Abstract
Although numerous RNAs have been detected in the phloem, only a few have been confirmed to move long distances. In potato, full-length mRNA of the BEL1-like transcription factor, StBEL5, moves from leaf veins through the phloem to stolon tips to activate tuber formation. BEL1-like transcription factors are ubiquitous in plants and interact with KNOTTED1-types to regulate numerous developmental processes. To explore the range of KNOTTED1- and BEL1-like mRNAs present in phloem, an analysis of the transcript profile of phloem sap was undertaken. Using a modified technique for the collection of phloem-enriched exudate from excised stems, numerous RNAs encoding these transcription factors were detected in the phloem sap from several solanceous species. All seven known BEL1-like RNAs of potato were detected in the phloem-enriched exudates of stem, whereas several stolon-abundant RNAs were not. After refining the technique to minimize the contamination from RNA arising from wounded cells, KNOTTED1-like RNAs were detected in phloem-enriched sap of potato and BEL5 RNA was detected in the sap collected from two closely related nontuber-bearing potato species and tomato. BEL5 RNA was also detected in RNA extracted from leaf veins of tobacco. The detection of these full-length mRNAs from the KNOTTED1- and BEL1-like families in phloem sap indicates that their potential role as long-distance signals seems to be much more extensive than previously known.
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Introduction
As a part of an elaborate long-distance communication system, plants have evolved a unique signaling pathway that takes advantage of connections in the vascular tissue, predominately the phloem. This information pathway has been implicated in regulating development, responding to biotic stress, delivering nutrients, and as a vehicle commandeered by viruses for spreading infections (Lough and Lucas 2006). Co-suppression mediated by systemic-acquired gene silencing and the transport of miRNAs also involves movement through the phloem (Sonoda and Nishiguchi 2000; Crete et al. 2001; Brosnan et al. 2007; Buhtz et al. 2008). Recently, the transport of full-length mRNA through the phloem has been identified as a key component of long-distance signaling (reviewed by Kehr and Buhtz 2008).
Whereas hundreds of RNAs have been identified in phloem (Asano et al. 2002; Vilaine et al. 2003; Doering-Saad et al. 2006; Omid et al. 2007), only six have been confirmed via grafting experiments to be transported (Kehr and Buhtz 2008). Of these, CmGAIP, CmNACP, and CmPP16 are from Cucurbita maxima (Haywood et al. 2005; Ruiz-Medrano et al. 1999; Xoconostle-Cazares et al. 1999), DELLA-GAI is from Arabidopsis (Haywood et al. 2005), PFP-LeT6 is from tomato (Kim et al. 2001), and StBEL5 is from potato (Banerjee et al. 2006). Using the Cucurbita species, scions of cucumber grafted onto pumpkin stocks provided direct evidence that specific pumpkin mRNAs were translocated through the heterograft via the phloem into the shoot apex (Ruiz-Medrano et al. 1999; Xoconostle-Cazares et al. 1999). The discovery of the RNA-binding protein, CmPP16, provided additional support for the long-distance transport of RNA in pumpkin (Xoconostle-Cazares et al. 1999). Of this group of six RNAs, however, information on the dynamics of movement is available only for StBEL5 RNA (Banerjee et al. 2006).
StBEL5 is a member of the TALE superfamily of transcription factors (Bürglin 1997). The BEL1-like family of transcription factors is ubiquitous among plant species and interact with KNOTTED1-types for targeting genes to regulate numerous developmental processes (Bellaoui et al. 2001; Müller et al. 2001; Smith et al. 2002; Smith and Hake 2003; Bhatt et al. 2004; Kanrar et al. 2006). In potato, the BEL1 transcription factor, StBEL5 and its Knox protein partner, POTH1, regulate tuber formation by mediating hormone levels in the stolon tip (Rosin et al. 2003; Chen et al. 2003; 2004).
RNA detection methods and heterografting experiments demonstrate that StBEL5 transcripts are present in phloem cells and move across a graft union to localize in stolon tips, the site of tuber induction (Banerjee et al. 2006). This movement of RNA to stolon tips is facilitated by a short-day photoperiod, mediated by sequence tags present in the untranslated regions of the StBEL5 transcript, and correlated with enhanced tuber production (Banerjee et al. 2006). Based on these results, the mRNA of StBEL5 appears to act as a mobile signal that is delivered to the stolon tip to induce tuber formation. Tuberization is a specialized developmental process, yet sequence motifs present in the 3′ UTR of StBEL5 are conserved in the BEL5 mRNAs of both tomato and tobacco. Are BEL5 RNAs present in the phloem of plant species that do not make tubers? Are other transcripts from the TALE superfamily of transcription factors present in phloem? The results of this study confirm the presence of several BEL1- and KNOTTED1-like RNAs in the phloem sap of potato and other nontuber-bearing solanaceous species suggesting that movement of these full-length mRNAs plays a much wider role in long-distance signaling than previously assumed.
Materials and methods
Plant materials
Solanum tuberosum ssp. andigena plants were generated from in vitro-grown plantlets established from tuber sprouts. All plantlets were grown on a media containing 2% sucrose plus MS basal salts (Murashige and Skoog 1962) under long-day conditions (16 h light, 8 h dark) at 21°C. Plants of Solanum etuberosum (PI 498311) and S. palustre (PI 558259) were generated from the seeds obtained from the Potato Introduction Station (Sturgeon Bay, WI, USA). Both of these Andean potato species do not form tubers. Rooted cuttings from healthy stock plants of S. etuberosum and S. palustre were transferred to growth chambers under long-day conditions for 2 weeks before starting any experiments. Nicotiana tabacum var. Petit Havana and S. lycopersicum plants were grown from seed and transferred to 10-cm pots. All plants were grown under long-day conditions (16 h light, 8 h dark) either in the greenhouse or in a growth chamber at approximately 21°C. Sap was collected from stock plants after they were grown for approximately 3 weeks.
RNA extraction, sap collection and RT-PCR
Total RNA was extracted from leaf, stem, and root samples according to the manufacturer’s instructions (RNeasy® Plant Mini Kit, Qiagen). Samples were incubated for 1 min at room temperature before eluting the column. RNA samples were treated with a RNase-free DNase (TURBO DNA free™ kit, Ambion) before PCR.
Two hours prior to collection of sap, source plants were thoroughly watered. Immediately, prior to harvest, plants were placed in a tray with excess water. Complete transverse cuts were made across the stem approximately 3.0 cm above the soil level with a clean razor blade. The stem exudate was blotted with a ChemWipe tissue for 5 min to minimize RNA contamination from disrupted stem cells and sap was collected for up to 30 min (Fig. 4). For harvest (Fig. 4), at least 200 μl of sap was collected across the stem with a 0.2 ml Pipetman and stored on ice. For analysis (Fig. 5), 200 μl of sap was collected during each time interval: immediately upon bleeding, 10–20 min from the onset of bleeding, 20–30 min, and after 30 min from the onset of stem bleeding, all from the same set of plants. Immediately after collection, 500 μl of Trizol reagent was added. The sample was vortexed for 30 s, 0.2 ml chloroform was added, and the sample was again vortexed for 30 s. The sample was then centrifuged (12 K rcf) for 15 min at 4°C. The aqueous phase was removed, placed in a separate tube, and then subjected to ethanol precipitation at −20°C overnight. After washing the pellet with 70% ethanol, the RNA sample was air-dried, resuspended in a minimum volume of nuclease-free water, and quantified using a GeneQuant spectrophotometer (Biochrom, Cambridge, England).
To detect specific mRNAs, sample RNA was reverse-transcribed using SuperScript™ III One Step RT-PCR System with Platinum® Taq DNA Polymerase Kit (Invitrogen, Carlsbad, CA, USA) with 0.25 μM gene-specific primers (Table 1). Primers for G2, NT2, POTH1, StBEL11, −13, −14, −22, −29, and −30 were previously described (Yu et al. 2007). All primers were synthesized at the DNA Facility, Iowa State University. The amount of RNA template used varied among reactions (20–200 ng) due to the estimated abundance level of the target RNA. PCR conditions were 50°C for 30 min; 94°C for 2 min; 38 cycles of 94°C for 30 s, 54–56°C for 30 s, 68°C for 30 s. G2 RNA (Access # TC118156) was used as a positive control for phloem sap (Zhao et al. 2005) and the root-specific potato RNA (Access # CK267169), homologue to the nitrate transporter (NT) gene of Arabidopsis (Nazoa et al. 2003), as a negative control.
Characterization of BEL cDNAs from Solanum etuberosum and S. palustre
Of the seven known BEL1-family members from S. tuberosum, four were chosen for further analysis in the two nontuber-bearing species: StBEL5, StBEL14, StBEL29, and StBEL30. The first four primer sets listed in Table 1 were used to construct full-length StBEL5, StBEL14, StBEL29, and StBEL30, respectively, from S. etuberosum and S. palustre. The RT-PCR conditions were similar to those described in the previous section, PCR products were cloned into the TOPO TA vector (Invitrogen, Carlsbad, CA, USA). After selection and plasmid isolation, clones were sequenced at the DNA Facility, Iowa State University. The sequence obtained was screened for matches using the basic local alignment search tool (BLAST). The percent nt matches were determined by a comparison to the known StBEL sequences (Table 2).
Wounding experiment
Solanum etuberosum and S. palustre stock plants were propagated from the cuttings rooted under mist and placed in a growth chamber under long-day conditions at 21°C until they reached the 10- to 12-leaf stage. Intact plants were then wounded by cutting stems 3.0 cm above the soil level superficially with a clean razor blade or using a hemostat to wound leaf mesophyll several times between major veins. Tissue samples were harvested from three plants per species per time point (0, 24, and 48 h post-wounding), pooled, frozen in liquid nitrogen, and stored at −80°C until RNA extraction.
RT-PCR was performed with 20 ng of total RNA as template and the BEL5-specific primers, StBEL5-F and StBEL5-R (Table 1). PCR conditions for BEL5 quantification were 50°C for 30 min; 94°C for 2 min; 32 cycles of 94°C for 30 s, 56°C for 30 s, 68°C for 30 s. The internal control for PCR reactions was rRNA. PCR conditions for the rRNA were 50°C for 30 min; 94°C for 2 min; 21 cycles of 94°C for 30 s, 56°C for 30 s, 68°C for 30 s. Homogenous PCR products were quantified using ImageJ software (Abramoff et al. 2004) and normalized using the rRNA values. PCRs for BEL5 and rRNA were standardized and optimized to yield product in the linear range (for example, 32 cycles for the BEL5 RNA and 21 cycles for the rRNA). Three quantitative RT-PCRs were performed and the standard error calculated.
Results
BEL1-like genes are present in nontuber-bearing Solanum species
Using RT-PCR with primers from potato BEL1-like genes, several BEL1-like cDNAs were identified and sequenced from two very closely related nontuber-bearing solanaceous species, S. etuberosum and S. palustre (Table 2). Three of these genes, BEL5, −14, -and −30, were selected as representatives of the major phylogenetic groups of potato (Chen et al. 2003). BEL29 is closely related to BEL5 in overall sequence. To compare to existing BEL1 genes, a phylogenetic analysis of these new genes based on their amino acid sequence was performed (Fig. 1). As expected, the S. etuberosum and S. palustre BEL1 proteins aligned very closely to their tuberosum (St) and tomato (Sl) counterparts. Arabidopsis BEL1 proteins are included in the dendrogram as a reference. These new BEL1-like cDNAs exhibited a very high level of nucleotide sequence match with their S. tuberosum counterparts (Table 2). The lengths of the 3′ UTRs were also similar as the 505 nt UTR of StBEL5 was matched by 454-nt and 514-nt 3′ UTRs for SeBEL5 and SpBEL5, respectively. Alignment of the available 3′ UTR revealed a very high level of nt sequence match (Fig. 2), 94 and 93% match for Sp and Se, respectively, for the StBEL5 UTR (Fig. 2a), a 87% match for both Sp and Se for the StBEL29 UTR (Fig. 2b), and a 97% match for both Sp and Se for the StBEL30 UTR (Fig. 2c). As expected, the conserved regions of the BEL1 family, the SKY box, the BELL domain, and the homeodomain, were also conserved in all eight of the new BEL1 proteins examined in this study.
Because of their importance in development (Banerjee et al. 2006), the BEL5 genes were selected for a more thorough characterization of their expression patterns. Similar to StBEL5, the BEL5 RNAs of S. etuberosum and S. palustre were ubiquitous. Using gene-specific primers and RT-PCR, these RNAs were detected in leaf, stem, and root RNA (Fig. 3a).
Wound induction of BEL5 genes
Previous studies have demonstrated that the promoter of StBEL5 was activated in response to wounding in stems but not leaves (Chatterjee et al. 2007). To determine if other solanaceous species exhibited a similar pattern of expression, wound induction for both leaves and stems was examined for S. etuberosum and S. palustre. Consistent with the induction pattern of the StBEL5 promoter, steady-state levels of BEL5 RNA in both the species were enhanced in wounded stems but not in leaves over 48 h (Fig. 3b, c).
BEL1-like mRNAs are present in phloem-enriched sap
Previous work using in situ hybridization and laser capture microdissection, demonstrated the presence of a several BEL1-like RNAs in phloem cells of potato (Banerjee et al. 2006; Yu et al. 2007). These protocols, however, are labor-intensive and time-consuming. To facilitate the analysis of mRNAs in phloem sap, a modified protocol adapted from Buhtz et al. (2008) was implemented. This technique involves collection of sap exudates from excised stems of potato. RT-PCR of RNA extracted from this stem sap revealed the presence of all seven BEL1-like mRNAs (Fig. 4a). G2 mRNA of potato (# TC118156) is a phloem-specific RNA (Zhao et al. 2005) and NT2 represents a xylem-specific transcript of roots (Nazoa et al. 2003). Several BEL RNAs of potato accumulate in stolons during tuber formation (Fig. 4a; Chen et al. 2003). To determine if other stolon RNAs are present in the sap RNA extracted here, several additional RNAs upregulated in stolon tips were assayed (Hannapel 2007). A Cen1-like (TIGR access # TC98831), tup1 (# TC95867), NAC1 (# TC96473), and ras-like (# TC67617) RNA were detected in the stolon tips during the onset of tuber formation but not in the harvested stem sap (Fig. 4b). Gigantea (Access # BF154299) was included because of its pivotal role in mediating photoperiod-regulated processes (Sawa et al. 2007). The integrity of the band detected for the ras-like transcript in sap RNA (Fig. 4b) could not be confirmed.
Because of the possibility of contamination from the wounded cells of the cut surface of the stem, RNA was extracted from the sap collected at several time intervals after bleeding was initiated. Even though the stem exudates collected in this protocol most certainly contain xylem sap, contamination via this source was not considered a factor because, in a previous study, no RNA was detected in xylem sap (Buhtz et al. 2008). Contamination from the rubisco small subunit RNA (# TC137121) was observed from 0 to 30 min after the initial bleed but not after 30 min (>30, Fig. 5). The two phloem RNAs, G2 and StBEL5, could still be detected even after 30 min of sap collection (Fig. 5). RNA yield from these harvests ranged from 1.4 to 4.2 ng/μl of sap. Based on these results, subsequent analyses of RNA were performed on sap collected after 30 min from the onset of bleeding.
Can other mRNAs be detected in phloem-enriched sap?
Results using the technique for sap collection described in Fig. 4 indicated the presence of all seven StBEL RNAs in phloem sap. RT-PCR of RNA extracted from phloem cells harvested using laser capture microdissection detected the presence of a KNOTTED1-like mRNA, POTH1 (Yu et al. 2007). To determine if other KNOTTED1-like mRNAs are present in the phloem sap of potato, RT-PCR was performed on total RNA extracted from sap collected at least 30 min after the initial bleed from S. tuberosum ssp. andigena (Fig. 6a). As expected, POTH1 RNA was detected in the phloem sap of potato. In addition, POTH15 RNA, a class-I KNOX gene (Tanaka-Ueguchi et al. 1998), and H09, a class-II KNOX gene (Resier et al. 2000), were also detected (Fig. 6a). Whereas StBEL14 RNA was not detected previously (Yu et al. 2007), the presence of a low level of its mRNA was observed in RNA from phloem-enriched sap collected in this study (Fig. 6a). This inconsistency is most likely explained by the amount of template used for the PCR. RNA yields from the LCM-harvested cells totaled approximately 30 ng and 2–3 ng of RNA template were used per RT-PCR (Yu et al. 2007). In the current study, RNA yields ranged from 280 to 840 ng/200 μl of harvested sap (Fig. 5) and approximately 70 ng of RNA extracted from sap was used as template for the StBEL14 assay (Fig. 6a).
To determine if BEL5-like RNAs are present in phloem RNA from nontuber-bearing solanaceous species, RT-PCR was again performed on RNA extracted from sap collected after 30 min of bleeding. BEL5-like RNA was detected in sap RNA from S. etuberosum and S. lycopersicum cv. BHN (Fig. 6b). BEL5 RNA was also detected in the phloem sap of S. palustre (data not shown). Phloem sap from tobacco (Nt) stems could not be obtained from stem excisions so primary midveins harvested from the abaxial (lower) side of the leaf blade were used instead. These prominent veins protrude 2–3 mm from the lower side of the leaf blade and were relatively easy to excise with a razor blade without any leaf mesophyll contamination. A large proportion of BEL5 RNA was detected in RNA from these veins relative to the amount detected in RNA from leaf (Fig. 6b, Nt). The lower section of midveins contains, in order, starting from the lamina, xylem, phloem, and collenchyma tissue (Esau 1977). Consistent with these results, in potato, the foliar midveins are the primary source of StBEL5 promoter activity (Chatterjee et al. 2007).
Discussion
KNOTTED1-like RNAs in the phloem
The presence of several KNOTTED1-like RNAs in phloem-enriched sap of potato, including both class-I and -II types, was confirmed in this study. Previous work using laser capture microdissection and in situ hybridization showed that the RNA of POTH1, a KNOX protein that interacts with StBEL5, could be detected in phloem cells of the stem (Yu et al. 2007) and the stolon tip (Rosin et al. 2003). The presence of a mRNA in the phloem sap, however, does not prove that it is transported. CmSTMP, a KNOTTED1-like gene involved in meristem maintenance, was identified in the phloem sap of Cucurbita maxima (pumpkin) but its movement into a cucumber scion could not be confirmed (Ruiz-Medrano et al. 1999). Other KNOTTED1-like transcripts have been transported across a graft union. A fusion of a phosphofructokinase-knotted1 transcript of tomato, PFP-LeT6, moved across a graft and induced a developmental phenotype (Kim et al. 2001). The best examples of KNOX RNA movement, however, are in association with its protein through the plasmodesmata of tobacco mesophyll cells (Lucas et al. 1995) and from cell-to-cell using a trafficking assay coupled to trichome rescue in Arabidopsis (Kim et al. 2005). In the latter study, the KNOX homeodomain of the protein promoted intercellular trafficking of both the KNOX protein and its associated mRNA. In this report, the observation that KNOTTED1-like mRNAs of potato are present in the phloem at the same time and location as the mRNAs from their protein partners of the BEL1-like family is intriguing. As previously reported (Chen et al. 2004), the tandem complex consisting of both protein types was necessary for regulating transcriptional activity in a target gene that affected tuber formation.
What is the function of BEL5 RNAs in the phloem of nontuber-bearing species?
Clearly, BEL1-like genes function in a wide variety of roles in plant development and metabolism. StBEL5 is unique in that it functions as a long-distance signal to regulate vegetative growth in a specialized underground organ, the tuber (Banerjee et al. 2006). But what is the function of BEL5-like RNAs in the phloem of nontuber-bearing species? Two BEL5 RNAs, from tobacco and tomato, even contain conserved sequence motifs in their respective 3′ UTRs that are also present in StBEL5 (data not shown). With StBEL5 RNA, the 3′ UTR has been implicated in mediating mobility (Banerjee et al. 2006). Based on the wound-induction pattern previously described (Chatterjee et al. 2007) and verified in the current study, it is plausible that BEL5-like genes function as a phloem defense signal responsive to mechanical or insect damage. A BEL1-like gene from rice, OsBIHD1, was identified that functions in disease resistance and pathogen defense (Luo et al. 2005). A protein partner of BEL1, Brevipedicellus, a KNOTTED1-like transcription factor of Arabidopsis, regulates several genes involved in lignin biosynthesis (Mele et al. 2003), implying that the BEL/KNOX complex may be involved in rebuilding cell walls in the vascular tissue after wounding or damage from insect predation.
Harvesting phloem RNAs from potato
Plant species from the Cucurbitaceae have been studied extensively for phloem analysis because of their propensity for releasing phloem-abundant sap from excised stems (Ruiz-Medrano et al. 1999). For Ricinus communis, phloem sap was collected from the cotyledon for 15 min following the excision of the seedlings (Doering-Saad et al. 2006). With Brassica napus, phloem samples were obtained by making small punctures with a hypodermic needle into inflorescence stems of 8- to 10-week-old plants. The first flowing droplet was discarded and the subsequent exudate was collected into sample buffer (Giavalisco et al. 2006). All of these collection techniques yielded phloem-enriched RNA populations.
After preliminary work, phloem-enriched sap was effectively harvested from excised potato stems in this study. By allowing flow to occur for up to 30 min, contaminants from wounded cells were reduced to a minimum. Even though it is assumed that this harvested sap contained xylem flow, based on previous analyses demonstrating that xylem sap contains no RNA (Buhtz et al. 2008) and the results of this study (Figs. 4, 5), it may be concluded that the harvested sap described here contains a phloem-enriched RNA population. In summary, these results indicate the presence of numerous transcripts from the TALE superfamily of transcription factors in phloem cells. The process of transporting full-length mRNAs through the phloem as long-distance signals seems to be much more extensive than previously assumed. What we know so far about this dynamic information system appears to be just the tip of the iceberg.
The following GenBank accession numbers have been assigned: EU686384 for SeBEL5, EU686378 for SeBEL14, EU686385 for SeBEL29, EU686379 for SeBEL30, EU686380 for SpBEL5, EU686381 for SpBEL14, EU686382 for SpBEL29, and EU686383 for SpBEL30.
Abbreviations
- UTR:
-
Untranslated regions
- TALE:
-
Three amino acid loop extesnion
References
Abramoff MD, Magelhaes PJ, Ram SJ (2004) Image processing with Image. J Biophotonics Int 11:36–42
Asano T, Masumura T, Kusano H, Kikuchi S, Kurita A, Shimada H, Kadowaki K (2002) Construction of a specialized cDNA library from plant cells isolated by laser capture microdissection: toward comprehensive analysis of the genes expressed in the rice phloem. Plant J 32:401–408
Banerjee AK, Chatterjee M, Yu YY, Suh SG, Miller WA, Hannapel DJ (2006) Dynamic of a mobile RNA of potato involved in a long-distance signaling pathway. Plant Cell 18:3443–3457
Bellaoui M, Pidkowich MS, Samach A, Kushalappa K, Kohalmi SE, Modrusan Z, Crosby WL, Haughn GW (2001) The Arabidopsis BELL1 and KNOX TALE homeodomain proteins interact through a domain conserved between plants and animals. Plant Cell 13:2455–2470
Bhatt AM, Etchells JP, Canales C, Lagodienko A, Dickinson H (2004) VAAMANA—a BEL1-like homeodomain protein, interacts with KNOX proteins BP and STM and regulates inflorescence stem growth in Arabidopsis. Gene 328:103–111
Brosnan CA, Mitter N, Christie M, Smith NA, Waterhouse PM, Carroll BJ (2007) Nuclear gene silencing directs reception of long-distance mRNA silencing in Arabidopsis. Proc Natl Acad Sci USA 104:14741–14746
Buhtz A, Springer F, Chappell L, Baulcombe DC, Kehr J (2008) Identification and characterization of small RNAs from the phloem of Brassica napus. Plant J 53:739–749
Bürglin TR (1997) Analysis of TALE superclass homeobox genes (MEIS, PBC, KNOX, Iroquois, TGIF) reveals a novel domain conserved between plants and animals. Nucleic Acids Res 25:4173–4180
Chatterjee M, Banerjee AK, Hannapel DJ (2007) A BELL1-like gene of potato is light activated and wound inducible. Plant Physiol 145:1435–1443
Chen H, Rosin FM, Prat S, Hannapel DJ (2003) Interacting transcription factors from the TALE superclass regulate tuber formation. Plant Physiol 132:1391–1404
Chen H, Banerjee AK, Hannapel DJ (2004) The tandem complex of BEL and KNOX partners is required for transcriptional repression of ga20ox1. Plant J 38:276–284
Crete P, Leuenberger S, Iglesias VA, Suarez V, Schob H, Holtorf H, van Eeden S, Meins F (2001) Graft transmission of induced and spontaneous post-transcriptional silencing of chitinase genes. Plant J 28:493–501
Doering-Saad C, Newbury HJ, Couldridge CE, Bale JS, Pritchard J (2006) A phloem-enriched cDNA library from Ricinus: insights into phloem function. J Exp Bot 57:3183–3193
Esau K (1977) The leaf: basic structure and development. In: Anatomy of seed plants. John Wiley and Sons, New York, pp 321–349
Giavalisco P, Kapitza K, Kolasa A, Buhtz A, Kehr J (2006) Towards the proteome of Brassica napus phloem sap. Proteomics 6:896–909
Hannapel DJ (2007) Signalling the induction of tuber formation. In: Gebhardt C, MacKerron D, Viola R, Vreugdenhil D (eds) Potato biology and biotechnology: advances and perspectives, Elsevier Publishing, Amsterdam, pp 237–256
Haywood V, Yu TS, Huang NC, Lucas WJ (2005) Phloem long-distance trafficking of GA-INSENSITIVE RNA regulates leaf development. Plant J 42:49–68
Kanrar S, Onguka O, Smith HM (2006) Arabidopsis inflorescence architecture requires the activities of KNOX-BELL homeodomain heterodimers. Planta 224:2263–2273
Kehr J, Buhtz A (2008) Long distance transport and movement of RNA through the phloem. J Exp Bot 59:85–92
Kim M, Canio W, Kessler S, Sinha N (2001) Developmental changes due to long-distance movement of a homeobox fusion transcript in tomato. Science 293:287–289
Kim JY, Rim Y, Wang J, Jackson D (2005) A novel cell-to-cell trafficking assay indicates that the KNOX homeodomain is necessary and sufficient for intercellular protein and mRNA trafficking. Genes Dev 19:788–793
Lough TJ, Lucas WJ (2006) Integrative plant biology: role of phloem long-distance macromolecular trafficking. Annu Rev Plant Biol 57:203–232
Lucas WJ, Bouché-Pillon S, Jackson DP, Nguyen L, Baker L, Ding B, Hake S (1995) Selective trafficking of KNOTTED1 homeodomain protein and its mRNA through plasmodesmata. Science 270:1943–1944
Luo H, Song F, Goodman RM, Zheng Z (2005) Up-regulation of OsBIHD1, a rice gene encoding BELL homeodomain transcriptional factor, in disease resistance responses. Plant Biol 7:459–468
Mele G, Ori N, Sato Y, Hake H (2003) The knotted1-like homeobox gene BREVIPEDICELLUS regulates cell differentiation by modulating metabolic pathways. Genes Dev 17:2088–2093
Müller J, Wang Y, Franzen R, Santi L, Salamini F, Rohde W (2001) In vitro interactions between barley TALE proteins suggest a role for protein–protein associations in the regulation of Knox gene function. Plant J 27:13–23
Murashige T, Skoog F (1962) A revised medium for rapid growth and bioassays with tobacco tissue cultures. Physiol Plant 15:473–497
Nazoa P, Vidmar JJ, Tranbarger TJ, Mouline K, Damiani I, Tillard P, Zhuo D, Glass AD, Touraine B (2003) Regulation of the nitrate transporter gene AtNRT2.1 in Arabidopsis thaliana: responses to nitrate, amino acids and developmental stage. Plant Mol Biol 52:689–703
Omid A, Keilin T, Glass A, Leshkowitz D, Wolf S (2007) Characterization of phloem-sap transcription profile in melon plants. J Exp Bot 58:3645–3656
Reiser L, Sánchez-Baracaldo P, Hake S (2000) Knots in the family tree: evolutionary relationships and functions of knox homeobox genes. Plant Mol Biol 42:151–166
Rosin FM, Hart JK, Horner HT, Davies PJ, Hannapel DJ (2003) Overexpression of a Knotted-like homeobox gene of potato alters vegetative development by decreasing gibberellin accumulation. Plant Physiol 132:106–117
Ruiz-Medrano R, Xoconostle-Cazares B, Lucas WJ (1999) Phloem long-distance transport of CmNACP mRNA: implications for supracellular regulation in plants. Development 126:4405–4419
Sawa M, Nusinow DA, Kay SA, Imaizumi T (2007) FKF1 and GIGANTEA complex formation is required for day-length measurement in Arabidopsis. Science 318:261–265
Smith HMS, Hake S (2003) The interaction of two homeobox genes, BREVIPEDICELLUS and PENNYWISE, regulates internode patterning in the Arabidopsis inflorescence. Plant Cell 15:1717–1727
Smith HM, Boschke I, Hake S (2002) Selective interaction of plant homeodomain proteins mediates high DNA-binding affinity. Proc Natl Acad Sci USA 99:9579–9584
Sonoda S, Nishiguchi M (2000) Graft transmission of post-transcriptional gene silencing: target specificity for RNA degradation is transmissible between silenced and non-silenced plants, but not between silenced plants. Plant J 21:1–8
Tamura K, Dudley J, Nei M, Kumar S (2007) MEGA4: molecular evolutionary genetics analysis (MEGA) software version 4.0. Mol Biol Evol 24:1596–1599
Tanaka-Ueguchi M, Itoh H, Oyama N, Koshioka M, Matsuoka M (1998) Over-expression of a tobacco homeobox gene, NTH15, decreases the expression of a gibberellin biosynthetic gene encoding GA 20-oxidase. Plant J 15:391–400
Vilaine F, Palauqui JC, Amselem J, Kusiak C, Lemoine R, Dinant S (2003) Towards deciphering phloem: a transcriptome analysis of the phloem of Apium graveolens. Plant J 36:67–81
Xoconostle-Cazares B, Xiang Y, Ruiz-Medrano R, Wang HL, Monzer J, Yoo BC, McFarland KC, Franceschi VR, Lucas WJ (1999) Plant paralog to viral movement protein that potentiates transport of mRNA into the phloem. Science 283:94–98
Yu YY, Lashbrook CC, Hannapel DJ (2007) Tissue integrity and RNA quality of laser microdissected phloem of potato. Planta 226:797–803
Zhao C, Craig JC, Petzold HE, Dickerman AW, Beers EP (2005) The xylem and phloem transcriptomes from secondary tissues of the Arabidopsis root-hypocotyl. Plant Physiol 138:803–818
Acknowledgments
Thanks to Dr. Anjan Banerjee for his steady and helpful technical assistance. This work was supported by National Science Foundation award no. 0344850 in the Division of Integrative Organismal Biology. BAC was supported by a fellowship from the Monsanto Company. JH was supported by a Research Experience for Undergraduates grant under NSF award no. 0344850 and a fellowship from Cornell College, Mt. Vernon, IA.
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Campbell, B.A., Hallengren, J. & Hannapel, D.J. Accumulation of BEL1-like transcripts in solanaceous species. Planta 228, 897–906 (2008). https://doi.org/10.1007/s00425-008-0780-7
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DOI: https://doi.org/10.1007/s00425-008-0780-7