Light Signaling in Bud Outgrowth and Branching in Plants

Photosynthesis regulates tillering bud elongation and nitrogen-use efficiency via sugar-induced NGR5 in rice

Rice tillering is one of the most important agronomical traits largely determining grain yield. Photosynthesis and nitrogen availability are two important factors affecting rice tiller bud elongation; however, underlying mechanism and their cross-talk is poorly understood. Here, we used map-based cloning, transcriptome profiling, phenotypic analysis, and molecular genetics to understand the roles of the Decreased Tiller Number 1 (DTN1) gene that encodes the fructose-1,6-bisphosphate aldolase and involves in photosynthesis required for light-induced axillary bud elongation in rice. Deficiency of DTN1 results in the reduced photosynthetic rate and decreased contents of sucrose and other sugars in both leaves and axillary buds, and the reduced tiller number in dtn1 mutant could be partially rescued by exogenous sucrose treatment. Furthermore, we found that the expression of nitrogen-mediated tiller growth response 5 (NGR5) was remarkably decreased in shoot base of dtn1-2, which can be activated by sucrose treatment. Overexpression of NGR5 in the dtn1-2 could partially rescue the reduced tiller number, and the tiller number of dtn1-2 was insensitive to nitrogen supply. This work demonstrated that the sugar level regulated by photosynthesis and DTN1 could positively regulate NGR5 expression, which coordinates the cross-talk between carbon and nitrate to control tiller bud outgrowth in rice.

Supplemental greenhouse lighting increased the water use efficiency, crop growth, and cutting production in Cannabis sativa

The expanding cannabis production sector faces economic challenges, intensified by freshwater scarcity in the main US production areas. Greenhouse cultivation harnesses sunlight to reduce production costs, yet the impact of greenhouse light levels on crucial production components, such as plant growth, branching, and water use efficiency (WUE), remains poorly understood. This study aimed to assess the effects of combined sunlight and supplemental lighting on the crop's main production components and leaf gas exchange of Cannabis sativa 'Suver Haze' in the vegetative stage. Within a greenhouse, LED lighting provided at intensities of ~150, 300, 500, and 700 µmol m-2 s-1 (18-hour photoperiod), combined with solar radiation, resulted in average daily light integrals of 17.9, 29.8, 39.5, and 51.8 mol m-2 d-1. Increasing light levels linearly increased biomass, leaf area, and the number of branches per plant and square meter, with respective rates of 0.26 g, 32.5 cm2, and 0.41 branches per mole of additional light. As anticipated, crop evapotranspiration increased by 1.8-fold with the increase in light intensity yet crop WUE improved by 1.6-fold when comparing the lowest and highest light treatments. Moreover, water requirements per unit of plant biomass decreased from 0.37 to 0.24 liters per gram when lighting increased from ~18 to 52 mol m-2 d-1, marking a 35% reduction in evapotranspiration. These results were supported by increments in leaf photosynthesis and WUE with light enhancement. Furthermore, our findings indicate that even 52 mol m-2 d-1 of supplemental lighting did not saturate any of the crop responses to light and can be economically viable for cannabis nurseries. In conclusion, light supplementation strongly enhanced photosynthesis and plant growth while increasing WUE. Additionally, a comprehensive discussion highlights the shared physiological mechanisms governing WUE in diverse plant species and their potential for water conservation under enhanced lighting conditions.

Far-red light-enhanced apical dominance stimulates flower and fruit abortion in sweet pepper

Far-red radiation affects many plant processes, including reproductive organ abortion. Our research aimed to determine the role of apical dominance in far-red light-induced flower and fruit abortion in sweet pepper (Capsicum annuum L.). We conducted several climate room experiments where plants were grown under white- or red-rich LED light, with or without additional far-red light. Additional far-red light enhanced apical dominance: it increased auxin levels in the apices of dominant shoots, and caused a greater difference in internode length and apical auxin levels between dominant and subordinate shoots. Additional far-red light stimulated fruit abortion in intact plants but not in decapitated plants, suggesting a crucial role of shoot apices in this effect. However, reducing basipetal auxin transport in the stems with N-1-naphthylphthalamic acid did not influence far-red light-stimulated fruit abortion, although auxin levels in the stem were largely reduced. Applying the synthetic auxin 1-naphthaleneacetic acid on decapitated apices did not influence fruit abortion. However, applying the auxin biosynthesis inhibitor yucasin to shoot apices reduced fruit abortion regardless of the light conditions, accompanied by slight shoot growth retardation. These findings suggest that the basipetal auxin stream does not mediate far-red light-stimulated fruit abortion. Far-red light-stimulated fruit abortion was associated with reduced sucrose accumulation and lower invertase activities in flowers. We suggest that under additional far-red light conditions, increased auxin levels in shoot apices promote fruit abortion probably through enhanced competition for assimilates between apices and flowers, which limits assimilate import into flowers.

Interaction of Phytohormones and External Environmental Factors in the Regulation of the Bud Dormancy in Woody Plants

Bud dormancy and release are essential phenomena that greatly assist in adapting to adverse growing conditions and promoting the holistic growth and development of perennial plants. The dormancy and release process of buds in temperate perennial trees involves complex interactions between physiological and biochemical processes influenced by various environmental factors, representing a meticulously orchestrated life cycle. In this review, we summarize the role of phytohormones and their crosstalk in the establishment and release of bud dormancy. External environmental factors, such as light and temperature, play a crucial role in regulating bud germination. We also highlight the mechanisms of how light and temperature are involved in the regulation of bud dormancy by modulating phytohormones. Moreover, the role of nutrient factors, including sugar, in regulating bud dormancy is also discussed. This review provides a foundation for enhancing our understanding of plant growth and development patterns, fostering agricultural production, and exploring plant adaptive responses to adversity.

Improving citrus bud grafting efficiency

Commercial citrus trees are composed of a scion grafted onto a rootstock. Because grafting is one of the most expensive methods of plant propagation, grafting efficiency is of large practical importance. The purpose of this study was to improve citrus bud-grafting efficiency. The effects of six factors that included BA, Tween-20, DMSO, type of solvent (water or EtOH), cardinal orientation of grafted bud, and type of supplemental light (LED, metal halide, none) on forty-four bud-grafting measures were determined using a multifactor design of experiment approach. Four measures useful for identifying treatments of practical value included the number of rootstock axial buds that formed shoots, the percentage of grafted buds that formed shoots, the length of the longest shoot formed from the grafted buds, and the total leaf area of the grafted bud shoots. The factors that most affected these responses were no supplemental light to minimize the number of shoots from rootstock axial buds, a south orientation and 5 mM BA to maximize the percentage of grafted buds that formed shoots, a north orientation and 5 mM BA to maximize the length of the longest grafted bud shoot, and 5 mM BA to maximize the leaf area of the grafted bud shoots.

Transcriptomic analysis implicates ABA signaling and carbon supply in the differential outgrowth of petunia axillary buds

BACKGROUND

Shoot branching of flowering plants exhibits phenotypic plasticity and variability. This plasticity is determined by the activity of axillary meristems, which in turn is influenced by endogenous and exogenous cues such as nutrients and light. In many species, not all buds on the main shoot develop into branches despite favorable growing conditions. In petunia, basal axillary buds (buds 1-3) typically do not grow out to form branches, while more apical axillary buds (buds 6 and 7) are competent to grow.

RESULTS

The genetic regulation of buds was explored using transcriptome analyses of petunia axillary buds at different positions on the main stem. To suppress or promote bud outgrowth, we grew the plants in media with differing phosphate (P) levels. Using RNA-seq, we found many (> 5000) differentially expressed genes between bud 6 or 7, and bud 2. In addition, more genes were differentially expressed when we transferred the plants from low P to high P medium, compared with shifting from high P to low P medium. Buds 6 and 7 had increased transcript abundance of cytokinin and auxin-related genes, whereas the basal non-growing buds (bud 2 and to a lesser extent bud 3) had higher expression of strigolactone, abscisic acid, and dormancy-related genes, suggesting the outgrowth of these basal buds was actively suppressed. Consistent with this, the expression of ABA associated genes decreased significantly in apical buds after stimulating growth by switching the medium from low P to high P. Furthermore, comparisons between our data and transcriptome data from other species suggest that the suppression of outgrowth of bud 2 was correlated with a limited supply of carbon to these axillary buds. Candidate genes that might repress bud outgrowth were identified by co-expression analysis.

CONCLUSIONS

Plants need to balance growth of axillary buds into branches to fit with available resources while allowing some buds to remain dormant to grow after the loss of plant parts or in response to a change in environmental conditions. Here we demonstrate that different buds on the same plant with different developmental potentials have quite different transcriptome profiles.

Spectral Light Treatment Influenced Morpho-Physiological Properties and Carvacrol Accumulation in Indian Borage

Light emitting diodes (LEDs) as an alternative light source for plants had shown to enhance the plant material quality. Indian borage or Plectranthus amboinicus (Lour.) Spreng, a medicinal herb produces carvacrol as the major volatile organic compound (VOC). Histolocalization of VOCs and expression pattern of the terpenoid biosynthesis genes after spectral light treatment is not yet reported in P. amboinicus. This work investigated the morpho-physiological, biochemical and transcriptional responses towards red, green, blue, warm white and red-blue (RB, 1:1) LEDs treatment at 40 ± 5 μmol m-2 s-1 light intensity after 40 days. Maximal growth index (GI), leaf fresh weight and dry weight were obtained in RB (1:1) treated plants. There was one-fold increase in phenolics content and 2.5-fold increase in antioxidant activity in comparison to warm white. High quantity of terpenes and phenolics deposition were observed in the glandular trichomes of RB (1:1). Maximum carvacrol accumulation (14.45 µmol g-1 FW) was also detected in RB (1:1). The transcript levels of early terpene biosynthesis genes PaDXS, PaDXR, PaHMGR and cytochrome P450 monooxygenase genes, PaCYP1 and PaCYP9 were highly upregulated in RB (1:1) and green. The overall results suggest RB (1:1) as the better lighting option amongst the studied spectral lights for obtaining maximum phytochemicals in P. amboinicus. Work is being continued with different spectral ratios of red and blue LED lights to maximize phytochemical accumulation, the outcome of which will be reported elsewhere in near future.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00344-023-11028-6.

Summer dormancy of Myricaria laxiflora to escape flooding stress: Changes in phytohormones and enzymes induced by environmental factors

Dormancy is an adaptation mechanism of plants to environmental stress. Myricaria laxiflora undergoes a long period of flooding stress every year. In order to determine whether this species escapes flooding stress through dormancy, young branches and leaves were collected at different time points before the onset of flooding, and changes in the content/activity of hormones/enzymes that are closely involved in plant growth were monitored. The inducing environmental factors of summer dormancy were identified. The branches and leaves of M. laxiflora showed the following trends as summer flooding approached: (1) gradual increase in the abscisic acid content; (2) gradual decrease in the gibberellin and cytokinin contents; and (3) a continuous decrease in the activities of malate dehydrogenase (MDH), ribulose diphosphate carboxylase (RuBisCo), and glycolate oxidase (GLO). Pearson correlation analysis revealed (1) daylight duration was highly correlated with the hormone content and enzyme activity; (2) the daily mean air temperature (DMAT) was significantly correlated with the cytokinin content. These findings suggest that daylight duration was the main environmental factor leading to changes in the phytohormone content and enzyme activity as well as leading to summer dormancy. M. laxiflora undergoes dormancy before the onset of summer flooding to escape summer flooding stress. Our data indicate that summer flooding does not impede the survival and growth of M. laxiflora.

When to branch: seasonal control of shoot architecture in trees

Long-lived perennial plants optimize their shoot architecture by responding to seasonal cues. The main strategy used by plants of temperate and boreal regions with respect to surviving the extremely unfavourable conditions of winter comprises the protection of their apical and lateral meristematic tissues. This involves myriads of transcriptional, translational and metabolic changes in the plants because shoot architecture is controlled by multiple pathways that regulate processes such as bud formation and flowering, small RNAs, environmental factors (especially light quality, photoperiod and temperature), hormones, and sugars. Recent studies have begun to reveal how these pathways are recruited for the seasonal adaptation and regulation of shoot architecture in perennial plants, including the role of a regulatory module consisting of antagonistic players terminal flower 1 (TFL1) and like-ap1 (LAP1) in the hybrid aspen. Here, we review recent progress in our understanding of the genetic control of shoot architecture in perennials compared to in annuals.

Flow similarity, stochastic branching, and quarter-power scaling in plants

The origin of allometric scaling patterns that are multiples of one-fourth has long fascinated biologists. While not universal, quarter-power scaling relationships are common and have been described in all major clades. Several models have been advanced to explain the origin of such patterns, but questions regarding the discordance between model predictions and empirical data have limited their widespread acceptance. Notable among these is a fractal branching model that predicts power-law scaling of both metabolism and physical dimensions. While a power law is a useful first approximation to some data sets, nonlinear data compilations suggest the possibility of alternative mechanisms. Here, we show that quarter-power scaling can be derived using only the preservation of volume flow rate and velocity as model constraints. Applying our model to land plants, we show that incorporating biomechanical principles and allowing different parts of plant branching networks to be optimized to serve different functions predicts nonlinearity in allometric relationships and helps explain why interspecific scaling exponents covary along a fractal continuum. We also demonstrate that while branching may be a stochastic process, due to the conservation of volume, data may still be consistent with the expectations for a fractal network when one examines sub-trees within a tree. Data from numerous sources at the level of plant shoots, stems, and petioles show strong agreement with our model predictions. This theoretical framework provides an easily testable alternative to current general models of plant metabolic allometry.

Integrated transcriptomic and gibberellin analyses reveal genes related to branch development in Eucalyptus urophylla

Tree branches affect the planting density and basal scab, which act as important attributes in the yield and quality of trees. Eucalyptus urophylla is an important pioneer tree with characteristics of strong adaptability, fast growth, short rotation period, and low disease and pest pressures. In this study, we collected ZQUC14 and LDUD26 clones and compared their transcriptomes and metabolomes from mature xylem, phloem, and developing tissues to identify factors that may influence branch development. In total, 32,809 differentially expressed genes (DEGs) and 18 gibberellin (GA) hormones were detected in the five sampled tissues. Searches of the kyoto Encyclopedia of Genes and Genomes pathways identified mainly genes related to diterpenoid biosynthesis, plant MAPK signaling pathways, plant hormone signal transduction, glycerolipid metabolism, peroxisome, phenylpropanoid biosynthesis, ABC transporters, and brassinosteroid biosynthesis. Furthermore, gene expression trend analysis and weighted gene co-expression network analysis revealed 13 genes likely involved in diterpenoid biosynthesis, including five members of the 2OG-Fe(II) oxygenase superfamily, four cytochrome P450 genes, and four novel genes. In GA signal transduction pathways, 24 DEGs were found to positively regulate branch formation. These results provide a comprehensive analysis of branch development based on the transcriptome and metabolome, and help clarify the molecular mechanisms of E. urophylla.

The Role of Transcription Factors in the Regulation of Plant Shoot Branching

Transcription factors, also known as trans-acting factors, balance development and stress responses in plants. Branching plays an important role in plant morphogenesis and is closely related to plant biomass and crop yield. The apical meristem produced during plant embryonic development repeatedly produces the body of the plant, and the final aerial structure is regulated by the branching mode generated by axillary meristem (AM) activities. These branching patterns are regulated by two processes: AM formation and axillary bud growth. In recent years, transcription factors involved in regulating these processes have been identified. In addition, these transcription factors play an important role in various plant hormone pathways and photoresponses regulating plant branching. In this review, we start from the formation and growth of axillary meristems, including the regulation of hormones, light and other internal and external factors, and focus on the transcription factors involved in regulating plant branching and development to provide candidate genes for improving crop architecture through gene editing or directed breeding.

Involvement of sugar and abscisic acid in the genotype-specific response of rose to far-red light

Plant architecture determines yield (fruit or flowers) and product quality in many horticultural species. It results from growth and branching processes and is dependent on genetic and environmental factors such as light quality. Highly significant genotype and light quality effects and their interaction have been demonstrated on the architecture of rose. Far-red (FR) light is known for its favourable effect on plant growth and development. We evaluated the effect of FR on rose growth and development and its interaction with the genotype through architectural, eco-physiological (net photosynthesis rate) and biochemical (sugar and hormone concentrations) approaches. Two cultivars ('The Fairy' - TF - and Knock Out^® Radrazz - KO) with contrasting architectures were grown in a climate chamber under FR or in the absence of FR at an average photosynthetic photon flux density (400-700 nm) of 181.7 ± 12.8 μmol m^-2 s^-1 for 16 h. A significant effect of FR on the architecture of TF was demonstrated, marked by greater stem elongation, shoot branching and flowering, while KO remained insensitive to FR, supporting a genotype x FR interaction. The response of TF to FR was associated with improved photosynthetic capabilities, while KO exhibited an elevated level of abscisic acid (ABA) in its leaves. FR-dependent ABA accumulation might inhibit photosynthesis and prevent the increased plant carbon status required for growth. From a practical perspective, these findings argue in favour of a better reasoning of the choice of the cultivars grown in lighted production systems. Further investigations will be necessary to better understand these genotype-specific responses to FR and to unravel their molecular determinants.

Genetic Diversity and Genome-Wide Association Study of Architectural Traits of Spray Cut Chrysanthemum Varieties

The architecture of spray cut chrysanthemum is crucial for the quality and quantity of cut flower production. However, the mechanism underlying plant architecture still needs to be clarified. In this study, we measured nine architecture-related traits of 195 spray cut chrysanthemum varieties during a two-year period. The results showed that the number of upper primary branches, number of lateral flower buds and primary branch length widely varied. Additionally, plant height had a significant positive correlation with number of leaf nodes and total number of lateral buds. Number of upper primary branches had a significant negative correlation with primary branch diameter, primary branch angle and primary branch length. Plant height, total number of lateral buds, number of upper primary branches, stem diameter, primary branch diameter and primary branch length were vulnerable to environmental impacts. All varieties could be divided into five categories according to cluster analysis, and the typical plant architecture of the varieties was summarized. Finally, a genome-wide association study (GWAS) was performed to find potential functional genes.

Shoot Branching Phenotyping in Arabidopsis and Tomato

Shoot branching is an important trait that depends on the activity of axillary meristems and buds and their outgrowth into branches. It is remarkably plastic, being influenced by a number of external cues, such as light, temperature, soil nutrients, and mechanical manipulation. These are transduced into an internal hormone signaling network where auxin, cytokinins, and strigolactones play leading regulatory roles. Recently, sugars have also emerged as important signals promoting bud activation. These signals are in part integrated by the bud-specific growth repressor BRANCHED1 (BRC1).To understand how shoot branching is affected by particular growth conditions or in specific plant lines, it is necessary to count the number of branches and/or quantify other branch-related parameters. Here we describe how to perform such quantifications in Arabidopsis and in tomato.

Molecular reprogramming in grapevine woody tissues at bud burst

Perennial woody plants undergo a period of dormancy from the beginning of autumn until the end of spring. Whereas the molecular and physiological events that characterize dormancy release of buds have been described in detail, those occurring in woody tissues underneath the buds are mostly unknown. To bridge this gap, the mRNA populations of cane segments located underneath the bud were analyzed at bud dormancy (E-L 1) and at bud burst (E-L 4). They revealed an important reprogramming of gene expression suggesting that cell division, cell wall metabolism and the mobilization of sugars are the main metabolic and cellular events occurring in cane woody tissues at bud burst. Also, the upregulation of several genes of sugar metabolism, encoding starch- and sucrose-degrading enzymes and sugar transporters, correlates with the decrease in starch and soluble sugars in woody tissues concomitant with increased sucrose synthase and α-amylolytic biochemical activities. The latter is likely due to the VviAMY2 gene that encodes a functional α-amylase as observed after its heterologous expression in yeast. Taken together, these results are consistent with starch and sugar mobilization in canes being primarily involved in grapevine secondary growth initiation and supporting the growth of the emerging bud.

HEXOKINASE1 signalling promotes shoot branching and interacts with cytokinin and strigolactone pathways

Plant architecture is controlled by several endogenous signals including hormones and sugars. However, only little information is known about the nature and roles of the sugar signalling pathways in this process. Here we test whether the sugar signalling pathway mediated by HEXOKINASE1 (HXK1) is involved in the control of shoot branching. To test the involvement of HXK1 in shoot branching and in the hormonal network controlling this process, we modulated the HXK1 pathway using physiological and genetic approaches in rose, pea and arabidopsis. Mannose-induced HXK signalling triggered bud outgrowth in rose and pea. In arabidopsis, both HXK1 deficiency and defoliation led to decreased shoot branching and conferred hypersensitivity to auxin. Complementation of the HXK1 knockout mutant gin2 with a catalytically inactive HXK1, restored shoot branching to the wild-type level. HXK1-deficient plants displayed decreased cytokinin levels and increased expression of MAX2, which is required for strigolactone signalling. The branching phenotype of HXK1-deficient plants could be partly restored by cytokinin treatment and strigolactone deficiency could override the negative impact of HXK1 deficiency on shoot branching. Our observations demonstrate that HXK1 signalling contributes to the regulation of shoot branching and interacts with hormones to modulate plant architecture.

LED Intercanopy Lighting in Blackberry During Spring Improves Yield as a Result of Increased Number of Fruiting Laterals and Has a Positive Carryover Effect on Autumn Yield

High market price and low availability of local winter and spring production has stimulated production of blackberries in glasshouses at northern latitudes. For this production, light is the main limiting factor. We investigated the potential of intercanopy lighting (ICL) using light emitting diodes (LEDs) to improve blackberry fruit yield in a crop with a spring and an autumn production cycle. During the spring production cycle three light treatments were applied: only natural light (no ICL), 93 or 185 μmol m^-2 s^-1 ICL In summer the lateral shoots were cut back and 93 μmol m^-2 s^-1 ICL was applied to all plants after cutting back, investigating a possible carryover effect of supplemental light in spring on autumn production. Fresh fruit yield in spring increased by 79 and 122% with 93 and 185 μmol m^-2 s^-1 ICL, respectively, compared to no ICL. This represents 3.6 and 2.8% increase in harvestable product for every additional 1% of light. A yield component analysis and leaf photosynthesis measurements were conducted. Maximum photosynthetic capacity (A _max) for leaves at 185 μmol m^-2 s^-1 ICL was about 50% higher, and LAI was 41% higher compared to no ICL. ICL increased the number of fruiting laterals per cane, and this explained 75% of the increase in yield. ICL at 185 μmol m^-2 s^-1 resulted in a higher yield compared to no ICL, primarily as a result of higher total dry matter production. Furthermore, a higher fraction of dry matter partitioned to the fruits (0.59 compared to 0.52) contributed to yield increase, whereas fruit dry matter content and fruit quality (sugar and acid content) was not affected by ICL. Averaged over the three light treatments autumn yield was 47% lower than spring yield. Autumn yield was 10% higher for plants at ICL 93 μmol m^-2 s^-1 in spring and 36% higher for plants at 185 μmol m^-2 s^-1 in spring compared to no ICL in spring. This increased autumn yield was caused by more fruiting laterals (less necrotic buds). It is concluded that management practices in spring can have a carryover effect on the autumn production. This is the first scientific paper on the potential for applying LED ICL in blackberries. Further research should focus on optimal intensity of ICL, positioning of supplementary lighting and economic feasibility.

Ascorbate-glutathione pathways mediated by cytokinin regulate H2O2 levels in light-controlled rose bud burst

Rosebush (Rosa "Radrazz") plants are an excellent model to study light control of bud outgrowth since bud outgrowth only arises in the presence of light and never occurs in darkness. Recently, we demonstrated high levels of hydrogen peroxide (H2O2) present in the quiescent axillary buds strongly repress the outgrowth process. In light, the outgrowing process occurred after H2O2 scavenging through the promotion of Ascorbic acid-Glutathione (AsA-GSH)-dependent pathways and the continuous decrease in H2O2 production. Here we showed Respiratory Burst Oxidase Homologs expression decreased in buds during the outgrowth process in light. In continuous darkness, the same decrease was observed although H2O2 remained at high levels in axillary buds, as a consequence of the strong inhibition of AsA-GSH cycle and GSH synthesis preventing the outgrowth process. Cytokinin (CK) application can evoke bud outgrowth in light as well as in continuous darkness. Furthermore, CKs are the initial targets of light in the photocontrol process. We showed CK application to cultured buds in darkness decreases bud H2O2 to a level that is similar to that observed in light. Furthermore, this treatment restores GSH levels and engages bud burst. We treated plants with buthionine sulfoximine, an inhibitor of GSH synthesis, to solve the sequence of events involving H2O2/GSH metabolisms in the photocontrol process. This treatment prevented bud burst, even in the presence of CK, suggesting the sequence of actions starts with the positive CK effect on GSH that in turn stimulates H2O2 scavenging, resulting in initiation of bud outgrowth.

Sucrose promotes stem branching through cytokinin

Shoot branching is an important aspect of plant architecture because it substantially affects plant biology and agricultural performance. Sugars play an important role in the induction of shoot branching in several species, including potato (Solanum tuberosum L.). However, the mechanism by which sugars affect shoot branching remains mostly unknown. In the present study, we addressed this question using sugar-mediated induction of bud outgrowth in potato stems under etiolated conditions. Our results indicate that sucrose feeding to detached stems promotes the accumulation of cytokinin (CK), as well as the expression of vacuolar invertase (VInv), an enzyme that contributes to sugar sink strength. These effects of sucrose were suppressed by CK synthesis and perception inhibitors, while CK supplied to detached stems induced bud outgrowth and VInv activity in the absence of sucrose. CK-induced bud outgrowth was suppressed in vinv mutants, which we generated by genome editing. Altogether, our results identify a branching-promoting module, and suggest that sugar-induced lateral bud outgrowth is in part promoted by the induction of CK-mediated VInv activity.

BcHTT4 Inhibits Branching of Non-Heading Chinese Cabbage at the Vegetative Stage

Branching is speculated to contribute to the plant architecture and crop yield. As a quantitative trait, branching is regulated by multiple genes in non-heading Chinese cabbage (NHCC). Several related candidate genes have been discovered in previous studies on the branching of NHCC, but their specific functions and regulatory mechanisms still need to be verified and explored. In this study, we found that the expression of BcHTT4, the ortholog to HEAT-INDUCED TAS1 TARGET4 (HTT4) in Arabidopsis, was significantly different between ‘Suzhouqing’ (common type) and ‘Maertou’ (multiple shoot branching type) in NHCC, which was consistent with the previous transcriptome sequencing results. The silencing of BcHTT4 expression in non-heading Chinese cabbage promotes axillary bud growth at the vegetative stage. When BcHTT4 is overexpressed in Arabidopsis, branching will decrease. In further study, we found that BcHTT4 interacts with immunophilin BcFKBP13 in vivo and in vitro through yeast two-hybrid analysis and bimolecular fluorescence complementation (BiFC) assays. Moreover, quantitative real-time PCR analysis showed that when the expression of BcHTT4 was silenced in ‘Suzhouqing’, the expression of BcFKBP13 also decreased significantly. Our findings reveal that BcHTT4 is involved in the branching mechanism and interacts with immunophilin BcFKBP13 in NHCC.

Photocontrol of Axillary Bud Outgrowth by MicroRNAs: Current State-of-the-Art and Novel Perspectives Gained From the Rosebush Model

Shoot branching is highly dependent on environmental factors. While many species show some light dependence for branching, the rosebush shows a strict requirement for light to allow branching, making this species an excellent model to further understand how light impinges on branching. Here, in the first part, we provide a review of the current understanding of how light may modulate the complex regulatory network of endogenous factors like hormones (SL, IAA, CK, GA, and ABA), nutrients (sugar and nitrogen), and ROS to control branching. We review the regulatory contribution of microRNAs (miRNAs) to branching in different species, highlighting the action of such evolutionarily conserved factors. We underline some possible pathways by which light may modulate miRNA-dependent regulation of branching. In the second part, we exploit the strict light dependence of rosebush for branching to identify putative miRNAs that could contribute to the photocontrol of branching. For this, we first performed a profiling of the miRNAs expressed in early light-induced rosebush buds and next tested whether they were predicted to target recognized regulators of branching. Thus, we identified seven miRNAs (miR156, miR159, miR164, miR166, miR399, miR477, and miR8175) that could target nine genes (CKX1/6, EXPA3, MAX4, CYCD3;1, SUSY, 6PFK, APX1, and RBOHB1). Because these genes are affecting branching through different hormonal or metabolic pathways and because expression of some of these genes is photoregulated, our bioinformatic analysis suggests that miRNAs may trigger a rearrangement of the regulatory network to modulate branching in response to light environment.

Photoselective Protective Netting Improves "Honeycrisp" Fruit Quality

High temperatures, wind, and excessive sunlight can negatively impact yield and fruit quality in semi-arid apple production regions. Netting was originally designed for hail protection, but it can modify the light spectrum and affect fruit quality. Here, pearl, blue, and red photoselective netting (≈20% shading factor) was installed in 2015 over a commercial “Cameron Select^® Honeycrisp” orchard. Our research objectives were to (1) describe the light quantity and quality under the colored nets compared to an uncovered control and (2) investigate the effect of Photoselective nets on “Honeycrisp” apple quality for two growing seasons. Light transmittance and scattering for each treatment were measured with a spectroradiometer, and samples for fruit quality analyses were collected at harvest. PAR (photosynthetic active radiation), UV, blue, red, and far-red light were lower underneath all netting treatments compared to an uncovered control. The scattered light was higher under the pearl net compared to other colors, while red and far-red light were lower under the blue net. For two consecutive years, trees grown under the photoselective nets intercepted more incoming light than the uncovered trees with no differences among the three colors. In both years, trees under red and blue nets had more sunburn-free (clean) apples than pearl and control. Red color development for fruit was lower when nets were used. Interestingly, bitter pit incidence was lower underneath red nets for both years. Other than red color development, “Honeycrisp” fruit quality was not appreciably affected by the use of netting. These results highlight the beneficial effect of nets in improving light quality in orchards and mitigating physiological disorders such as bitter pit in “Honeycrisp” apple.

Regulation of ARGONAUTE10 Expression Enables Temporal and Spatial Precision in Axillary Meristem Initiation in Arabidopsis

Axillary meristems (AMs) give rise to lateral shoots and are critical to plant architecture. Understanding how developmental cues and environmental signals impact AM development will enable the improvement of plant architecture in agriculture. Here, we show that ARGONAUTE10 (AGO10), which sequesters miR165/166, promotes AM development through the miR165/166 target gene REVOLUTA. We reveal that AGO10 expression is precisely controlled temporally and spatially by auxin, brassinosteroids, and light to result in AM initiation only in the axils of leaves at a certain age. AUXIN RESPONSE FACTOR 5 (ARF5) activates while BRASSINAZOLE-RESISTANT 1 (BZR1) and PHYTOCHROME-INTERACTING FACTOR 4 (PIF4) repress AGO10 transcription directly. In axils of young leaves, BZR1 and PIF4 repress AGO10 expression to prevent AM initiation. In axils of older leaves, ARF5 upregulates AGO10 expression to promote AM initiation. Our results uncover the spatiotemporal control of AM development through the cooperation of hormones and light converging on a regulator of microRNA.

Proteomic analysis of a clavata-like phenotype mutant in Brassica napus

Rapeseed is one of important oil crops in China. Better understanding of the regulation network of main agronomic traits of rapeseed could improve the yielding of rapeseed. In this study, we obtained an influrescence mutant that showed a fusion phenotype, similar with the Arabidopsis clavata-like phenotype, so we named the mutant as Bnclavata-like (Bnclv-like). Phenotype analysis illustrated that abnormal development of the inflorescence meristem (IM) led to the fused-inflorescence phenotype. At the stage of protein abundance, major regulators in metabolic processes, ROS metabolism, and cytoskeleton formation were seen to be altered in this mutant. These results not only revealed the relationship between biological processes and inflorescence meristem development, but also suggest bioengineering strategies for the improved breeding and production of Brassica napus.

The initiation of bud burst in grapevine features dynamic regulation of the apoplastic pore size

The physiological constraints on bud burst in woody perennials, including vascular development and oxygenation, remain unresolved. Both light and tissue oxygen status have emerged as important cues for vascular development in other systems; however, grapevine buds have only a facultative light requirement, and data on the tissue oxygen status have been confounded by the spatial variability within the bud. Here, we analysed apoplastic development at early stages of grapevine bud burst and combined molecular modelling with histochemical techniques to determine the pore size of cell walls in grapevine buds. The data demonstrate that quiescent grapevine buds were impermeable to apoplastic dyes (acid fuchsin and eosin Y) until after bud burst was established. The molecular exclusion size was calculated to be 2.1 nm, which would exclude most macromolecules except simple sugars and phytohormones until after bud burst. We used micro-computed tomography to demonstrate that tissue oxygen partial pressure data correlated well with structural heterogeneity of the bud and differences in tissue density, confirming that the primary bud complex becomes rapidly and preferentially oxygenated during bud burst. Taken together, our results reveal that the apoplastic porosity is highly regulated during the early stages of bud burst, suggesting a role for vascular development in the initial, rapid oxygenation of the primary bud complex.

Light Regulation of Axillary Bud Outgrowth Along Plant Axes: An Overview of the Roles of Sugars and Hormones

Apical dominance, the process by which the growing apical zone of the shoot inhibits bud outgrowth, involves an intricate network of several signals in the shoot. Auxin originating from plant apical region inhibits bud outgrowth indirectly. This inhibition is in particular mediated by cytokinins and strigolactones, which move from the stem to the bud and that respectively stimulate and repress bud outgrowth. The action of this hormonal network is itself modulated by sugar levels as competition for sugars, caused by the growing apical sugar sink, may deprive buds from sugars and prevents bud outgrowth partly by their signaling role. In this review, we analyze recent findings on the interaction between light, in terms of quantity and quality, and apical dominance regulation. Depending on growth conditions, light may trigger different pathways of the apical dominance regulatory network. Studies pinpoint to the key role of shoot-located cytokinin synthesis for light intensity and abscisic acid synthesis in the bud for R:FR in the regulation of bud outgrowth by light. Our analysis provides three major research lines to get a more comprehensive understanding of light effects on bud outgrowth. This would undoubtedly benefit from the use of computer modeling associated with experimental observations to deal with a regulatory system that involves several interacting signals, feedbacks, and quantitative effects.

Functional analysis of the GbDWARF14 gene associated with branching development in cotton

Plant architecture, including branching pattern, is an important agronomic trait of cotton crops. In recent years, strigolactones (SLs) have been considered important plant hormones that regulate branch development. In some species such as Arabidopsis, DWARF14 is an unconventional receptor that plays an important role in the SL signaling pathway. However, studies on SL receptors in cotton are still lacking. Here, we cloned and analysed the structure of the GbD14 gene in Gossypium barbadense and found that it contains the domains necessary for a SL receptor. The GbD14 gene was expressed primarily in the roots, leaves and vascular bundles, and the GbD14 protein was determined via GFP to localize to the cytoplasm and nucleus. Gene expression analysis revealed that the GbD14 gene not only responded to SL signals but also was differentially expressed between cotton plants whose types of branching differed. In particular, GbD14 was expressed mainly in the axillary buds of normal-branching cotton, while it was expressed the most in the leaves of nulliplex-branch cotton. In cotton, the GbD14 gene can be induced by SL and other plant hormones, such as indoleacetic acid, abscisic acid, and jasmonic acid. Compared with wild-type Arabidopsis, GbD14-overexpressing Arabidopsis responded more rapidly to SL signals. Moreover, we also found that GbD14 can rescue the multi-branched phenotype of Arabidopsis Atd14 mutants. Our results indicate that the function of GbD14 is similar to that of AtD14, and GbD14 may be a receptor for SL in cotton and involved in regulating branch development. This research provides a theoretical basis for a profound understanding of the molecular mechanism of branch development and ideal plant architecture for cotton breeding improvements.

Effects of Continuous or End-of-Day Far-Red Light on Tomato Plant Growth, Morphology, Light Absorption, and Fruit Production

Shading by sunlit leaves causes a low red (R) to far-red (FR) ratio that results in a low phytochrome stationary state (PSS). A low PSS induces an array of shade avoidance responses that influence plant architecture and development. It has often been suggested that this architectural response is advantageous for plant growth due to its positive effect on light interception. In contrast to sunlight, artificial light sources such as LEDs often lack FR, resulting in a PSS value higher than solar light (∼0.70). The aim of this study was to investigate how PSS values higher than solar radiation influence the growth and development of tomato plants. Additionally, we investigated whether a short period of FR at the end of the day (EOD-FR) could counteract any potentially negative effects caused by a lack of FR during the day. Tomato plants were grown at four PSS levels (0.70, 0.73, 0.80, and 0.88), or with a 15-min end-of-day far-red (EOD-FR) application (PSS 0.10). Photosynthetic Active Radiation (PAR; 150 μmol m-2 s-1) was supplied using red and blue (95/5%) LEDs. In an additional experiment, the same treatments were applied to plants receiving supplementary low-intensity solar light. Increasing PSS above solar PSS resulted in increased plant height. Leaf area and plant dry mass were lower in the treatments completely lacking FR than treatments with FR. EOD-FR-treated plants responded almost similarly to plants grown without FR, except for plant height, which was increased. Simulations with a 3D-model for light absorption revealed that the increase in dry mass was mainly related to an increase in light absorption due to a higher total leaf area. Increased petiole angle and internode length had a negative influence on total light absorption. Additionally, the treatments without FR and the EOD-FR showed strongly reduced fruit production due to reduced fruit growth associated with reduced source strength and delayed flowering. We conclude that growing tomato plants under artificial light without FR during the light period causes a range of inverse shade avoidance responses, which result in reduced plant source strength and reduced fruit production, which cannot be compensated by a simple EOD-FR treatment.

An Update on the Signals Controlling Shoot Branching

Many new questions on the regulation of shoot branching have been raised in recent years, prompting a review and reassessment of the role of each signal involved. Sugars and their signaling networks have been attributed a major role in the early events of axillary bud outgrowth, whereas cytokinin appears to play a critical role in the modulation of this process in response to the environment. Perception of the recently discovered hormone strigolactone is now quite well understood, while the downstream targets remain largely unknown. Recent literature has highlighted that auxin export from a bud is important for its subsequent growth.

BRANCHED1: A Key Hub of Shoot Branching

Shoot branching is a key process for plant growth and fitness. Newly produced axes result from axillary bud outgrowth, which is at least partly mediated through the regulation of BRANCHED1 gene expression (BRC1/TB1/FC1). BRC1 encodes a pivotal bud-outgrowth-inhibiting transcription factor belonging to the TCP family. As the regulation of BRC1 expression is a hub for many shoot-branching-related mechanisms, it is influenced by endogenous (phytohormones and nutrients) and exogenous (light) inputs, which involve so-far only partly identified molecular networks. This review highlights the central role of BRC1 in shoot branching and its responsiveness to different stimuli, and emphasizes the different knowledge gaps that should be addressed in the near future.

Using light to improve commercial value

The plasticity of plant morphology has evolved to maximize reproductive fitness in response to prevailing environmental conditions. Leaf architecture elaborates to maximize light harvesting, while the transition to flowering can either be accelerated or delayed to improve an individual's fitness. One of the most important environmental signals is light, with plants using light for both photosynthesis and as an environmental signal. Plants perceive different wavelengths of light using distinct photoreceptors. Recent advances in LED technology now enable light quality to be manipulated at a commercial scale, and as such opportunities now exist to take advantage of plants' developmental plasticity to enhance crop yield and quality through precise manipulation of a crops' lighting regime. This review will discuss how plants perceive and respond to light, and consider how these specific signaling pathways can be manipulated to improve crop yield and quality.

ScGAI is a key regulator of culm development in sugarcane

Sugarcane contributes more than 70% of sugar production and is the second largest feedstock for ethanol production globally. Since sugar accumulates in sugarcane culms, culm biomass and sucrose content are the most commercially important traits. Despite extensive breeding, progress in both cane yield and sugar content remains very slow in most countries. We hypothesize that manipulating the genetic elements controlling culm growth will alter source-sink regulation and help break down the yield barriers. In this study, we investigate the role of sugarcane ScGAI, an ortholog of SLR1/D8/RHT1/GAI, on culm development and source-sink regulation through a combination of molecular techniques and transgenic strategies. We show that ScGAI is a key molecular regulator of culm growth and development. Changing ScGAI activity created substantial culm growth and carbon allocation changes for structural molecules and storage. ScGAI regulates spatio-temporal growth of sugarcane culm and leaf by interacting with ScPIF3/PIF4 and ethylene signaling elements ScEIN3/ScEIL1, and its action appears to be regulated by SUMOylation in leaf but not in the culm. Collectively, the remarkable culm growth variation observed suggests that ScGAI could be used as an effective molecular breeding target for breaking the slow yield gain in sugarcane.

Developmental control of hypoxia during bud burst in grapevine

Dormant or quiescent buds of woody perennials are often dense and in the case of grapevine (Vitis vinifera L.) have a low tissue oxygen status. The precise timing of the decision to resume growth is difficult to predict, but once committed, the increase in tissue oxygen status is rapid and developmentally regulated. Here, we show that more than a third of the grapevine homologues of widely conserved hypoxia-responsive genes and nearly a fifth of all grapevine genes possessing a plant hypoxia-responsive promoter element were differentially regulated during bud burst, in apparent harmony with resumption of meristem identity and cell-cycle gene regulation. We then investigated the molecular and biochemical properties of the grapevine ERF-VII homologues, which in other species are oxygen labile and function in transcriptional regulation of hypoxia-responsive genes. Each of the 3 VvERF-VIIs were substrates for oxygen-dependent proteolysis in vitro, as a function of the N-terminal cysteine. Collectively, these data support an important developmental function of oxygen-dependent signalling in determining the timing and effective coordination bud burst in grapevine. In addition, novel regulators, including GASA-, TCP-, MYB3R-, PLT-, and WUS-like transcription factors, were identified as hallmarks of the orderly and functional resumption of growth following quiescence in buds.

Asparagine and sugars are both required to sustain secondary axis elongation after bud outgrowth in Rosa hybrida

Nitrogen is required for optimal plant growth, especially in young organs such as secondary axes (axes II) after axillary bud outgrowth. Several studies have shown an increase of nitrogen concentration in xylem sap concomitantly with bud outgrowth, but the relation between nitrogen, sugars and plant hormones in axis II still remains unclear. We investigated in Rosa hybrida the involvement of nitrogen nutrition in axis II elongation in relation with sugars and cytokinins using ¹⁵N-labeled nitrate and sugars, amino acids and cytokinin quantifications. Besides, we measured the effect of the exogenous supply of these compounds on axis II elongation using in vitro excised bud culture. We demonstrated that nitrogen in the axis II comes mainly from new root uptake after decapitation. Asparagine, which concentration increases in sap exudates and tissues during axis II elongation, was the sole amino acid able to sustain an efficient elongation in vitro when supplied in combination with sucrose.

Integrative models for joint analysis of shoot growth and branching patterns

Plants exhibit dependences between shoot growth and branching that generate highly structured patterns. The characterization of the patterning mechanism is still an open issue because of the developmental processes involved with both succession of events (e.g. internode elongation, axillary shoot initiation and elongation) and complex dependences among neighbouring positions along the parent shoot. Statistical models called semi-Markov switching partitioned conditional generalized linear models were built on the basis of apple and pear tree datasets. In these models, the semi-Markov chain represents both the succession and lengths of branching zones, whereas the partitioned conditional generalized linear models represent the influence of parent shoot growth variables on axillary productions within each branching zone. Parent shoot growth variables were shown to influence specific developmental events. On this basis, the growth and branching patterns of two apple tree (Malus domestica) cultivars, as well as of pear trees (Pyrus spinosa) between two successive growing cycles, were compared. The proposed integrative statistical models were able to decipher the roles of successive developmental events in the growth and branching patterning mechanisms. These models could incorporate other parent shoot explanatory variables, such as the local curvature or the maximum growth rate of the leaf.

Cell cycle arrest in plants: what distinguishes quiescence, dormancy and differentiated G1?

BACKGROUND

Quiescence is a fundamental feature of plant life, which enables plasticity, renewal and fidelity of the somatic cell line. Cellular quiescence is defined by arrest in a particular phase of the cell cycle, typically G1 or G2; however, the regulation of quiescence and proliferation can also be considered across wider scales in space and time. As such, quiescence is a defining feature of plant development and phenology, from meristematic stem cell progenitors to terminally differentiated cells, as well as dormant or suppressed seeds and buds. While the physiology of each of these states differs considerably, each is referred to as 'cell cycle arrest' or 'G1 arrest'.

SCOPE

Here the physiology and molecular regulation of (1) meristematic quiescence, (2) dormancy and (3) terminal differentiation (cell cycle exit) are considered in order to determine whether and how the molecular decisions guiding these nuclear states are distinct. A brief overview of the canonical cell cycle regulators is provided, and the genetic and genomic, as well as physiological, evidence is considered regarding two primary questions: (1) Are the canonical cell cycle regulators superior or subordinate in the regulation of quiescence? (2) Are these three modes of quiescence governed by distinct molecular controls?

CONCLUSION

Meristematic quiescence, dormancy and terminal differentiation are each predominantly characterized by G1 arrest but regulated distinctly, at a level largely superior to the canonical cell cycle. Meristematic quiescence is intrinsically linked to non-cell-autonomous regulation of meristem cell identity, and particularly through the influence of ubiquitin-dependent proteolysis, in partnership with reactive oxygen species, abscisic acid and auxin. The regulation of terminal differentiation shares analogous features with meristematic quiescence, albeit with specific activators and a greater role for cytokinin signalling. Dormancy meanwhile appears to be regulated at the level of chromatin accessibility, by Polycomb group-type histone modifications of particular dormancy genes.

Cytokinins and Abscisic Acid Act Antagonistically in the Regulation of the Bud Outgrowth Pattern by Light Intensity

Bud outgrowth is a key process in the elaboration of yield and visual quality in rose crops. Although light intensity is well known to affect bud outgrowth, little is known on the mechanisms involved in this regulation. The objective of this work was to test if the control of bud outgrowth pattern along the stem by photosynthetic photon flux density (PPFD) is mediated by sugars, cytokinins and/or abscisic acid in intact rose plants. Rooted cuttings of Rosa hybrida 'Radrazz' were grown in growth chambers under high PPFD (530 μmol m-2 s-1) until the floral bud visible stage. Plants were then either placed under low PPFD (90 μmol m-2 s-1) or maintained under high PPFD. Bud outgrowth inhibition by low PPFD was associated with lower cytokinin and sugar contents and a higher abscisic acid content in the stem. Interestingly, cytokinin supply to the stem restored bud outgrowth under low PPFD. On the other hand, abscisic acid supply inhibited outgrowth under high PPFD and antagonized bud outgrowth stimulation by cytokinins under low PPFD. In contrast, application of sugars did not restore bud outgrowth under low PPFD. These results suggest that PPFD regulation of bud outgrowth in rose involves a signaling pathway in which cytokinins and abscisic acid play antagonistic roles. Sugars can act as nutritional and signaling compounds and may be involved too, but do not appear as the main regulator of the response to PPFD.

Comparative transcriptome analysis of axillary buds in response to the shoot branching regulators gibberellin A3 and 6-benzyladenine in Jatropha curcas

Cytokinin (CK) is the primary hormone that positively regulates axillary bud outgrowth. However, in many woody plants, such as Jatropha curcas, gibberellin (GA) also promotes shoot branching. The molecular mechanisms underlying GA and CK interaction in the regulation of bud outgrowth in Jatropha remain unclear. To determine how young axillary buds respond to GA₃ and 6-benzyladenine (BA), we performed a comparative transcriptome analysis of the young axillary buds of Jatropha seedlings treated with GA₃ or BA. Two hundred and fifty genes were identified to be co-regulated in response to GA₃ or BA. Seven NAC family members were down-regulated after treatment with both GA₃ and BA, whereas these genes were up-regulated after treatment with the shoot branching inhibitor strigolactone. The expressions of the cell cycle genes CDC6, CDC45 and GRF5 were up-regulated after treatment with both GA₃ and BA, suggesting they may promote bud outgrowth via regulation of the cell cycle machinery. In the axillary buds, BA significantly increased the expression of GA biosynthesis genes JcGA20oxs and JcGA3ox1, and down-regulated the expression of GA degradation genes JcGA2oxs. Overall, the comprehensive transcriptome data set provides novel insight into the responses of young axillary buds to GA and CK.

Discrimination of wild types and hybrids of Duboisia myoporoides and Duboisia leichhardtii at different growth stages using 1H NMR-based metabolite profiling and tropane alkaloids-targeted HPLC-MS analysis

Duboisia species, which belong to the family of Solanaceae, are commercially cultivated in large scale, as they are main source of the pharmaceutically-used active compound scopolamine. In this study, ¹H NMR-based metabolite profiling linking primary with secondary metabolism and additional quantification via HPCL-MS with special focus on the tropane alkaloids were applied to compare leaf and root extracts of three wild types and two hybrids of Duboisia myoporoides and D. leichhardtii at different developmental stages grown under controlled conditions in climate chambers and under agricultural field plantation. Based on the leaf extracts, a clear distinction between the Duboisia hybrids and the wild types Duboisia myoporoides and D. leichhardtii using principal component analysis of ¹H NMR data was observed. The average content in scopolamine in the hybrids of Duboisia cultivated in climate chambers increased significantly from month 3-6 after potting of the rooted cuttings, however not so for the examined wild types. The Duboisia hybrids grown in climate chambers showed higher growth and contained more sugars and amino acids than Duboisia hybrids grown in the field, which in contrast showed an enhanced flux towards tropane alkaloids as well as flavonoids. For a more detailed analysis of tropane alkaloids, an appropriate HPLC-MS method was developed and validated. The measurements revealed large differences in the alkaloid pattern within the different genotypes under investigation, especially regarding the last enzymatic step, the conversion from hyoscamine to scopolamine by the hyoscyamine 6β-hydroxylase. Scopolamine was found in highest concentrations in Duboisia hybrids (20.04 ± 4.05 and 17.82 ± 3.52 mg/g dry wt) followed by Duboisia myoporoides (12.71 ± 2.55 mg/g dry wt), both showing a high selectivity for scopolamine in contrast to Duboisia leichhardtii (3.38 ± 0.59 and 5.09 ± 1.24 mg/g dry wt) with hyoscyamine being the predominant alkaloid.

Cytokinins Are Initial Targets of Light in the Control of Bud Outgrowth

Bud outgrowth is controlled by environmental and endogenous factors. Through the use of the photosynthesis inhibitor norflurazon and of masking experiments, evidence is given here that light acts mainly as a morphogenic signal in the triggering of bud outgrowth and that initial steps in the light signaling pathway involve cytokinins (CKs). Indeed, in rose (Rosa hybrida), inhibition of bud outgrowth by darkness is suppressed solely by the application of CKs. In contrast, application of sugars has a limited effect. Exposure of plants to white light (WL) induces a rapid (after 3-6 h of WL exposure) up-regulation of CK synthesis (RhIPT3 and RhIPT5), of CK activation (RhLOG8), and of CK putative transporter RhPUP5 genes and to the repression of the CK degradation RhCKX1 gene in the node. This leads to the accumulation of CKs in the node within 6 h and in the bud at 24 h and to the triggering of bud outgrowth. Molecular analysis of genes involved in major mechanisms of bud outgrowth (strigolactone signaling [RwMAX2], metabolism and transport of auxin [RhPIN1, RhYUC1, and RhTAR1], regulation of sugar sink strength [RhVI, RhSUSY, RhSUC2, and RhSWEET10], and cell division and expansion [RhEXP and RhPCNA]) reveal that, when supplied in darkness, CKs up-regulate their expression as rapidly and as intensely as WL Additionally, up-regulation of CKs by WL promotes xylem flux toward the bud, as evidenced by Methylene Blue accumulation in the bud after CK treatment in the dark. Altogether, these results suggest that CKs are initial components of the light signaling pathway that controls the initiation of bud outgrowth.

Ready, steady, go! A sugar hit starts the race to shoot branching

In the classical theory of apical dominance, auxin depletion from the stem releases bud dormancy. Recent studies have revealed a poor correlation between the initial bud release and auxin depletion from the stem after decapitation. Sucrose mobility in plants and its accumulation in buds correlates well with the onset of bud release and is able to trigger bud outgrowth. The diversion of sugars away from axillary buds decreases bud release even where hormones are at levels generally considered conducive to bud release. This impact of sugars on bud outgrowth may be mediated by specific sugar and hormonal signalling pathways.

Genotypic differences in architectural and physiological responses to water restriction in rose bush

The shape and, therefore, the architecture of the plant are dependent on genetic and environmental factors such as water supply. The architecture determines the visual quality, a key criterion underlying the decision to purchase an ornamental potted plant. The aim of this study was to analyze genotypic responses of eight rose bush cultivars to alternation of water restriction and re-watering periods, with soil water potential of -20 and -10 kPa respectively. Responses were evaluated at the architectural level through 3D digitalization using six architectural variables and at the physiological level by measuring stomatal conductance, water content, hormones [abscisic acid (ABA), auxin, cytokinins, jasmonic acid, and salicylic acid (SA)], sugars (sucrose, fructose, and glucose), and proline. Highly significant genotype and watering effects were revealed for all the architectural variables measured, as well as genotype × watering interaction, with three distinct genotypic architectural responses to water restriction - weak, moderate and strong - represented by Hw336, 'Baipome' and 'The Fairy,' respectively. The physiological analysis explained, at least in part, the more moderate architectural response of 'Baipome' compared to 'The Fairy,' but not that of Hw336 which is an interspecific hybrid. Such physiological responses in 'Baipome' could be related to: (i) the maintenance of the stimulation of budbreak and photosynthetic activity during water restriction periods due to a higher concentration in conjugated cytokinins (cCK) and to a lower concentration in SA; (ii) a better resumption of budbreak during the re-watering periods due to a lower concentration in ABA during this period. When associated with the six architectural descriptors, cCK, SA and ABA, which explained the genotypic differences in this study, could be used as selection criteria for breeding programs aimed at improving plant shape and tolerance to water restriction.

Multiple pathways regulate shoot branching

Shoot branching patterns result from the spatio-temporal regulation of axillary bud outgrowth. Numerous endogenous, developmental and environmental factors are integrated at the bud and plant levels to determine numbers of growing shoots. Multiple pathways that converge to common integrators are most probably involved. We propose several pathways involving not only the classical hormones auxin, cytokinins and strigolactones, but also other signals with a strong influence on shoot branching such as gibberellins, sugars or molecular actors of plant phase transition. We also deal with recent findings about the molecular mechanisms and the pathway involved in the response to shade as an example of an environmental signal controlling branching. We propose the TEOSINTE BRANCHED1, CYCLOIDEA, PCF transcription factor TB1/BRC1 and the polar auxin transport stream in the stem as possible integrators of these pathways. We finally discuss how modeling can help to represent this highly dynamic system by articulating knowledges and hypothesis and calculating the phenotype properties they imply.