The default growth pattern of primary roots of land plants is directed by gravity. However, roots possess the ability to sense and respond directionally to other chemical and physical stimuli, separately and in combination. Therefore, these root tropic responses must be antagonistic to gravitropism. The role of reactive oxygen species (ROS) in gravitropism of maize and Arabidopsis (Arabidopsis thaliana) roots has been previously described. However, which cellular signals underlie the integration of the different environmental stimuli, which lead to an appropriate root tropic response, is currently unknown. In gravity-responding roots, we observed, by applying the ROS-sensitive fluorescent dye dihydrorhodamine-123 and confocal microscopy, a transient asymmetric ROS distribution, higher at the concave side of the root. The asymmetry, detected at the distal elongation zone, was built in the first 2 h of the gravitropic response and dissipated after another 2 h. In contrast, hydrotropically responding roots show no transient asymmetric distribution of ROS. Decreasing ROS levels by applying the antioxidant ascorbate, or the ROS-generation inhibitor diphenylene iodonium attenuated gravitropism while enhancing hydrotropism. Arabidopsis mutants deficient in Ascorbate Peroxidase 1 showed attenuated hydrotropic root bending. Mutants of the root-expressed NADPH oxidase RBOH C, but not rbohD, showed enhanced hydrotropism and less ROS in their roots apices (tested in tissue extracts with Amplex Red). Finally, hydrostimulation prior to gravistimulation attenuated the gravistimulated asymmetric ROS and auxin signals that are required for gravity-directed curvature. We suggest that ROS, presumably H2O2, function in tuning root tropic responses by promoting gravitropism and negatively regulating hydrotropism.
The switch from vegetative growth to reproductive growth, i.e. flowering, is the critical event in a plant’s life. Flowering is regulated either autonomously or by environmental factors; photoperiodic flowering, which is regulated by the duration of the day and night periods, and vernalization, which is regulated by low temperature, have been well studied. Additionally, it has become clear that stress also regulates flowering. Diverse stress factors can induce or accelerate flowering, or inhibit or delay it, in a wide range of plant species. This article focuses on the positive regulation of flowering via stress, i.e. the induction or acceleration of flowering in response to stress that is known as stress-induced flowering – a new category of flowering response. This review aims to clarify the concept of stress-induced flowering and to summarize the full range of characteristics of stress-induced flowering from a predominately physiological perspective.
Plant responses to drought stress require the regulation of transcriptional networks via drought-responsive transcription factors, which mediate a range of morphological and physiological changes. AP2/ERF transcription factors are known to act as key regulators of drought resistance transcriptional networks; however, little is known about the associated molecular mechanisms that give rise to specific morphological and physiological adaptations. In this study, we functionally characterized the rice (Oryza sativa) drought-responsive AP2/ERF transcription factor OsERF71, which is expressed predominantly in the root meristem, pericycle, and endodermis. Overexpression of OsERF71, either throughout the entire plant or specifically in roots, resulted in a drought resistance phenotype at the vegetative growth stage, indicating that overexpression in roots was sufficient to confer drought resistance. The root-specific overexpression was more effective in conferring drought resistance at the reproductive stage, such that grain yield was increased by 23% to 42% over wild-type plants or whole-body overexpressing transgenic lines under drought conditions. OsERF71 overexpression in roots elevated the expression levels of genes related to cell wall loosening and lignin biosynthetic genes, which correlated with changes in root structure, the formation of enlarged aerenchyma, and high lignification levels. Furthermore, OsERF71 was found to directly bind to the promoter of OsCINNAMOYL-COENZYME A REDUCTASE1, a key gene in lignin biosynthesis. These results indicate that the OsERF71-mediated drought resistance pathway recruits factors involved in cell wall modification to enable root morphological adaptations, thereby providing a mechanism for enhancing drought resistance.
Plants have evolved a remarkable faculty of adaptation to deal with various and changing environmental conditions. In this context, the roots have taken over nutritional aspects and the root system architecture can be modulated in response to nutrient availability or biotic interactions with soil microorganisms. This adaptability requires a fine tuning of gene expression. Indeed, root specification and development are highly complex processes requiring gene regulatory networks involved in hormonal regulations and cell identity. Among the different molecular partners governing root development, microRNAs (miRNAs) are key players for the fast regulation of gene expression. miRNAs are small RNAs involved in most developmental processes and are required for the normal growth of organisms, by the negative regulation of key genes, such as transcription factors and hormone receptors. Here, we review the known roles of miRNAs in root specification and development, from the embryonic roots to the establishment of root symbioses, highlighting the major roles of miRNAs in these processes.
Last September, world leaders made a commitment to end hunger by 2030, as part of the United Nations Sustainable Development Goals (SDGs). It sounds like a massive undertaking. In fact, the world already produces enough food to feed everyone. So why does the problem persist?
Plant cell walls provide stability and protection to plant cells. During growth and development the composition of cell walls changes, but provides enough strength to withstand the turgor of the cells. Hence, cell walls are highly flexible and diverse in nature. These characteristics are important during root growth, as plant roots consist of radial patterns of cells that have diverse functions and that are at different developmental stages along the growth axis. Young stem cell daughters undergo a series of rapid cell divisions, during which new cell walls are formed that are highly dynamic, and that support rapid anisotropic cell expansion. Once the cells have differentiated, the walls of specific cell types need to comply with and support different cell functions. For example, a newly formed root hair needs to be able to break through the surrounding soil, while endodermal cells modify their walls at distinct positions to form Casparian strips between them. Hence, the cell walls are modified and rebuilt while cells transit through different developmental stages. In addition, the cell walls of roots readjust to their environment to support growth and to maximize nutrient uptake. Many of these modifications are likely driven by different developmental and stress signalling pathways. However, our understanding of how such pathways affect cell wall modifications and what enzymes are involved remain largely unknown. In this review we aim to compile data linking cell wall content and re-modelling to developmental stages of root cells, and dissect how root cell walls respond to certain environmental changes.
Breeding crops with more biomass produced per drop of water transpired is a key challenge in the context of climate change. However, the tight coupling between transpiration and carbon assimilation during the day makes it challenging to decrease water loss without altering photosynthesis and reducing crop yield. We tested whether reducing transpiration at night when photosynthesis is inactive could substantially reduce water loss without altering growth—a hypothesis that, to our knowledge, has never been genetically addressed in any species. By studying a whole progeny in grapevine, a major crop for drought-prone areas, we identified genomic regions where selection could be operated to reduce transpiration at night and maintain growth. This opens new horizons for breeding crops with higher water-use efficiency.
In this study we test the hypothesis that maize genotypes with reduced crown root number (CN) will have greater root depth and improved water acquisition from drying soil. Maize recombinant inbred lines with contrasting CN were evaluated under water stress in greenhouse mesocosms and field rainout shelters. CN varied from 25 to 62 among genotypes. Under water stress in the mesocosms, genotypes with low CN had 31% fewer crown roots, 30% deeper rooting, 56% greater stomatal conductance, 45% greater leaf CO2 assimilation, 61% net canopy CO2 assimilation, and 55% greater shoot biomass than genotypes with high CN at 35 days after planting. Under water stress in the field, genotypes with low CN had 21% fewer crown roots, 41% deeper rooting, 48% lighter stem water oxygen isotope enrichment (δ18O) signature signifying deeper water capture, 13% greater leaf relative water content, 33% greater shoot biomass at anthesis, and 57% greater yield than genotypes with high CN. These results support the hypothesis that low CN improves drought tolerance by increasing rooting depth and water acquisition from the subsoil.
Future rice (Oryza sativa) crops will likely experience a range of growth conditions, and root architectural plasticity will be an important characteristic to confer adaptability across variable environments. In this study, the relationship between root architectural plasticity and adaptability (i.e. yield stability) was evaluated in two traditional × improved rice populations (Aus 276 × MTU1010 and Kali Aus × MTU1010). Forty contrasting genotypes were grown in direct-seeded upland and transplanted lowland conditions with drought and drought + rewatered stress treatments in lysimeter and field studies and a low-phosphorus stress treatment in a Rhizoscope study. Relationships among root architectural plasticity for root dry weight, root length density, and percentage lateral roots with yield stability were identified. Selected genotypes that showed high yield stability also showed a high degree of root plasticity in response to both drought and low phosphorus. The two populations varied in the soil depth effect on root architectural plasticity traits, none of which resulted in reduced grain yield. Root architectural plasticity traits were related to 13 (Aus 276 population) and 21 (Kali Aus population) genetic loci, which were contributed by both the traditional donor parents and MTU1010. Three genomic loci were identified as hot spots with multiple root architectural plasticity traits in both populations, and one locus for both root architectural plasticity and grain yield was detected. These results suggest an important role of root architectural plasticity across future rice crop conditions and provide a starting point for marker-assisted selection for plasticity.
Background Morphogenesis depends on the concerted modulation of cell proliferation and differentiation. Such modulation is dynamically adjusted in response to various external and internal signals via complex transcriptional regulatory networks that mediate between such signals and regulation of cell-cycle and cellular responses (proliferation, growth, differentiation). In plants, which are sessile, the proliferation/differentiation balance is plastically adjusted during their life cycle and transcriptional networks are important in this process. MADS-box genes are key developmental regulators in eukaryotes, but their role in cell proliferation and differentiation modulation in plants remains poorly studied.
Methods We characterize the XAL1 loss-of-function xal1-2 allele and overexpression lines using quantitative cellular and cytometry analyses to explore its role in cell cycle, proliferation, stem-cell patterning and transition to differentiation. We used quantitative PCR and cellular markers to explore if XAL1 regulates cell-cycle components and PLETHORA1 (PLT1) gene expression, as well as confocal microscopy to analyse stem-cell niche organization.
Key Results We previously showed that XAANTAL1 (XAL1/AGL12) is necessary for Arabidopsis root development as a promoter of cell proliferation in the root apical meristem. Here, we demonstrate that XAL1 positively regulates the expression of PLT1 and important components of the cell cycle: CYCD3;1, CYCA2;3, CYCB1;1, CDKB1;1 and CDT1a. In addition, we show that xal1-2 mutant plants have a premature transition to differentiation with root hairs appearing closer to the root tip, while endoreplication in these plants is partially compromised. Coincidently, the final size of cortex cells in the mutant is shorter than wild-type cells. Finally, XAL1 overexpression-lines corroborate that this transcription factor is able to promote cell proliferation at the stem-cell niche.
Conclusion XAL1 seems to be an important component of the networks that modulate cell proliferation/differentiation transition and stem-cell proliferation during Arabidopsis root development; it also regulates several cell-cycle components.
GRAS domain genes are a group of important plant-specific transcription factors that have been reported to be involved in plant development. In order to know the roles of GRAS genes in grapevine, a widely cultivated fruit crop, the study on grapevine GRAS (VvGRAS) was carried out, and from which, 43 were identified from 12× assemble grapevine genomic sequences. Further, the genomic structures, synteny, phylogeny, expression profiles in different tissues of these genes, and their roles in response to stress were investigated. Among the genes, two potential target genes (VvSCL15 and VvSCL22) for VvmiR171 were experimentally verified by PPM-RACE and RLM-RACE, in that not only the cleavage sites of miR171 on the target mRNA were mapped but also the cleaved fragments and their expressing patterns were detected. Transgenic Arabidopsis plants over expression VvSCL15 showed lower tolerance to drought and salt treatments.
Long distance transport in plants occurs in sieve tubes of the phloem. The pressure flow hypothesis introduced by Ernst Münch in 1930 describes a mechanism of osmotically generated pressure differentials that are supposed to drive the movement of sugars and other solutes in the phloem, but this hypothesis has long faced major challenges. The key issue is whether the conductance of sieve tubes, including sieve plate pores, is sufficient to allow pressure flow. We show that with increasing distance between source and sink, sieve tube conductivity and turgor increases dramatically in Ipomoea nil. Our results provide strong support for the Münch hypothesis, while providing new tools for the investigation of one of the least understood plant tissues.
Since the Industrial Revolution began approximately 200 years ago, global atmospheric carbon dioxide concentration ([CO2]) has increased from 270 to 401 µL L−1, and average global temperatures have risen by 0.85°C, with the most pronounced effects occurring near the poles (IPCC, 2013). In addition, the last 30 years were the warmest decades in 1,400 years (PAGES 2k Consortium, 2013). By the end of this century, [CO2] is expected to reach at least 700 µL L−1, and global temperatures are projected to rise by 4°C or more based on greenhouse gas scenarios (IPCC, 2013). Precipitation regimes also are expected to shift on a regional scale as the hydrologic cycle intensifies, resulting in greater extremes in dry versus wet conditions (Medvigy and Beaulieu, 2012). Such changes already are having profound impacts on the physiological functioning of plants that scale up to influence interactions between plants and other organisms and ecosystems as a whole (Fig. 1). Shifts in climate also may alter selective pressures on plants and, therefore, have the potential to influence evolutionary processes. In some cases, evolutionary responses can occur as rapidly as only a few generations (Ward et al., 2000; Franks et al., 2007; Lau and Lennon, 2012), but there is still much to learn in this area, as pointed out by Franks et al. (2014). Such responses have the potential to alter ecological processes, including species interactions, via ecoevolutionary feedbacks (Shefferson and Salguero-Gómez, 2015). In this review, we discuss microevolutionary and macroevolutionary processes that can shape plant responses to climate change as well as direct physiological responses to climate change during the recent geologic past as recorded in the fossil record. We also present work that documents how plant physiological and evolutionary responses influence interactions with other organisms as an example of how climate change effects on plants can scale to influence higher order processes within ecosystems. Thus, this review combines findings in plant physiological ecology and evolutionary biology for a comprehensive view of plant responses to climate change, both past and present.
As plant tissues dehydrate, water transport efficiency declines, a process typically attributed to air obstruction (embolism) in the xylem. Trifiló et al. (pages 5029–5039) dissect leaf hydraulic vulnerability and show that both xylem and living tissues may be important. If confirmed and clarified, an important role for outside-xylem hydraulic decline will change our understanding of how plants transport water and control biosphere carbon and water fluxes.
Circadian clocks allow the temporal compartmentalization of biological processes. In Arabidopsis, circadian rhythms display organ specificity but the underlying molecular causes have not been identified. We investigated the mechanisms responsible for the similarities and differences between the clocks of mature shoots and roots in constant conditions and in light : dark cycles. We developed an imaging system to monitor clock gene expression in shoots and light- or dark-grown roots, modified a recent mathematical model of the Arabidopsis clock and used this to simulate our new data. We showed that the shoot and root circadian clocks have different rhythmic properties (period and amplitude) and respond differently to light quality. The root clock was entrained by direct exposure to low-intensity light, even in antiphase to the illumination of shoots. Differences between the clocks were more pronounced in conditions where light was present than in constant darkness, and persisted in the presence of sucrose. We simulated the data successfully by modifying those parameters of a clock model that are related to light inputs. We conclude that differences and similarities between the shoot and root clocks can largely be explained by organ-specific light inputs. This provides mechanistic insight into the developing field of organ-specific clocks.
Assimilate partitioning to the root system is a desirable developmental trait to control but little is known of the signaling pathway underlying partitioning. A null mutation in the gene encoding the Gβ subunit of the heterotrimeric G protein complex, a nexus for a variety of signaling pathways, confers altered sugar partitioning in roots. While fixed carbon rapidly reached the roots of wild type and agb1-2 mutant seedlings, agb1 roots had more of this fixed carbon in the form of glucose, fructose, and sucrose which manifested as a higher lateral root density. Upon glucose treatment, the agb1-2 mutant had abnormal gene expression in the root tip validated by transcriptome analysis. In addition, PIN2 membrane localization and level was altered in the agb1-2 mutant. The heterotrimeric G protein complex integrates photosynthesis-derived sugar signaling incorporating both membrane-and transcriptional-based mechanisms. The time constants for these signaling mechanisms are in the same range as photosynthate delivery to the root, raising the possibility that root cells are able to use changes in carbon fixation in real time to adjust growth behavior.
Einkorn, emmer, and spelt are old wheat species that have fed the world for centuries before they have nearly completely been replaced by modern bread wheat. Nowadays, the diversity of these old species lies frozen in gene banks and rare attempts aim to exploit them as a source for genetic diversity in modern wheat breeding. Here, we want to raise a debate on a more holistic exploitation of ancient species via their direct introduction to the consumer market as high quality products. Although exemplified only for ancient wheat species, this innovative self-financing strategy can be directly extended to other species.
The present work investigates the interactions between soil content, rootstock and scion by focusing on the effects of roostocks and nitrogen supply on grape berry content. Scions of Cabernet Sauvignon (CS) and Pinot Noir (PN) varieties were grafted either on Riparia Gloire de Montpellier (RGM) or 110 Richter (110R) rootstock. The 4 rooststock/scion combinations were fertilized with 3 different levels of nitrogen after fruit set. Both in 2013 and 2014, N supply increased N uptake by the plants, and N content both in vegetative and reproductory organs. Rootstock, variety and year affected berry weight at harvest, while nitrogen did not affect significantly this parameter. Grafting on RGM consistently increased berry weight compared to 110R. PN consistently produced bigger berries than CS. CS berries were heavier in 2014 than in 2013, but the year effect was less marked for PN berries. The berries were collected between veraison and maturity, separated in skin and pulp, and their content was analyzed by conventional analytical procedures and untargeted metabolomics. For anthocyanins, the relative quantitation was fairly comparable with both LC-MS determination and HPLC-DAD, which is a fully quantitative technique. The data show complex responses of the metabolite content (sugars, organic acids, amino acids, anthocyanins, flavonols, flavan-3-ols/procyanidins, stilbenes, hydroxycinnamic and hydroxybenzoic acids.) that depend on the rootstock, the scion, the vintage, the nitrogen level, the berry compartment. This opens a wide range of possibilities to adjust the content of these compounds through the choice of the roostock, variety and nitrogen fertilization.
This review presents recent progress in various aspects of plant carotenoids research, along with the knowledge on carotenoid metabolism and regulation in plants. It also covers the most recent progress in the field of carotenoid metabolism, sequestration, and degradation.
Root hairs are fast growing, ephemeral tubular extensions of the root epidermis. They arise in the unsuberized maturation zone of the root, effectively increasing the root surface area in the region over which nutrient and water uptake occur. Variation in root hair length (RHL) between varieties has been shown to be genetically determined, and could, therefore, have consequences for nutrient capture and yield potential in crops. We describe the development of a medium-to-high throughput screening method for assessing RHL in wheat at the seedling stage. This method was used to screen a number of wheat mapping population parental lines for variation in RHL. Parents of two populations derived from inter-varietal crosses differed for RHL: Spark vs Rialto and Charger vs Badger. We identified quantitative trait loci (QTLs) for RHL in the populations derived from these crosses. In Spark × Rialto, QTLs on chromosomes 1A, 2A and 6A were associated with variation in RHL, whilst in Charger × Badger, a QTL for RHL was identified on 2BL. The QTLs on 2A and 6A co-localized with previously described QTLs for yield components. Longer root hairs may confer an advantage by exploiting limiting mineral and water resources. This first QTL analysis of root hair length in wheat identifies loci that could usefully be further investigated for their role in tolerance to limiting conditions.
Mitogen-activated protein kinase (MAPK) cascades play critical roles in signal transduction processes in eukaryotes. The MAPK kinases (MAPKKs) that link MAPKK kinases (MAPKKKs) and MAPKs are key components of MAPK cascades. However, the intricate regulatory mechanisms that control MAPKKs under drought stress conditions are not fully understood, especially in cotton (Gossypium hirsutum). Here, we isolated and characterized the cotton group B MAPKK gene GhMKK3. Overexpressing GhMKK3 in Nicotiana benthamiana enhanced tolerance to drought, and the results of RNA sequencing (RNA-seq) and quantitative real-time PCR (qRT-PCR) assays suggest that GhMKK3 plays an important role in responses to abiotic stresses by regulating stomatal responses and root hair growth. Further evidence demonstrated that overexpressing GhMKK3 promoted root growth and ABA-induced stomatal closure. In contrast, silencing GhMKK3 in cotton using virus-induced gene silencing (VIGS) resulted in the opposite phenotypes. More importantly, we identified an ABA- and drought-induced MAPK cascade that is composed of GhMKK3, GhMPK7 and GhPIP1 that compensates for deficiency in the MAPK cascade pathway in cotton under drought stress conditions. Together, these findings significantly improve our understanding of the mechanism by which GhMKK3 positively regulates drought stress responses.
Phytohormone ethylene has previously been known to play an important role in mediating root hair development induced by phosphate starvation; however, the underlying molecular mechanism is not understood. Using combined molecular, genetic, and genomic approaches, we identify a group of genes that affect root hair development by regulating cell wall modifications. Pi starvation increase the stability of EIN3 protein, a key component in the ethylene signaling pathway. The expression of the identified root hair-related genes is enhanced in the EIN3-overexpressing line, but suppressed in the ein3 mutant. Furthermore, EIN3 protein directly binds to the promoter of these genes which are also targeted by a key transcription factor that regulates root hair development. This work not only explains how ethylene mediates root hair responses to phosphate starvation, but may provide a general mechanism for how ethylene regulates root hair development under both stress and non-stress conditions.
Sensitivity to variability in resources has been documented in humans, primates, birds, and social insects, but the fit between empirical results and the predictions of risk sensitivity theory (RST), which aims to explain this sensitivity in adaptive terms, is weak . RST predicts that agents should switch between risk proneness and risk aversion depending on state and circumstances, especially according to the richness of the least variable option . Unrealistic assumptions about agents’ information processing mechanisms and poor knowledge of the extent to which variability imposes specific selection in nature are strong candidates to explain the gap between theory and data. RST’s rationale also applies to plants, where it has not hitherto been tested. Given the differences between animals’ and plants’ information processing mechanisms, such tests should help unravel the conflicts between theory and data. Measuring root growth allocation by split-root pea plants, we show that they favor variability when mean nutrient levels are low and the opposite when they are high, supporting the most widespread RST prediction. However, the combination of non-linear effects of nitrogen availability at local and systemic levels may explain some of these effects as a consequence of mechanisms not necessarily evolved to cope with variance [3 and 4]. This resembles animal examples in which properties of perception and learning cause risk sensitivity even though they are not risk adaptations .
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