How plants protect themselves from ultraviolet-B radiation stress.

Ultraviolet-B (UV-B) radiation has a wavelength range of 280-315 nm. Plants perceive UV-B as an environmental signal and a potential abiotic stress factor that affects development and acclimation. UV-B regulates photomorphogenesis including hypocotyl elongation inhibition, cotyledon expansion, and flavonoid accumulation, but high intensity UV-B can also harm plants by damaging DNA, triggering accumulation of reactive oxygen species, and impairing photosynthesis. Plants have evolved "sunscreen" flavonoids that accumulate under UV-B stress to prevent or limit damage. The UV-B receptor UV RESISTANCE LOCUS 8 (UVR8) plays a critical role in promoting flavonoid biosynthesis to enhance UV-B stress tolerance. Recent studies have clarified several UVR8-mediated and UVR8-independent pathways that regulate UV-B stress tolerance. Here, we review these additions to our understanding of the molecular pathways involved in UV-B stress tolerance, highlighting the important roles of ELONGATED HYPOCOTYL 5, BRI1-EMS-SUPPRESSOR1, MYB DOMAIN PROTEIN 13, MAP KINASE PHOSPHATASE 1, and ATM- and RAD3-RELATED. We also summarize the known interactions with visible light receptors and the contribution of melatonin to UV-B stress responses. Finally, we update a working model of the UV-B stress tolerance pathway.

Transcriptome analysis of Clavibacter michiganensis subsp. michiganensis-infected tomatoes: a role of salicylic acid in the host response.

Bacterial canker of tomato (Solanum lycopersicon) caused by the Gram-positive bacterium Clavibacter michiganensis subsp. michiganensis (Cmm) is an economically important disease. To understand the host defense response to Cmm infection, transcriptome sequences in tomato cotyledons were analyzed by RNA-seq. Overall, 1788 and 540 genes were upregulated and downregulated upon infection, respectively. Gene Ontology enrichment analysis revealed that genes involved in the defense response, phosphorylation, and hormone signaling were over-represented by the infection. Induced expression of defense-associated genes suggested that the tomato response to Cmm showed similarities to common plant disease responses. After infection, many resistance gene analogs (RGAs) were transcriptionally upregulated, including the expressions of some receptor-like kinases (RLKs) involved in pattern-triggered immunity. The expressions of WRKYs, NACs, HSFs, and CBP60s encoding transcription factors (TFs) reported to regulate defense-associated genes were induced after infection with Cmm. Tomato genes orthologous to Arabidopsis EDS1, EDS5/SID1, and PAD4/EDS9, which are causal genes of salicylic acid (SA)-deficient mutants, were upregulated after infection with Cmm. Furthermore, Cmm infection drastically stimulated SA accumulation in tomato cotyledons. Genes involved in the phenylalanine ammonia lyase pathway were upregulated, whereas metabolic enzyme gene expression in the isochorismate synthase pathway remained unchanged. Exogenously applied SA suppressed bacterial growth and induced the expression of WRKYs, suggesting that some Cmm-responsive genes are regulated by SA signaling, and SA signaling activation should improve tomato immunity against Cmm.

The PUB4 E3 Ubiquitin Ligase Is Responsible for the Variegated Phenotype Observed upon Alteration of Chloroplast Protein Homeostasis in Arabidopsis Cotyledons.

During a plant's life cycle, plastids undergo several modifications, from undifferentiated pro-plastids to either photosynthetically-active chloroplasts, ezioplasts, chromoplasts or storage organelles, such as amyloplasts, elaioplasts and proteinoplasts. Plastid proteome rearrangements and protein homeostasis, together with intracellular communication pathways, are key factors for correct plastid differentiation and functioning. When plastid development is affected, aberrant organelles are degraded and recycled in a process that involves plastid protein ubiquitination. In this study, we have analysed the Arabidopsis gun1-102 ftsh5-3 double mutant, lacking both the plastid-located protein GUN1 (Genomes Uncoupled 1), involved in plastid-to-nucleus communication, and the chloroplast-located FTSH5 (Filamentous temperature-sensitive H5), a metalloprotease with a role in photosystem repair and chloroplast biogenesis. gun1-102 ftsh5-3 seedlings show variegated cotyledons and true leaves that we attempted to suppress by introgressing second-site mutations in genes involved in: (i) plastid translation, (ii) plastid folding/import and (iii) cytosolic protein ubiquitination. Different phenotypic effects, ranging from seedling-lethality to partial or complete suppression of the variegated phenotype, were observed in the corresponding triple mutants. Our findings indicate that Plant U-Box 4 (PUB4) E3 ubiquitin ligase plays a major role in the target degradation of damaged chloroplasts and is the main contributor to the variegated phenotype observed in gun1-102 ftsh5-3 seedlings.

An auxin signaling network translates low-sugar-state input into compensated cell enlargement in the fugu5 cotyledon.

In plants, the effective mobilization of seed nutrient reserves is crucial during germination and for seedling establishment. The Arabidopsis H+-PPase-loss-of-function fugu5 mutants exhibit a reduced number of cells in the cotyledons. This leads to enhanced post-mitotic cell expansion, also known as compensated cell enlargement (CCE). While decreased cell numbers have been ascribed to reduced gluconeogenesis from triacylglycerol, the molecular mechanisms underlying CCE remain ill-known. Given the role of indole 3-butyric acid (IBA) in cotyledon development, and because CCE in fugu5 is specifically and completely cancelled by ech2, which shows defective IBA-to-indoleacetic acid (IAA) conversion, IBA has emerged as a potential regulator of CCE. Here, to further illuminate the regulatory role of IBA in CCE, we used a series of high-order mutants that harbored a specific defect in IBA-to-IAA conversion, IBA efflux, IAA signaling, or vacuolar type H+-ATPase (V-ATPase) activity and analyzed the genetic interaction with fugu5-1. We found that while CCE in fugu5 was promoted by IBA, defects in IBA-to-IAA conversion, IAA response, or the V-ATPase activity alone cancelled CCE. Consistently, endogenous IAA in fugu5 reached a level 2.2-fold higher than the WT in 1-week-old seedlings. Finally, the above findings were validated in icl-2, mls-2, pck1-2 and ibr10 mutants, in which CCE was triggered by low sugar contents. This provides a scenario in which following seed germination, the low-sugar-state triggers IAA synthesis, leading to CCE through the activation of the V-ATPase. These findings illustrate how fine-tuning cell and organ size regulation depend on interplays between metabolism and IAA levels in plants.

Fusicoccin (FC)-Induced Rapid Growth, Proton Extrusion and Membrane Potential Changes in Maize (Zea mays L.) Coleoptile Cells: Comparison to Auxin Responses.

The fungal toxin fusicoccin (FC) induces rapid cell elongation, proton extrusion and plasma membrane hyperpolarization in maize coleoptile cells. Here, these three parameters were simultaneously measured using non-abraded and non-peeled segments with the incubation medium having access to their lumen. The dose-response curve for the FC-induced growth was sigmoidal shaped with the maximum at 10-6 M over 10 h. The amplitudes of the rapid growth and proton extrusion were significantly higher for FC than those for indole-3-acetic acid (IAA). The differences between the membrane potential changes that were observed in the presence of FC and IAA relate to the permanent membrane hyperpolarization for FC and transient hyperpolarization for IAA. It was also found that the lag times of the rapid growth, proton extrusion and membrane hyperpolarization were shorter for FC compared to IAA. At 30 °C, the biphasic kinetics of the IAA-induced growth rate could be changed into a monophasic (parabolic) one, which is characteristic for FC-induced rapid growth. It has been suggested that the rates of the initial phase of the FC- and IAA-induced growth involve two common mechanisms that consist of the proton pumps and potassium channels whose contribution to the action of both effectors on the rapid growth is different.

Cotyledons facilitate the adaptation of early-maturing soybean varieties to high-latitude long-day environments.

Soybean (Glycine max), a typical short-day plant (SDP) domesticated in temperate regions, has expanded to high latitudes where daylengths are long from soybean emergence to bloom, but rapidly decrease from seed filling to maturity. Cotyledons are well known as the major storage organs in seeds, but it is unclear whether developing cotyledons store flowering substances at filling stage in SD for upcoming seedlings, or instead respond to photoperiod for floral induction after emergence of matured seeds in long-day (LD). Here, we report that cotyledons accelerate flowering of early-maturing varieties not resulting from stored floral stimuli but by perceiving photoperiod after emergence. We found that light signal is indispensable to activate cotyledons for floral induction, and flowering promoting gene GmFT2a is required for cotyledon-dependent floral induction via upregulation of floral identity gene GmAP1. Interestingly, cotyledons are competent to support the entire life cycle of a cotyledon-only plant to produce seeds, underlying a new photoperiod study system in soybean and other dicots. Taken together, these results demonstrate a substantial role for cotyledons in flowering process, whereby we propose a 'cotyledon-based self-reliance' model highlighting floral induction from emergence as a key ecological adaptation for rapid flowering of SDPs grown in LD environments at high latitudes.

Mitochondrial heat-shock cognate protein 70 contributes to auxin-mediated embryo development.

In Arabidopsis thaliana, mitochondrial-localized heat-shock cognate protein 70-1 (mtHSC70-1) plays an important role in vegetativegrowth. However, whether mtHSC70-1 affects reproductive growth remains unknown. Here, we found that the mtHSC70-1 gene was expressed in the provascular cells of the embryo proper from the early heart stage onward during embryogenesis. Phenotypic analyses of mthsc70-1 mutants revealed that mtHSC70 deficiency leads to defective embryo development and that this effect is mediated by auxin. In addition to a dwarf phenotype, the mthsc70-1 mutant displayed defects in flower morphology, anther development, and embryogenesis. At early developmental stages, the mthsc70-1 embryos exhibited abnormal cell divisions in both embryo proper and suspensor cells. From heart stage onward, they displayed an abnormal shape such as with no or very small cotyledon protrusions, had aberrant number of cotyledons, or were twisted. These embryo defects were associated with reduced or ectopic expression of auxin responsive reporter DR5rev:GFP. Consistently, the expression of auxin biosynthesis and polar auxin transport genes were markedly altered in mthsc70-1. On the other hand, mitochondrial retrograde regulation (MRR) was enhanced in mthsc70-1. Treatment of wild-type plants with an inhibitor that activates mitochondrial retrograde signaling reduced the expression level of auxin biosynthesis and polar auxin transport genes and induced phenotypes similar to those of mthsc70-1. Taken together, our data reveal that loss of function of mtHSC70-1 induces MRR, which inhibits auxin biosynthesis and polar auxin transport, leading to abnormal auxin gradients and defective embryo development.

A BIN2-GLK1 Signaling Module Integrates Brassinosteroid and Light Signaling to Repress Chloroplast Development in the Dark.

Arabidopsis GLYCOGEN SYNTHASE KINASE 3 (GSK3)-like kinases play various roles in plant development, including chloroplast development, but the underlying molecular mechanism is not well defined. Here, we demonstrate that transcription factors GLK1 and GLK2 interact with and are phosphorylated by the BRASSINOSTEROID insensitive2 (BIN2). The loss-of-function mutant of BIN2 and its homologs, bin2-3 bil1 bil2, displays abnormal chloroplast development, whereas the gain-of-function mutant, bin2-1, exhibits insensitivity to BR-induced de-greening and reduced numbers of thylakoids per granum, suggesting that BIN2 positively regulates chloroplast development. Furthermore, BIN2 phosphorylates GLK1 at T175, T238, T248, and T256, and mutations of these phosphorylation sites alter GLK1 protein stability and DNA binding and impair plant responses to BRs/darkness. On the other hand, BRs and darkness repress the BIN2-GLK module to enhance BR/dark-mediated de-greening and impair the formation of the photosynthetic apparatus. Our results thus provide a mechanism by which BRs modulate photomorphogenesis and chloroplast development.

SAUR17 and SAUR50 Differentially Regulate PP2C-D1 during Apical Hook Development and Cotyledon Opening in Arabidopsis.

Following germination in the dark, Arabidopsis (Arabidopsis thaliana) seedlings undergo etiolation and develop apical hooks, closed cotyledons, and rapidly elongating hypocotyls. Upon light perception, the seedlings de-etiolate, which includes the opening of apical hooks and cotyledons. Here, we identify Arabidopsis Small Auxin Up RNA17 (SAUR17) as a downstream effector of etiolation, which serves to bring about apical hook formation and closed cotyledons. SAUR17 is highly expressed in apical hooks and cotyledons and is repressed by light. The apical organs also express a group of light-inducing SAURs, as represented by SAUR50, which promote hook and cotyledon opening. The development of etiolated or de-etiolated apical structures requires asymmetric differential cell growth. We present evidence that the opposing actions of SAUR17 and SAUR50 on apical development largely result from their antagonistic regulation of Protein Phosphatase 2C D-clade 1 (PP2C-D1), a phosphatase that suppresses cell expansion and promotes apical hook development in the dark. SAUR50 inhibits PP2C-D1, whereas SAUR17 has a higher affinity for PP2C-D1 without inhibiting its activity. PP2C-D1 predominantly associates with SAUR17 in etiolated seedlings, which shields it from inhibitory SAURs such as SAUR50. Light signals turn off SAUR17 and upregulate a subgroup of SAURs including SAUR50 at the inner side of the hook and cotyledon cells, leading to cell expansion and unfolding of the hook and cotyledons.

Transcriptomic Profiling of Young Cotyledons Response to Chilling Stress in Two Contrasting Cotton (Gossypium hirsutum L.) Genotypes at the Seedling Stage.

Young cotyledons of cotton seedlings are most susceptible to chilling stress. To gain insight into the potential mechanism of cold tolerance of young cotton cotyledons, we conducted physiological and comparative transcriptome analysis of two varieties with contrasting phenotypes. The evaluation of chilling injury of young cotyledons among 74 cotton varieties revealed that H559 was the most tolerant and YM21 was the most sensitive. The physiological analysis found that the ROS scavenging ability was lower, and cell membrane damage was more severe in the cotyledons of YM21 than that of H559 under chilling stress. RNA-seq analysis identified a total of 44,998 expressed genes and 19,982 differentially expressed genes (DEGs) in young cotyledons of the two varieties under chilling stress. Weighted gene coexpression network analysis (WGCNA) of all DEGs revealed four significant modules with close correlation with specific samples. The GO-term enrichment analysis found that lots of genes in H559-specific modules were involved in plant resistance to abiotic stress. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis revealed that pathways such as plant hormone signal transduction, MAPK signaling, and plant-pathogen interaction were related to chilling stress response. A total of 574 transcription factors and 936 hub genes in these modules were identified. Twenty hub genes were selected for qRT-PCR verification, revealing the reliability and accuracy of transcriptome data. These findings will lay a foundation for future research on the molecular mechanism of cold tolerance in cotyledons of cotton.

Genome-wide association mapping reveals novel genes associated with coleoptile length in a worldwide collection of barley.

BACKGROUND: Drought is projected to become more frequent and severe in a changing climate, which requires deep sowing of crop seeds to reach soil moisture. Coleoptile length is a key agronomic trait in cereal crops such as barley, as long coleoptiles are linked to drought tolerance and improved seedling establishment under early water-limited growing conditions. RESULTS: In this study, we detected large genetic variation in a panel of 328 diverse barley (Hordeum vulgare L.) accessions. To understand the overall genetic basis of barley coleoptile length, all accessions were germinated in the dark and phenotyped for coleoptile length after 2 weeks. The investigated barleys had significant variation for coleoptile length. We then conducted genome-wide association studies (GWASs) with more than 30,000 molecular markers and identified 8 genes and 12 intergenic loci significantly associated with coleoptile length in our barley panel. The Squamosa promoter-binding-like protein 3 gene (SPL3) on chromosome 6H was identified as a major candidate gene. The missense variant on the second exon changed serine to alanine in the conserved SBP domain, which likely impacted its DNA-binding activity. CONCLUSION: This study provides genetic loci for seedling coleoptile length along with candidate genes for future potential incorporation in breeding programmes to enhance early vigour and yield potential in water-limited environments.

Spatial and developmental synthesis of endogenous sesquiterpene lactones supports function in growth regulation of sunflower.

MAIN CONCLUSION: Tissue-specific occurrence and formation of endogenous sesquiterpene lactones has been assessed and suggests physiological function as antagonists of auxin-induced plant growth in sunflower. Sunflower, Helianthus annuus, accumulate high concentrations of bioactive sesquiterpene lactones (STL) in glandular trichomes, but in addition, structurally different STL occur in only trace amounts in the inner tissues. The spatial and temporal production of these endogenous STL during early phases of plant development is widely unknown and their physiological function as putative natural growth regulators is yet speculative. By means of HPLC and MS analysis it was shown that costunolide, dehydrocostuslactone, 8-epixanthatin and tomentosin are already present in dry seeds and can be extracted in low amounts from cotyledons, hypocotyls and roots of seedlings during the first days after germination. Semi-quantitative and RT-qPCR experiments with genes of the key enzymes of two independent routes of the endogenous STL biosynthesis confirmed the early and individual expression in these organs and revealed a gradual down regulation during the first 72-96 h after germination. Light irradiation of the plants led to a fast, but transient increase of STL in parts of the hypocotyl which correlated with growth retardation of the stem. One-sided external application of costunolide on hypocotyls conferred reduced growth of the treated side, thus resulting in the curving of the stem towards the side of the application. This indicates the inhibiting effects of STL on plant growth. The putative function of endogenous STL in sunflower as antagonists of auxin in growth processes is discussed.

High-Throughput Identification of Resistance to Pseudomonas syringae pv. Tomato in Tomato using Seedling Flood Assay.

Tomato is an agronomically important crop that can be infected by Pseudomonas syringae, a Gram-negative bacterium, resulting in bacterial speck disease. The tomato-P. syringae pv. tomato pathosystem is widely used to dissect the genetic basis of plant innate responses and disease resistance. While disease was successfully managed for many decades through the introduction of the Pto/Prf gene cluster from Solanum pimpinellifolium into cultivated tomato, race 1 strains of P. syringae have evolved to overcome resistance conferred by the Pto/Prf gene cluster and occur worldwide. Wild tomato species are important reservoirs of natural diversity in pathogen recognition, because they evolved in diverse environments with different pathogen pressures. In typical screens for disease resistance in wild tomato, adult plants are used, which can limit the number of plants that can be screened due to their extended growth time and greater growth space requirements. We developed a method to screen 10-day-old tomato seedlings for resistance, which minimizes plant growth time and growth chamber space, allows a rapid turnover of plants, and allows large sample sizes to be tested. Seedling outcomes of survival or death can be treated as discrete phenotypes or on a resistance scale defined by amount of new growth in surviving seedlings after flooding. This method has been optimized to screen 10-day-old tomato seedlings for resistance to two P. syringae strains and can easily be adapted to other P. syringae strains.

Quantitative proteomic, physiological and biochemical analysis of cotyledon, embryo, leaf and pod reveals the effects of high temperature and humidity stress on seed vigor formation in soybean.

BACKGROUND: Soybean developing seed is susceptible to high temperature and humidity (HTH) stress in the field, resulting in vigor reduction. Actually, the HTH in the field during soybean seed growth and development would also stress the whole plant, especially on leaf and pod, which in turn affect seed growth and development as well as vigor formation through nutrient supply and protection. RESULTS: In the present study, using a pair of pre-harvest seed deterioration-sensitive and -resistant cultivars Ningzhen No. 1 and Xiangdou No. 3, the comprehensive effects of HTH stress on seed vigor formation during physiological maturity were investigated by analyzing cotyledon, embryo, leaf, and pod at the levels of protein, ultrastructure, and physiology and biochemistry. There were 247, 179, and 517 differentially abundant proteins (DAPs) identified in cotyledon, embryo, and leaf of cv. Xiangdou No. 3 under HTH stress, while 235, 366, and 479 DAPs were identified in cotyledon, embryo, and leaf of cv. Ningzhen No. 1. Moreover, 120, 144, and 438 DAPs between the two cultivars were identified in cotyledon, embryo, and leaf under HTH stress, respectively. Moreover, 120, 144, and 438 DAPs between the two cultivars were identified in cotyledon, embryo, and leaf under HTH stress, respectively. Most of the DAPs identified were found to be involved in major metabolic pathways and cellular processes, including signal transduction, tricarboxylic acid cycle, fatty acid metabolism, photosynthesis, protein processing, folding and assembly, protein biosynthesis or degradation, plant-pathogen interaction, starch and sucrose metabolism, and oxidative stress response. The HTH stress had less negative effects on metabolic pathways, cell ultrastructure, and physiology and biochemistry in the four organs of Xiangdou No. 3 than in those of Ningzhen No. 1, leading to produce higher vigor seeds in the former. CONCLUSION: High seed vigor formation is enhanced by increasing protein biosynthesis and nutrient storage in cotyledon, stronger stability and viability in embryo, more powerful photosynthetic capacity and nutrient supply in leaf, and stronger protection in pod under HTH stress. These results provide comprehensive characteristics of leaf, pod and seed (cotyledon and embryo) under HTH stress, and some of them can be used as selection index in high seed vigor breeding program in soybean.

Hormesis in Plants: The Role of Oxidative Stress, Auxins and Photosynthesis in Corn Treated with Cd or Pb.

Hormesis, which describes the stimulatory effect of low doses of toxic substances on growth, is a well-known phenomenon in the plant and animal kingdoms. However, the mechanisms that are involved in this phenomenon are still poorly understood. We performed preliminary studies on corn coleoptile sections, which showed a positive correlation between the stimulation of growth by Cd or Pb and an increase in the auxin and H2O2 content in the coleoptile sections. Subsequently, we grew corn seedlings in hydroponic culture and tested a wide range of Cd or Pb concentrations in order to determine hormetic growth stimulation. In these seedlings the gas exchange and the chlorophyll a fluorescence, as well as the content of chlorophyll, flavonol, auxin and hydrogen peroxide, were measured. We found that during the hormetic stimulation of growth, the response of the photosynthetic apparatus to Cd and Pb differed significantly. While the application of Cd mostly caused a decrease in various photosynthetic parameters, the application of Pb stimulated some of them. Nevertheless, we discovered that the common features of the hormetic stimulation of shoot growth by heavy metals are an increase in the auxin and flavonol content and the maintenance of hydrogen peroxide at the same level as the control plants.

Auxin-Abscisic Acid Interactions in Plant Growth and Development.

Plant hormones regulate many aspects of plant growth, development, and response to biotic and abiotic stress. Much research has gone into our understanding of individual plant hormones, focusing primarily on their mechanisms of action and the processes that they regulate. However, recent research has begun to focus on a more complex problem; how various plant hormones work together to regulate growth and developmental processes. In this review, we focus on two phytohormones, abscisic acid (ABA) and auxin. We begin with brief overviews of the hormones individually, followed by in depth analyses of interactions between auxin and ABA, focusing on interactions in individual tissues and how these interactions are occurring where possible. Finally, we end with a brief discussion and future prospects for the field.

Enzymes contributing to the hydrogen peroxide signal dynamics that regulate wall labyrinth formation in transfer cells.

Transfer cells are characterized by an amplified plasma membrane area supported on a wall labyrinth composed of a uniform wall layer (UWL) from which wall ingrowth (WI) papillae arise. Adaxial epidermal cells of developing Vicia faba cotyledons, when placed in culture, undergo a rapid (hours) trans-differentiation to a functional epidermal transfer cell (ETC) phenotype. The trans-differentiation event is controlled by a signalling cascade comprising auxin, ethylene, apoplasmic reactive oxygen species (apoROS), and cytosolic Ca2+. Apoplasmic hydrogen peroxide (apoH2O2) was confirmed as the apoROS regulating UWL and WI papillae formation. Informed by an ETC-specific transcriptome, a pharmacological approach identified a temporally changing cohort of H2O2 biosynthetic enzymes. The cohort contained a respiratory burst oxidase homologue, polyamine oxidase, copper amine oxidase, and a suite of class III peroxidases. Collectively these generated two consecutive bursts in apoH2O2 production. Spatial organization of biosynthetic/catabolic enzymes was deduced from responses to pharmacologically blocking their activities on the cellular and subcellular distribution of apoH2O2. The findings were consistent with catalase activity constraining the apoH2O2 signal to the outer periclinal wall of the ETCs. Strategic positioning of class III peroxidases in this outer domain shaped subcellular apoH2O2 signatures that differed during assembly of the UWL and WI papillae.

One acorn produces two seedlings in Chinese cork oak Quercus variabilis.

Although the early germination of white oaks has long been considered an adaptation to counter animal predation, the role of the robust radicles of white oak acorns has been largely neglected in the interaction between acorns and seed-eating animals. Here, the regeneration capabilities of the pruned radicles of intermediate oak Quercus variabilis left by the acorn-eating animals were compared to test the 'one acorn produce two seedlings' hypothesis stating that the remnant radicles exhibit the capability to produce normal seedlings owning to their atypical nutritional and anatomical characteristics. Our results first showed that both pruned radicle and cotyledons from single acorn successfully established seedlings, providing strong evidence on an old puzzle that the robust radicles of early-germinating oak acorns act as reserved propagules, allowing them to escape animals' predation. Our study also suggests that much of the investment in the food reserves of an acorn may be important as a food reward for the animals that disperse the acorns, rather than simply serving as energy for the young seedling.

A Mobile Auxin Signal Connects Temperature Sensing in Cotyledons with Growth Responses in Hypocotyls.

Plants have a remarkable capacity to adjust their growth and development to elevated ambient temperatures. Increased elongation growth of roots, hypocotyls, and petioles in warm temperatures are hallmarks of seedling thermomorphogenesis. In the last decade, significant progress has been made to identify the molecular signaling components regulating these growth responses. Increased ambient temperature utilizes diverse components of the light sensing and signal transduction network to trigger growth adjustments. However, it remains unknown whether temperature sensing and responses are universal processes that occur uniformly in all plant organs. Alternatively, temperature sensing may be confined to specific tissues or organs, which would require a systemic signal that mediates responses in distal parts of the plant. Here, we show that Arabidopsis (Arabidopsis thaliana) seedlings show organ-specific transcriptome responses to elevated temperatures and that thermomorphogenesis involves both autonomous and organ-interdependent temperature sensing and signaling. Seedling roots can sense and respond to temperature in a shoot-independent manner, whereas shoot temperature responses require both local and systemic processes. The induction of cell elongation in hypocotyls requires temperature sensing in cotyledons, followed by the generation of a mobile auxin signal. Subsequently, auxin travels to the hypocotyl, where it triggers local brassinosteroid-induced cell elongation in seedling stems, which depends upon a distinct, permissive temperature sensor in the hypocotyl.

The Transcription Factors TCP4 and PIF3 Antagonistically Regulate Organ-Specific Light Induction of SAUR Genes to Modulate Cotyledon Opening during De-Etiolation in Arabidopsis.

Light elicits different growth responses in different organs of plants. These organ-specific responses are prominently displayed during de-etiolation. While major light-responsive components and early signaling pathways in this process have been identified, this information has yet to explain how organ-specific light responses are achieved. Here, we report that members of the TEOSINTE BRANCHED1, CYCLOIDEA, and PCF (TCP) transcription factor family participate in photomorphogenesis and facilitate light-induced cotyledon opening in Arabidopsis (Arabidopsis thaliana). Chromatin immunoprecipitation sequencing and RNA sequencing analyses indicated that TCP4 targets a number of SMALL AUXIN UPREGULATED RNA (SAUR) genes that have previously been shown to exhibit organ-specific, light-responsive expression. We demonstrate that TCP4-like transcription factors, which are predominantly expressed in the cotyledons of both light- and dark-grown seedlings, activate SAUR16 and SAUR50 expression in response to light. Light regulates the binding of TCP4 to the promoters of SAUR14, SAUR16, and SAUR50 through PHYTOCHROME-INTERACTING FACTORs (PIFs). PIF3, which accumulates in etiolated seedlings and its levels rapidly decline upon light exposure, also binds to the SAUR16 and SAUR50 promoters, while suppressing the binding of TCP4 to these promoters in the dark. Our study reveals that the interplay between light-responsive factors PIFs and the developmental regulator TCP4 determines the cotyledon-specific light regulation of SAUR16 and SAUR50, which contributes to cotyledon closure and opening before and after de-etiolation.