Development of aerial and belowground tubers in potato is governed by photoperiod and epigenetic mechanism.

Plants exhibit diverse developmental plasticity and modulate growth responses under various environmental conditions. Potato (Solanum tuberosum), a modified stem and an important food crop, serves as a substantial portion of the world's subsistence food supply. In the past two decades, crucial molecular signals have been identified that govern the tuberization (potato development) mechanism. Interestingly, microRNA156 overexpression in potato provided the first evidence for induction of profuse aerial stolons and tubers from axillary meristems under short-day (SD) photoperiod. A similar phenotype was noticed for overexpression of epigenetic modifiers-MUTICOPY SUPRESSOR OF IRA1 (StMSI1) or ENAHNCER OF ZESTE 2 (StE[z]2), and knockdown of B-CELL-SPECIFIC MOLONEY MURINE LEUKEMIA VIRUS INTEGRATION SITE 1 (StBMI1). This striking phenotype represents a classic example of modulation of plant architecture and developmental plasticity. Differentiation of a stolon to a tuber or a shoot under in vitro or in vivo conditions symbolizes another example of organ-level plasticity and dual fate acquisition in potato. Stolon-to-tuber transition is governed by SD photoperiod, mobile RNAs/proteins, phytohormones, a plethora of small RNAs and their targets. Recent studies show that polycomb group proteins control microRNA156, phytohormone metabolism/transport/signaling and key tuberization genes through histone modifications to govern tuber development. Our comparative analysis of differentially expressed genes between the overexpression lines of StMSI1, StBEL5 (BEL1-LIKE transcription factor [TF]), and POTATO HOMEOBOX 15 TF revealed more than 1,000 common genes, indicative of a mutual gene regulatory network potentially involved in the formation of aerial and belowground tubers. In this review, in addition to key tuberization factors, we highlight the role of photoperiod and epigenetic mechanism that regulates the development of aerial and belowground tubers in potato.

Quantifying risk factors associated with light-induced potato tuber greening in retail stores.

Light conditions in retail stores may contribute to potato greening. In this study, we aimed to develop a potato tuber greening risk rating model for retail stores based on light quality and intensity parameters. This was achieved by firstly exposing three potato varieties (Nicola, Maranca and Kennebec) to seven specific light wavelengths (370, 420, 450, 530, 630, 660 and 735 nm) to determine the tuber greening propensity. Detailed light quality and intensity measurements from 25 retail stores were then combined with the greening propensity data to develop a tuber greening risk rating model. Our study showed that maximum greening occurred under blue light (450 nm), while 53%, 65% and 75% less occurred under green (530 nm), red (660 nm) and orange (630 nm) light, respectively. Greening risk, which varied between stores, was found to be related to light intensity level, and partially explained potato stock loss in stores. Our results from this study suggested that other in-store management practices, including lighting duration, average potato turnover, and light protection during non-retail periods, likely influence tuber greening risk.

An analysis of cellulose- and dextrose-based radicals in sweet potatoes as irradiation markers.

Dried sweet potatoes (SPs) are often irradiated for improved safety and shelf life. Formation of irradiation-derived radicals was analyzed using electron paramagnetic resonance (EPR) spectroscopy. These irradiation-specific radicals can be used to characterize the irradiation history of dry plant-based foods containing cellulose and sugars. The signal characteristics (intensity and peak shape) were evaluated at different sample locations (skin and flesh), as a function of sample preparation method (grinding, sieving, and pelletizing). The signal intensity was quantified using a double integration method of the peaks based on the area under the curve. The sieving caused ca. 50% decrease in total signal intensity as compared to nonsieved samples due to loss of cellulose-based radicals. The flesh of irradiated SP showed complex EPR spectra with multiple satellite peaks of cellulose radicals (333.5 and 338.8 mT) and split peak of dextrose radicals (337.4 mT); while skin spectra were distinctive of cellulose radicals. In this study, we demonstrated the effects of sample composition and preparation method on formation and analysis of irradiation-specific radicals based on EPR. PRACTICAL APPLICATION: In the last decade or so, there have been health concerns related to the consumption of irradiated pet food products. Electron paramagnetic resonance spectroscopy can be used to analyze the irradiation history of dry products containing cellulose and sugar, such as the popular dog treat dried sweet potatoes, to ensure the products were irradiated within safe limits. This work demonstrates that the formation of irradiation-specific radicals is affected by the sample location (skin and flesh) and moisture content.

Effects of low doses of UV-B radiation supplementation on tuber quality in purple potato (Solanum tuberosum L.).

UV-B is an important environmental factor that differentially affects plant growth and secondary metabolites. However, our knowledge regarding the physiological and biochemical changes in under-ground plant organs responded to UV-B treatment remains limited. In this study, we investigated potato plant (Solanum tuberosum L.) and tuber responses to short-term supplemental UV-B exposure performed during tuber development. Our results indicated that the supplemental UV-B radiation with relative low dose had no obvious adverse impact on plant growth or tuber production. Nutritional composition analyses of tubers revealed that the contents of starch, soluble sugars, and proteins were significantly increased under lower UV-B radiation relative to controls. Similarly, low dose of UV-B treatment promoted the health-promoting compounds, including anthocyanin, phenols, and flavonoids in tubers. Moreover, higher activities of antioxidant enzymes were significantly induced in tubers in response to lower UV-B radiation. These findings suggest that short-term UV-B radiation supplementation at relative low doses can improve the tuber quality in potato plants.

Suberin deposition in potato periderm: a novel resistance mechanism against tuber greening.

Light-induced tuber greening is one of the most important quality defects of potato. Although varietal and maturity factors are known to affect greening resistance, physiological mechanisms of resistance are poorly understood. We proposed that physiological and biochemical factors within the tuber periderm provide resistance and hypothesised that resistance is primarily related to suberin content. We investigated differences in the tuber periderm between genotypes and tuber maturities that varied in greening propensity. We examined suberin and light-induced pigment accumulation, and phellem cell development and studied greening propensity in mutant and chemically treated tubers with enhanced suberisation. Resistance to greening was strongly linked to increased suberin in the periderm, which varied with variety and tuber maturity. Furthermore, greening was reduced in mutant and chemically treated tubers with enhanced suberisation. Increases in phellem cell layers and light-induced carotenoids and anthocyanins were identified as secondary resistance factors. Our work represents the first physiological mechanism of varietal and tuber maturity resistance to greening, expanding the known functionality of suberin and providing for the first time a biomarker that will aid producers and breeders in selection and improvement of potato varieties for greening resistance.

RNA Sequencing Reveals That Both Abiotic and Biotic Stress-Responsive Genes are Induced during Expression of Steroidal Glycoalkaloid in Potato Tuber Subjected to Light Exposure.

Steroidal glycoalkaloids (SGAs), which are widely produced by potato, even in other Solanaceae plants, are a class of potentially toxic compounds, but are beneficial to host resistance. However, changes of the other metabolic process along with SGA accumulation are still poorly understood and researched. Based on RNA sequencing (RNA-seq) and bioinformatics analysis, the global gene expression profiles of potato variety Helan 15 (Favorita) was investigated at four-time points during light exposure. The data was further verified by using quantitative Real-time PCR (qRT-PCR). When compared to the control group, 1288, 1592, 1737, and 1870 differentially expressed genes (DEGs) were detected at 6 h, 24 h, 48 h, and 8 d, respectively. The results of both RNAseq and qRT-PCR showed that SGA biosynthetic genes were up-regulated in the potato tuber under light exposure. Functional enrichment analysis revealed that genes related to PS light reaction and Protein degradation were significantly enriched in most time points of light exposure. Additionally, enriched Bins included Receptor kinases, Secondary metabolic process in flavonoids, Abiotic stress, and Biotic stress in the early stage of light exposure, but PS Calvin cycle, RNA regulation of transcription, and UDP glucosyl and glucoronyl transferases in the later stage. Most of the DEGs involved in PS light reaction and Abiotic stress were up-regulated at all four time points, whereas DEGs that participated in biotic stresses were mainly up-regulated at the later stage (48 h and 8 d). Cis-element prediction and co-expression assay were used to confirm the expressional correlation between genes that are responsible for SGA biosynthesis and disease resistance. In conclusion, the expressions of genes involved in PS light reaction, Abiotic stress, and Biotic stress were obviously aroused during the accumulation of SGAs induced by light exposure. Moreover, an increased defense response might contribute to the potato resistance to the infection by phytopathogenic microorganisms.

Effect of infrared drying with multifrequency ultrasound pretreatments on the stability of phytochemical properties, antioxidant potential, and textural quality of dried sweet potatoes.

The study aims to determine drying of sweet potatoes using multifrequency ultrasound (US) pretreatments (20, 40, and 60 kHz) at three different infrared (IR) drying temperatures (60, 70, and 80°C) and evaluate the phytochemical and textural quality of the dried product. Drying time was significantly decreased in moderate US frequency (40 kHz) at 70°C with the increasing drying temperature. Comparing to the fresh samples, the dried samples showed the highest amount of phytochemical contents. The antioxidant activity of the samples increased especially at 60 kHz and 80°C, while US-IR treatments shown a positive impact on total carotenoids contents and ß-carotene. For phenolic compounds, Ellagic acid and Rutin were quantified in higher amount while Quercetin-3-rhamnoside and Quercetin 3-ß-D-glucoside were two new compounds identified for the first time in sweet potatoes. FTIR spectra showed the successful synthesis of OH group and phenolics in samples treated with the US at 20 kHz. PRACTICAL APPLICATIONS: This study investigated the effects of multifrequency ultrasound with different infrared drying temperatures. The study provides evidence that infrared drying application in synergy with ultrasonic pretreatments can improve drying efficiency and food quality much better than using each method alone. Total phenolic contents and total flavonoid contents remained stable at US 40 kHz and 60°C conditions. The findings showed that moderate ultrasound frequency (40 kHz) at 60°C improved phytochemical properties while antioxidant activities showed better preservation response at 80°C with 60 kHz. In addition, the samples treated with the same US treatment at 40 kHz showed less cell breakage in SEM analysis.

Phytochrome F plays critical roles in potato photoperiodic tuberization.

The transition to tuberization contributes greatly to the adaptability of potato to a wide range of environments. Phytochromes are important light receptors for the growth and development of plants, but the detailed functions of phytochromes remain unclear in potato. In this study, we first confirmed that phytochrome F (StPHYF) played essential roles in photoperiodic tuberization in potato. By suppressing the StPHYF gene, the strict short-day potato genotype exhibited normal tuber formation under long-day (LD) conditions, together with the degradation of the CONSTANTS protein StCOL1 and modulation of two FLOWERING LOCUS T (FT) paralogs, as demonstrated by the repression of StSP5G and by the activation of StSP6A during the light period. The function of StPHYF was further confirmed through grafting the scion of StPHYF-silenced lines, which induced the tuberization of untransformed stock under LDs, suggesting that StPHYF was involved in the production of mobile signals for tuberization in potato. We also identified that StPHYF exhibited substantial interaction with StPHYB both in vitro and in vivo. Therefore, our results indicate that StPHYF plays a role in potato photoperiodic tuberization, possibly by forming a heterodimer with StPHYB.

Coincidence of potato CONSTANS (StCOL1) expression and light cannot explain night-break repression of tuberization.

In the obligate short-day potato Solanum tuberosum group Andigena (Solanum andigena), short days, or actually long nights, induce tuberization. Applying a night break in the middle of this long night represses tuberization. However, it is not yet understood how this repression takes place. We suggest a coincidence model, similar to the model explaining photoperiodic flowering in Arabidopsis. We hypothesize that potato CONSTANS (StCOL1), expressed in the night of a short day, is stabilized by the light of the night break. This allows for StCOL1 to repress tuberization through induction of StSP5G, which represses the tuberization signal StSP6A. We grew S. andigena plants in short days, with night breaks applied at different time points during the dark period, either coinciding with StCOL1 expression or not. StCOL1 protein presence, StCOL1 expression and expression of downstream targets StSP5G and StSP6A were measured during a 24-h time course. Our results show that a night break applied during peak StCOL1 expression is unable to delay tuberization, while coincidence with low or no StCOL1 expression leads to severely repressed tuberization. These results imply that coincidence between StCOL1 expression and light does not explain why a night break represses tuberization in short days. Furthermore, stable StCOL1 did not always induce StSP5G, and upregulated StSP5G did not always lead to fully repressed StSP6A. Our findings suggest there is a yet unknown level of control between StCOL1, StSP5G and StSP6A expression, which determines whether a plant tuberizes.

Pilot-scale radiofrequency blanching of potato cuboids: heating uniformity.

BACKGROUND: Traditional hot water blanching has a slow heat transfer rate, whereas radiofrequency (RF) heating has the advantages of a much faster heating rate and a higher penetration depth. In the present study, RF heating was applied to improve heating uniformity for subsequent blanching experiments involving potato cuboids. Potato cuboids were treated in a pilot-scale, RF heating system (27.12 MHz, 6 kW) under different operating conditions. RESULTS: The dielectric constant increased first and then decreased with temperature, whereas the loss factor increased as the temperature increased. The results of the present study reveal that the electrode gap, sample height and NaCl solution had significant effects (P < 0.05) on the temperature distribution and heating uniformity of the sample after RF heating. The optimum RF heating uniformity was obtained at an electrode gap of 120 mm, a sample height of 60 mm and when immersed in NaCl solution of 0.5 s m-1 . The central heating pattern was presented in a sample. Cold spots were located at the edge of the top surface of the sample. CONCLUSION: The present study shows the great potential of RF heating for the blanching of vegetables. Future studies should aim to determine changes in the texture and nutrient contents of vegetables during RF heating. © 2017 Society of Chemical Industry.

Influence of altitude and enhanced ultraviolet-B radiation on tuber production, seed viability, leaf pigments and morphology in the wild potato species Solanum kurtzianum Bitter & Wittm collected from an elevational gradient.

Climate change could lead to an upward shift in plant distribution, exposing populations to higher levels of ultraviolet (UV)-B radiation. In the framework of an in situ strategy for conserving potato wild relatives, we evaluated the effect of high UV-B levels on natural population of Solanum kurtzianum. The hypothesis is that plants from naturally higher altitudes are more adapted to increased UV-B radiation. Two populations from low and high altitudes were field supplemented using UV-B-lamps (+UV-B) or excluded from it with plastic filters. Additionally, to assess in which extent the plant responses to these artificial experimental conditions are reproducible in natural conditions, three genotypes were cultivated in two mountain experimental gardens (EG) at different elevations. +UV-B treatment induced changes in leaf morphology and increases in phenolic compounds in both populations, indicating plant adaptation, since chlorophylls and reproductive structures were not negatively affected. These results indicate that this environmental factor may not limit the displacement of populations towards sites with higher UV-B levels. Meanwhile, in higher-altitude EG a tubers yield reduction, mainly through a decreased tuber number and a bigger accumulation of phenolic compounds than in +UV-B treatment were observed, suggesting that UV-B is not the only factor involved in plants adaptation to high altitude environments.

Characterisation of potato crisp effective porosity using micro-CT.

BACKGROUND: The effective porosity is an important quantitative parameter for food products that has a significant effect on taste and quality. It is challenging to quantify the apparent porosity of fried potato crisps as they have a thin irregularly shaped cross section containing oil and water. This study uses a novel micro-CT technique to determine the solid volume fraction and hence the effective porosity of three types of potato crisps: standard continuously fried crisps, microwaved crisps, and continuously fried 'kettle' crisps. RESULTS: It was found that continuously fried kettle crisps had the lowest effective porosity at 0.54, providing the desired crunchy taste and lower oil contents. Crisps produced using a microwave process designed to mimic the dehydration process of standard continuous fried crisps had an effective porosity of 0.65, which was very similar to the effective porosity of 0.63 for standard continuously fried crisps. The results were supported by the findings of a forced preference consumer test. CONCLUSION: The effective porosity affects the product taste and is therefore a critical parameter. This study shows that micro-CT analysis can be used to characterise the change in effective porosity of a thin irregularly shaped food product, caused by a change of cooking procedure. © 2016 Society of Chemical Industry.

Glycoalkaloid and calystegine levels in table potato cultivars subjected to wounding, light, and heat treatments.

Potato tubers naturally contain a number of defense substances, some of which are of major concern for food safety. Among these substances are the glycoalkaloids and calystegines. We have here analyzed levels of glycoalkaloids (α-chaconine and α-solanine) and calystegines (A3, B2, and B4) in potato tubers subjected to mechanical wounding, light exposure, or elevated temperature: stress treatments that are known or anticipated to induce glycoalkaloid levels. Basal glycoalkaloid levels in tubers varied between potato cultivars. Wounding and light exposure, but not heat, increased tuber glycoalkaloid levels, and the relative response differed among the cultivars. Also, calystegine levels varied between cultivars, with calystegine B4 showing the most marked variation. However, the total calystegine level was not affected by wounding or light exposure. The results demonstrate a strong variation among potato cultivars with regard to postharvest glycoalkaloid increases, and they suggest that the biosynthesis of glycoalkaloids and calystegines occurs independently of each other.

Naturally occurring allele diversity allows potato cultivation in northern latitudes.

Potato (Solanum tuberosum L.) originates from the Andes and evolved short-day-dependent tuber formation as a vegetative propagation strategy. Here we describe the identification of a central regulator underlying a major-effect quantitative trait locus for plant maturity and initiation of tuber development. We show that this gene belongs to the family of DOF (DNA-binding with one finger) transcription factors and regulates tuberization and plant life cycle length, by acting as a mediator between the circadian clock and the StSP6A mobile tuberization signal. We also show that natural allelic variants evade post-translational light regulation, allowing cultivation outside the geographical centre of origin of potato. Potato is a member of the Solanaceae family and is one of the world's most important food crops. This annual plant originates from the Andean regions of South America. Potato develops tubers from underground stems called stolons. Its equatorial origin makes potato essentially short-day dependent for tuberization and potato will not make tubers in the long-day conditions of spring and summer in the northern latitudes. When introduced in temperate zones, wild material will form tubers in the course of the autumnal shortening of day-length. Thus, one of the first selected traits in potato leading to a European potato type is likely to have been long-day acclimation for tuberization. Potato breeders can exploit the naturally occurring variation in tuberization onset and life cycle length, allowing varietal breeding for different latitudes, harvest times and markets.

Control of flowering and storage organ formation in potato by FLOWERING LOCUS T.

Seasonal fluctuations in day length regulate important aspects of plant development such as the flowering transition or, in potato (Solanum tuberosum), the formation of tubers. Day length is sensed by the leaves, which produce a mobile signal transported to the shoot apex or underground stems to induce a flowering transition or, respectively, a tuberization transition. Work in Arabidopsis, tomato and rice (Oryza sativa) identified the mobile FLOWERING LOCUS T (FT) protein as a main component of the long-range 'florigen', or flowering hormone, signal. Here we show that expression of the Hd3a gene, the FT orthologue in rice, induces strict short-day potato types to tuberize in long days. Tuber induction is graft transmissible and the Hd3a-GFP protein is detected in the stolons of grafted plants, transport of the fusion protein thus correlating with tuber formation. We provide evidence showing that the potato floral and tuberization transitions are controlled by two different FT-like paralogues (StSP3D and StSP6A) that respond to independent environmental cues, and show that an autorelay mechanism involving CONSTANS modulates expression of the tuberization-control StSP6A gene.

Effect of gamma-ray radiation on physiological, morphological characters and chromosome aberrations of minitubers in Solanum tuberosum L.

PURPOSE: To investigate the effects of gamma-ray radiation on the physiological, morphological characters and chromosome aberrations of minitubers. MATERIALS AND METHODS: Minitubers of one potato cultivar, 'Shepody', were irradiated with 8 doses of gamma-rays (0, 10, 20, 30, 40, 50, 60, 70 and 80 Gy [Gray]) to investigate the effects of radiation on emergence ability, plant height and root length, morphological variations, chromosome aberrations, M(1) (first generation mutants) tuber number and size of minituber plants. RESULTS: Compared with the non-irradiated controls, the whole period of emergence was prolonged by 10-15 days for minitubers treated with gamma-ray radiation, but low doses of radiation (10, 20 and 30 Gy) promoted the emergence percentage of minitubers. With an increase in radiation dose, the emergence percentage, plant height and root length of minituber plants were significantly inhibited at 40 and 50 Gy. No emergence occurred at 60 Gy and higher doses. After radiation, a series of morphological variations and chromosome aberrations appeared in minituber plants. Radiation with 20 Gy promoted tuber formation, and the average number and diameter of M(1) tubers per plant were significantly increased over the control by 71% and 34%, respectively. CONCLUSION: Low doses of radiation (10-30 Gy) might be used as a valuable parameter to study the improvement of minitubers by gamma-ray radiation treatment.

Morphological and molecular characterization of a spontaneously tuberizing potato mutant: an insight into the regulatory mechanisms of tuber induction.

BACKGROUND: Tuberization in potato (Solanum tuberosum L.) represents a morphogenetic transition of stolon growth to tuber formation, which is under complex environmental and endogenous regulation. In the present work, we studied the regulatory mechanisms and the role of different morphogenetic factors in a newly isolated potato mutant, which exhibited spontaneous tuberization (ST). The ST mutant was characterized in detail at morphological, physiological and biochemical levels. RESULTS: Tuberization of the ST mutant grown in the soil was photoperiod-insensitive; predominantly sessile tubers formed directly from axillary buds even under continuous light. Single-node cuttings of the ST mutant cultured in vitro frequently formed tubers or basal tuber-like swellings instead of normal shoots under conditions routinely used for shoot propagation. The tuberization response of ST cuttings under light was dependent on sucrose, the concentration of which had to exceed certain threshold that inversely correlated with irradiance. Gibberellic acid prevented tuberization of ST cuttings, but failed to restore normal shoot phenotype and caused severe malformations. Carbohydrate analysis showed increased levels of both soluble sugars and starch in ST plants, with altered carbohydrate partitioning and metabolism. Comparative proteomic analysis revealed only a few differences between ST- and wild-type plants, primary amongst which seemed to be the absence of an isoform of manganese-stabilizing protein, a key subunit of photosystem II. CONCLUSION: ST mutant exhibits complex developmental and phenotypic modifications, with features that are typical for plants strongly induced to tuberize. These changes are likely to be related to altered regulation of photosynthesis and carbohydrate metabolism rather than impaired transduction of inhibitory gibberellin or photoperiod-based signals. The effect of gibberellins on tuberization of ST mutant suggests that gibberellins inhibit tuberization downstream of the inductive effects of sucrose and other positive factors.

Quinoid radio-toxin (QRT) induced metabolic changes in mice: an ex vivo and in vivo EPR investigation.

Radio-toxins are toxic metabolites produced by ionizing irradiation and have toxic effects similar to those caused by direct irradiation. We have investigated the effect of a quinoid radio-toxin (QRT) obtained from gamma-irradiated potato tuber on various organs in mice using ex vivo and in vivo EPR spectroscopy. Results indicate a decrease in the activity of ribonucleotide reductase enzyme in spleen of mice treated with 0.2mg QRT. A dose of 2mg QRT was fatal to mice within 45-60 min of treatment. Nitrosyl hemoglobin complexes alpha-(Fe(2+)-NO)alpha-(Fe(2+))beta-(Fe(2+))(2) were detected from spleen, blood, liver, kidney, heart, and lung tissue samples of mice treated with lethal doses of QRT. A significant decrease of pO(2) in liver and brain was observed after administration of QRT at the lethal dose. The time of the appearance of the nitrosyl hemoglobin complex and its intensity varied with the dose of QRT and the type of tissue. These results indicate that the effect of the QRT is more prominent in spleen and to a lesser extent in liver and blood. The QRT action at the lethal doses resulted in an increased hypoxia over time with disruption of compensatory adaptive response. The results indicate similar outcome of QRT as observed with gamma-irradiation.

Dynamics of a mobile RNA of potato involved in a long-distance signaling pathway.

BEL1-like transcription factors interact with Knotted1 types to regulate numerous developmental processes. In potato (Solanum tuberosum), the BEL1 transcription factor St BEL5 and its protein partner POTH1 regulate tuber formation by mediating hormone levels in the stolon tip. The accumulation of St BEL5 RNA increases in response to short-day photoperiods, inductive for tuber formation. RNA detection methods and heterografting experiments demonstrate that BEL5 transcripts are present in phloem cells and move across a graft union to localize in stolon tips, the site of tuber induction. This movement of RNA to stolon tips is correlated with enhanced tuber production. Overexpression of BEL5 transcripts that include the untranslated sequences of the BEL5 transcript endows transgenic lines with the capacity to overcome the inhibitory effects of long days on tuber formation. Addition of the untranslated regions leads to preferential accumulation of the BEL5 RNA in stolon tips under short-day conditions. Using a leaf-specific promoter, the movement of BEL5 RNA to stolon tips was facilitated by a short-day photoperiod, and this movement was correlated with enhanced tuber production. These results implicate the transcripts of St BEL5 in a long-distance signaling pathway that are delivered to the target organ via the phloem stream.