Scales

Plants use light as a signal to determine neighbour proximity in dense vegetation. Far-red (FR) light reflected from neighbour plants elicits an array of growth responses throughout the plant. Recently, various light quality-induced signals have been discovered that travel between organs and tissue layers. These signals share upstream and downstream components, but can have opposing effects on cell growth. The question is how plants can coordinate these spatial signals into various growth responses in remote tissues. This coordination allows plants to adapt to the environment, and understanding the underlying mechanisms could allow precision engineering of crops. To achieve this understanding, plant photobiology research will need to focus increasingly on spatial signalling at the whole-plant level.

Distribution of perioperative stroke in cardiac surgery

Background and purpose

The recent literature suggests that a cardiac origin in ischaemic stroke is more frequent than previously assumed. However, it is not always clear which patients benefit from additional cardiac investigations if obvious cardiac pathology is absent.

Methods

A single‐center retrospective observational study was performed with 7454 consecutive patients admitted to the intensive care unit after cardiac surgery in the period 2006–2015 and who had postoperative brain imaging. Cerebral imaging was studied for the occurrence of stroke including subtype and involved vascular territory. It was assumed that all perioperative thromboembolic strokes are of cardiac origin. Data obtained from a hospital cohort of consecutive patients who received a diagnosis of ischaemic stroke were used for comparison.

Results

Thromboembolic stroke occurred in 135 cardiac surgery patients in 56 (41%) of whom the posterior cerebral circulation was involved. In the control group, 100 out of 503 strokes (20%) were located in the posterior cerebral circulation. The relative risk for a posterior location for stroke after cardiac surgery compared to patients with ischaemic stroke without prior cardiac surgery was 2.09; 95% confidence interval 1.60–2.72.

Conclusions

Thromboembolic stroke after cardiac surgery occurs twice as often in the posterior cerebral circulation compared to ischaemic strokes in the general population. If confirmed in general stroke cohorts, the consequence of this finding may be that in patients with an ischaemic stroke that involves the posterior cerebral circulation the chance of a cardiac origin is increased and therefore might trigger additional cardiac investigations such as long‐term heart rhythm monitoring or echocardiography.

Organ-specific phytohormone synthesis in two Geranium species with antithetical responses to far-red light enrichment

Plants growing in high densities experience a reduced red (R) to far-red (FR) light ratio and shade-intolerant species respond with accelerated elongation growth to reach the top of the canopy: the shade avoidance syndrome (SAS). FR-enriched light inactivates phytochrome photoreceptors, which results in subsequent action of several plant hormones regulating growth. SAS is adaptive for shade-intolerant plants, but is suppressed in shade-tolerant plant species. Inspired by a previously published transcriptome analysis, we use two species of the genus Geranium here to study the involvement of auxin, brassinosteroids (BRs), and gibberellins (GAs) in supplemental FR-induced elongation growth. G. pyrenaicum, a shade-avoiding species, strongly induces auxin and gibberellin levels, but not BR, in elongating petioles. We show that, in this species, FR light perception, hormone synthesis, and growth are local and restricted to the petiole, and not the leaf lamina. Using chemical hormone inhibitors, we confirm the essential role of auxin and GAs in supplemental FR-induced elongation growth. Shade-tolerant G. robertianum does not display the change in hormone levels upon FR light enrichment, resulting in the lack of a shade avoidance response.

Subtle variation in shade avoidance responses may have profound consequences for plant competitiveness

BACKGROUND AND AIMS

Although phenotypic plasticity has been shown to be beneficial for plant competitiveness for light, there is limited knowledge on how variation in these plastic responses plays a role in determining competitiveness.

METHODS

A combination of detailed plant experiments and functional-structural plant (FSP) modelling was used that captures the complex dynamic feedback between the changing plant phenotype and the within-canopy light environment in time and 3-D space. Leaf angle increase (hyponasty) and changes in petiole elongation rates in response to changes in the ratio between red and far-red light, two important shade avoidance responses in Arabidopsis thaliana growing in dense population stands, were chosen as a case study for plant plasticity. Measuring and implementing these responses into an FSP model allowed simulation of plant phenotype as an emergent property of the underlying growth and response mechanisms.

KEY RESULTS

Both the experimental and model results showed that substantial differences in competitiveness may arise between genotypes with only marginally different hyponasty or petiole elongation responses, due to the amplification of plant growth differences by small changes in plant phenotype. In addition, this study illustrated that strong competitive responses do not necessarily have to result in a tragedy of the commons; success in competition at the expense of community performance.

CONCLUSIONS

Together, these findings indicate that selection pressure could probably have played a role in fine-tuning the sensitive shade avoidance responses found in plants. The model approach presented here provides a novel tool to analyse further how natural selection could have acted on the evolution of plastic responses.

Light Signaling, Root Development, and Plasticity

Light signaling can affect root development and plasticity, either directly or through shoot-root communication via sugars, hormones, light, or other mobile factors.

Far-Red Light Detection in the Shoot Regulates Lateral Root Development through the HY5 Transcription Factor

Plants in dense vegetation compete for resources and detect competitors through reflection of far-red (FR) light from surrounding plants. This reflection causes a reduced red (R):FR ratio, which is sensed through phytochromes. Low R:FR induces shade avoidance responses of the shoot and also changes the root system architecture, although this has received little attention so far. Here, we investigate the molecular mechanisms through which light detection in the shoot regulates root development in Arabidopsis thaliana We do so using a combination of microscopy, gene expression, and mutant study approaches in a setup that allows root imaging without exposing the roots to light treatment. We show that low R:FR perception in the shoot decreases the lateral root (LR) density by inhibiting LR emergence. This decrease in LR emergence upon shoot FR enrichment is regulated by phytochrome-dependent accumulation of the transcription factor ELONGATED HYPOCOTYL5 (HY5) in the LR primordia. HY5 regulates LR emergence by decreasing the plasma membrane abundance of PIN-FORMED3 and LIKE-AUX1 3 auxin transporters. Accordingly, FR enrichment reduces the auxin signal in the overlaying cortex cells, and this reduces LR outgrowth. This shoot-to-root communication can help plants coordinate resource partitioning under competition for light in high density fields.

The shade-avoidance syndrome: multiple signals and ecological consequences

Plants use photoreceptor proteins to detect the proximity of other plants and to activate adaptive responses. Of these photoreceptors, phytochrome B (phyB), which is sensitive to changes in the red (R) to far-red (FR) ratio of sunlight, is the one that has been studied in greatest detail. The molecular connections between the proximity signal (low R:FR) and a model physiological response (increased elongation growth) have now been mapped in considerable detail in Arabidopsis seedlings. We briefly review our current understanding of these connections and discuss recent progress in establishing the roles of other photoreceptors in regulating growth-related pathways in response to competition cues. We also consider processes other than elongation that are controlled by photoreceptors and contribute to plant fitness under variable light conditions, including photoresponses that optimize the utilization of soil resources. In examining recent advances in the field, we highlight emerging roles of phyB as a major modulator of hormones related to plant immunity, in particular salicylic acid and jasmonic acid (JA). Recent attempts to manipulate connections between light signals and defence in Arabidopsis suggest that it might be possible to improve crop health at high planting densities by targeting links between phyB and JA signalling.

Plant Phenotypic and Transcriptional Changes Induced by Volatiles from the Fungal Root Pathogen Rhizoctonia solani

Beneficial soil microorganisms can affect plant growth and resistance by the production of volatile organic compounds (VOCs). Yet, little is known on how VOCs from soil-borne plant pathogens affect plant growth and resistance. Here we show that VOCs released from mycelium and sclerotia of the fungal root pathogen Rhizoctonia solani enhance growth and accelerate development of Arabidopsis thaliana. Seedlings briefly exposed to the fungal VOCs showed similar phenotypes, suggesting that enhanced biomass and accelerated development are primed already at early developmental stages. Fungal VOCs did not affect plant resistance to infection by the VOC-producing pathogen itself but reduced aboveground resistance to the herbivore Mamestra brassicae. Transcriptomics of A. thaliana revealed that genes involved in auxin signaling were up-regulated, whereas ethylene and jasmonic acid signaling pathways were down-regulated by fungal VOCs. Mutants disrupted in these pathways showed similar VOC-mediated growth responses as the wild-type A. thaliana, suggesting that other yet unknown pathways play a more prominent role. We postulate that R. solani uses VOCs to predispose plants for infection from a distance by altering root architecture and enhancing root biomass. Alternatively, plants may use enhanced root growth upon fungal VOC perception to sacrifice part of the root biomass and accelerate development and reproduction to survive infection.

Molecular Profiles of Contrasting Shade Response Strategies in Wild Plants: Differential Control of Immunity and Shoot Elongation

Plants growing at high densities elongate their shoots to reach for light, a response known as the shade avoidance syndrome (SAS). Phytochrome-mediated detection of far-red light reflection from neighboring plants activates growth-promoting molecular pathways leading to SAS However, it is unknown how plants that complete their life cycle in the forest understory and are shade tolerant prevent SAS when exposed to shade. Here, we show how two wild Geranium species from different native light environments regulate contrasting responses to light quality cues. A comparative RNA sequencing approach unveiled the molecular underpinnings of their contrasting growth responses to far-red light enrichment. It also identified differential phytochrome control of plant immunity genes and confirmed that far-red enrichment indeed contrastingly affects resistance against Botrytis cinerea between the two species. Furthermore, we identify a number of candidate regulators of differential shade avoidance. Three of these, the receptor-like kinases FERONIA and THESEUS1 and the non-DNA binding bHLH protein KIDARI, are functionally validated in Arabidopsis thaliana through gene knockout and/or overexpression studies. We propose that these components may be associated with either showing or not showing shade avoidance responses.

Integration of Phytochrome and Cryptochrome Signals Determines Plant Growth during Competition for Light

Plants in dense vegetation perceive their neighbors primarily through changes in light quality. Initially, the ratio between red (R) and far-red (FR) light decreases due to reflection of FR by plant tissue well before shading occurs. Perception of low R:FR by the phytochrome photoreceptors induces the shade avoidance response [1], of which accelerated elongation growth of leaf-bearing organs is an important feature. Low R:FR-induced phytochrome inactivation leads to the accumulation and activation of the transcription factors PHYTOCHROME-INTERACTING FACTORs (PIFs) 4, 5, and 7 and subsequent expression of their growth-mediating targets [2, 3]. When true shading occurs, transmitted light is especially depleted in red and blue (B) wavelengths, due to absorption by chlorophyll [4]. Although the reduction of blue wavelengths alone does not occur in nature, long-term exposure to low B light induces a shade avoidance-like response that is dependent on the cryptochrome photoreceptors and the transcription factors PIF4 and PIF5 [5-7]. We show in Arabidopsis thaliana that low B in combination with low R:FR enhances petiole elongation similar to vegetation shade, providing functional context for a low B response in plant competition. Low B potentiates the low R:FR response through PIF4, PIF5, and PIF7, and it involves increased PIF5 abundance and transcriptional changes. Low B attenuates a low R:FR-induced negative feedback loop through reduced gene expression of negative regulators and reduced HFR1 levels. The enhanced response to combined phytochrome and cryptochrome inactivation shows how multiple light cues can be integrated to fine-tune the plant's response to a changing environment.

Ethylene- and Shade-Induced Hypocotyl Elongation Share Transcriptome Patterns and Functional Regulators

Plants have evolved shoot elongation mechanisms to escape from diverse environmental stresses such as flooding and vegetative shade. The apparent similarity in growth responses suggests a possible convergence of the signaling pathways. Shoot elongation is mediated by passive ethylene accumulating to high concentrations in flooded plant organs and by changes in light quality and quantity under vegetation shade. Here, we study hypocotyl elongation as a proxy for shoot elongation and delineate Arabidopsis (Arabidopsis thaliana) hypocotyl length kinetics in response to ethylene and shade. Based on these kinetics, we further investigated ethylene- and shade-induced genome-wide gene expression changes in hypocotyls and cotyledons separately. Both treatments induced a more extensive transcriptome reconfiguration in the hypocotyls compared with the cotyledons. Bioinformatics analyses suggested contrasting regulation of growth promotion- and photosynthesis-related genes. These analyses also suggested an induction of auxin, brassinosteroid, and gibberellin signatures and the involvement of several candidate regulators in the elongating hypocotyls. Pharmacological and mutant analyses confirmed the functional involvement of several of these candidate genes and physiological control points in regulating stress-escape responses to different environmental stimuli. We discuss how these signaling networks might be integrated and conclude that plants, when facing different stresses, utilize a conserved set of transcriptionally regulated genes to modulate and fine-tune growth.

Plant Life without Ethylene

We propose that the ability to synthesize ethylene was selectively lost in evolution when the ancestors of fully aquatic higher plants lost their terrestrial lifestyle. We suggest that there has been negative selection on ethylene in these submerged species because it might interfere with growth in permanently deluged environments.

Red:far-red light conditions affect the emission of volatile organic compounds from barley (Hordeum vulgare), leading to altered biomass allocation in neighbouring plants

Since the publication of this paper it has become apparent that an error was made in the scale of the vertical axis in Fig. 6I. This has no impact at all on any of the conclusions in the paper since the differences between the treatments remain as published. The authors apologise for this error and a corrected version is reproduced below.

An Ancestral Role for CONSTITUTIVE TRIPLE RESPONSE1 Proteins in Both Ethylene and Abscisic Acid Signaling

Land plants have evolved adaptive regulatory mechanisms enabling the survival of environmental stresses associated with terrestrial life. Here, we focus on the evolution of the regulatory CONSTITUTIVE TRIPLE RESPONSE1 (CTR1) component of the ethylene signaling pathway that modulates stress-related changes in plant growth and development. First, we compare CTR1-like proteins from a bryophyte, Physcomitrella patens (representative of early divergent land plants), with those of more recently diverged lycophyte and angiosperm species (including Arabidopsis [Arabidopsis thaliana]) and identify a monophyletic CTR1 family. The fully sequenced P. patens genome encodes only a single member of this family (PpCTR1L). Next, we compare the functions of PpCTR1L with that of related angiosperm proteins. We show that, like angiosperm CTR1 proteins (e.g. AtCTR1 of Arabidopsis), PpCTR1L modulates downstream ethylene signaling via direct interaction with ethylene receptors. These functions, therefore, likely predate the divergence of the bryophytes from the land-plant lineage. However, we also show that PpCTR1L unexpectedly has dual functions and additionally modulates abscisic acid (ABA) signaling. In contrast, while AtCTR1 lacks detectable ABA signaling functions, Arabidopsis has during evolution acquired another homolog that is functionally distinct from AtCTR1. In conclusion, the roles of CTR1-related proteins appear to have functionally diversified during land-plant evolution, and angiosperm CTR1-related proteins appear to have lost an ancestral ABA signaling function. Our study provides new insights into how molecular events such as gene duplication and functional differentiation may have contributed to the adaptive evolution of regulatory mechanisms in plants.

Ethylene-Mediated Regulation of A2-Type CYCLINs Modulates Hyponastic Growth in Arabidopsis

Upward leaf movement (hyponastic growth) is frequently observed in response to changing environmental conditions and can be induced by the phytohormone ethylene. Hyponasty results from differential growth (i.e. enhanced cell elongation at the proximal abaxial side of the petiole relative to the adaxial side). Here, we characterize Enhanced Hyponasty-d, an activation-tagged Arabidopsis (Arabidopsis thaliana) line with exaggerated hyponasty. This phenotype is associated with overexpression of the mitotic cyclin CYCLINA2;1 (CYCA2;1), which hints at a role for cell divisions in regulating hyponasty. Indeed, mathematical analysis suggested that the observed changes in abaxial cell elongation rates during ethylene treatment should result in a larger hyponastic amplitude than observed, unless a decrease in cell proliferation rate at the proximal abaxial side of the petiole relative to the adaxial side was implemented. Our model predicts that when this differential proliferation mechanism is disrupted by either ectopic overexpression or mutation of CYCA2;1, the hyponastic growth response becomes exaggerated. This is in accordance with experimental observations on CYCA2;1 overexpression lines and cyca2;1 knockouts. We therefore propose a bipartite mechanism controlling leaf movement: ethylene induces longitudinal cell expansion in the abaxial petiole epidermis to induce hyponasty and simultaneously affects its amplitude by controlling cell proliferation through CYCA2;1. Further corroborating the model, we found that ethylene treatment results in transcriptional down-regulation of A2-type CYCLINs and propose that this, and possibly other regulatory mechanisms affecting CYCA2;1, may contribute to this attenuation of hyponastic growth.

RNAseq reveals weed-induced PIF3-like as a candidate target to manipulate weed stress response in soybean

Weeds reduce yield in soybeans (Glycine max) through incompletely defined mechanisms. The effects of weeds on the soybean transcriptome were evaluated in field conditions during four separate growing seasons. RNASeq data were collected from six biological samples of soybeans growing with or without weeds. Weed species and the methods to maintain weed-free controls varied between years to mitigate treatment effects, and to allow detection of general soybean weed responses. Soybean plants were not visibly nutrient- or water-stressed. We identified 55 consistently downregulated genes in weedy plots. Many of the downregulated genes were heat shock genes. Fourteen genes were consistently upregulated. Several transcription factors including a PHYTOCHROME INTERACTING FACTOR 3-like gene (PIF3) were included among the upregulated genes. Gene set enrichment analysis indicated roles for increased oxidative stress and jasmonic acid signaling responses during weed stress. The relationship of this weed-induced PIF3 gene to genes involved in shade avoidance responses in Arabidopsis provide evidence that this gene may be important in the response of soybean to weeds. These results suggest that the weed-induced PIF3 gene will be a target for manipulating weed tolerance in soybean.

Red:far-red light conditions affect the emission of volatile organic compounds from barley (Hordeum vulgare), leading to altered biomass allocation in neighbouring plants

BACKGROUND AND AIMS

Volatile organic compounds (VOCs) play various roles in plant-plant interactions, and constitutively produced VOCs might act as a cue to sense neighbouring plants. Previous studies have shown that VOCs emitted from the barley (Hordeum vulgare) cultivar 'Alva' cause changes in biomass allocation in plants of the cultivar 'Kara'. Other studies have shown that shading and the low red:far-red (R:FR) conditions that prevail at high plant densities can reduce the quantity and alter the composition of the VOCs emitted by Arabidopsis thaliana, but whether this affects plant-plant signalling remains unknown. This study therefore examines the effects of far-red light enrichment on VOC emissions and plant-plant signalling between 'Alva' and 'Kara'.

METHODS

The proximity of neighbouring plants was mimicked by supplemental far-red light treatment of VOC emitter plants of barley grown in growth chambers. Volatiles emitted by 'Alva' under control and far-red light-enriched conditions were analysed using gas chromatography-mass spectrometry (GC-MS). 'Kara' plants were exposed to the VOC blend emitted by the 'Alva' plants that were subjected to either of the light treatments. Dry matter partitioning, leaf area, stem and total root length were determined for 'Kara' plants exposed to 'Alva' VOCs, and also for 'Alva' plants exposed to either control or far-red-enriched light treatments.

KEY RESULTS

Total VOC emissions by 'Alva' were reduced under low R:FR conditions compared with control light conditions, although individual volatile compounds were found to be either suppressed, induced or not affected by R:FR. The altered composition of the VOC blend emitted by 'Alva' plants exposed to low R:FR was found to affect carbon allocation in receiver plants of 'Kara'.

CONCLUSIONS

The results indicate that changes in R:FR light conditions influence the emissions of VOCs in barley, and that these altered emissions affect VOC-mediated plant-plant interactions.

The art of being flexible: how to escape from shade, salt, and drought

Environmental stresses, such as shading of the shoot, drought, and soil salinity, threaten plant growth, yield, and survival. Plants can alleviate the impact of these stresses through various modes of phenotypic plasticity, such as shade avoidance and halotropism. Here, we review the current state of knowledge regarding the mechanisms that control plant developmental responses to shade, salt, and drought stress. We discuss plant hormones and cellular signaling pathways that control shoot branching and elongation responses to shade and root architecture modulation in response to drought and salinity. Because belowground stresses also result in aboveground changes and vice versa, we then outline how a wider palette of plant phenotypic traits is affected by the individual stresses. Consequently, we argue for a research agenda that integrates multiple plant organs, responses, and stresses. This will generate the scientific understanding needed for future crop improvement programs aiming at crops that can maintain yields under variable and suboptimal conditions.

Ecology of plant volatiles: taking a plant community perspective

Although plants are sessile organisms, they can modulate their phenotype so as to cope with environmental stresses such as herbivore attack and competition with neighbouring plants. Plant-produced volatile compounds mediate various aspects of plant defence. The emission of volatiles has costs and benefits. Research on the role of plant volatiles in defence has focused primarily on the responses of individual plants. However, in nature, plants rarely occur as isolated individuals but are members of plant communities where they compete for resources and exchange information with other plants. In this review, we address the effects of neighbouring plants on plant volatile-mediated defences. We will outline the various roles of volatile compounds in the interactions between plants and other organisms, address the mechanisms of plant neighbour perception in plant communities, and discuss how neighbour detection and volatile signalling are interconnected. Finally, we will outline the most urgent questions to be addressed in the future.

Shade avoidance: phytochrome signalling and other aboveground neighbour detection cues

Plants compete with neighbouring vegetation for limited resources. In competition for light, plants adjust their architecture to bring the leaves higher in the vegetation where more light is available than in the lower strata. These architectural responses include accelerated elongation of the hypocotyl, internodes and petioles, upward leaf movement (hyponasty), and reduced shoot branching and are collectively referred to as the shade avoidance syndrome. This review discusses various cues that plants use to detect the presence and proximity of neighbouring competitors and respond to with the shade avoidance syndrome. These cues include light quality and quantity signals, mechanical stimulation, and plant-emitted volatile chemicals. We will outline current knowledge about each of these signals individually and discuss their possible interactions. In conclusion, we will make a case for a whole-plant, ecophysiology approach to identify the relative importance of the various neighbour detection cues and their possible interactions in determining plant performance during competition.

Interactions between auxin, microtubules and XTHs mediate green shade- induced petiole elongation in arabidopsis

Plants are highly attuned to translating environmental changes to appropriate modifications in growth. Such phenotypic plasticity is observed in dense vegetations, where shading by neighboring plants, triggers rapid unidirectional shoot growth (shade avoidance), such as petiole elongation, which is partly under the control of auxin. This growth is fuelled by cellular expansion requiring cell-wall modification by proteins such as xyloglucan endotransglucosylase/hydrolases (XTHs). Cortical microtubules (cMTs) are highly dynamic cytoskeletal structures that are also implicated in growth regulation. The objective of this study was to investigate the tripartite interaction between auxin, cMTs and XTHs in shade avoidance. Our results indicate a role for cMTs to control rapid petiole elongation in Arabidopsis during shade avoidance. Genetic and pharmacological perturbation of cMTs obliterated shade-induced growth and led to a reduction in XTH activity as well. Furthermore, the cMT disruption repressed the shade-induced expression of a specific set of XTHs. These XTHs were also regulated by the hormone auxin, an important regulator of plant developmental plasticity and also of several shade avoidance responses. Accordingly, the effect of cMT disruption on the shade enhanced XTH expression could be rescued by auxin application. Based on the results we hypothesize that cMTs can mediate petiole elongation during shade avoidance by regulating the expression of cell wall modifying proteins via control of auxin distribution.