The Companion Cell-Specific Arabidopsis Disaccharide Carrier AtSUC2 Is Expressed in Nematode-Induced Syncytia

Abstract

Cyst nematodes induce a metabolically highly active syncytial cell complex in host roots. The syncytia are symplastically isolated. Because they form a strong sink, assimilates must be imported via the apoplast, thus suggesting that specific membrane-bound sugar transport proteins are expressed and activated. To identify possible candidate genes, transgenic Arabidopsis plants expressing different reporter genes under the control of different promoters from Arabidopsis sugar transporter genes were infected with the beet cyst nematode (Heterodera schachtii). With polymerase chain reaction, 13 additional sugar transporters were tested for their presence in the syncytia through the use of a syncytium-specific cDNA library. Analysis of the infected roots showed that the promoter of the sucrose (Suc) transporter AtSUC2 gene that codes for a companion cell-specific Suc transporter in noninfected plants was found to be expressed in syncytia. Its expression patterns in ␤-glucuronidase and green fluorescent protein plants were monitored. Syncytium-specific gene expression was confirmed by reverse transcriptase-polymerase chain reaction. Results support the idea that AtSUC2 mediates the transmembrane transfer of Suc. AtSUC2 is the first disaccharide carrier described to be activated by pathogens.Sedentary nematodes are a group of economically important plant parasites that cause profound changes in root anatomy and plant physiology. The so-called cyst nematodes induce syncytial feeding sites in the root vascular cylinder. The nematode juveniles invade the roots and migrate through the cortical tissue by piercing cells with a stylet and releasing cell wall-degrading enzymes (Smant et al., 1998;Popeijus et al., 2000). Eventually, the juveniles enter the vascular cylinder searching for procambial or pericycle cells. In Arabidopsis, the cellular events during feeding site induction and expansion by the beet cyst nematode (Heterodera schachtii) are well described. Initiating from one single cell, a feeding site is induced by releasing secretions from the hollow stylet that is inserted into the initial cell without perforating the plasma membrane (Golinowski et al., 1996). The secretions are supposed to affect the selected cell by inducing a number of changes that finally lead to the fusion of the cell with its neighboring cells. This process continues until a syncytial feeding cell is formed that includes several hundred strongly hypertrophied cells. The syncytium provides the nutritional basis of the developing nematodes, which are strictly biotrophic parasites. Although syncytia are greatly enlarged cell systems, the developing nematodes were calculated to take up several times the syncytium content (Sijmons et al., 1991). Nematode feeding is a highly sophisticated behavioral program that meets the demands of the nematode without destroying the affected host cells (Wyss, 1992). With the aid of so-called feeding tubes, which are most probably the product of stylet secretions, nutrients are taken from the syncytial cytoplasm i...

Differential vascularization of nematode-induced feeding sites

“…When tmGFP9 was expressed from the same promoter, fluorescence remained limited to the phloem outside the RK. Essentially the same result was obtained when the AtSUC2 promoter was used to drive GUS expression (10). This suggests that that CCs are absent from RKs or are degraded or lose their identity, indicated by termination of AtSUC2 expression, during the process of feeding site formation.…”

“…After loading of [ 14 C]-sucrose to source leaves of Arabidopsis, radioactive signals could be monitored along the roots, within syncytia, and in the associated nematodes (8). It was postulated that sucrose transporters are responsible for this accumulation (9,10). However, despite of the presence of transcripts for these transporters in syncytia, the proteins were not detected (11).…”

Starch Serves as Carbohydrate Storage in Nematode-Induced Syncytia

“…In fact, a massive transfer of sugars from the host plant's phloem into syncytia occurs, which leads to the necessary high levels of sugars (Hofmann et al, 2007). Sugar import has been shown to be performed via transporters at the beginning of syncytium formation and via plasmodesmata at later stages (starting around 10 dai; Juergensen et al, 2003;Hoth et al, 2005;Grundler, 2006, 2007a;Hofmann et al, 2007). It could be anticipated that continuous sugar demand induces an equivalent transport, thus establishing physiological equilibrium.…”

“…The cytoplasm of syncytia was extracted with a microcapillary and a micromanipulator without contamination from uninfected root cells or nematodes (Juergensen et al, 2003). Noninfected root pieces cut from the elongation zone without root tips or lateral root primordia were used as control samples.…”

The Time Required for Dormancy Release in Arabidopsis Is Determined by DELAY OF GERMINATION1 Protein Levels in Freshly Harvested Seeds

“…These were the endosperm-specific promoters Pro At-FIS2, Pro At-ZHOUPI, and Pro Nt-CAT1 (Suzuki et al, 1995;Luo et al, 2000;Yang et al, 2008), the epidermal layer of the embryo-specific promoter Pro At-ML1 (Sessions et al, 1999), the meristem-specific promoters Pro At-STM and Pro At-CLV3 (Long et al, 1996), and the phloem-specific promoter Pro At-SUC2 (Juergensen et al, 2003). At-STM, At-ML1, and At-SUC2 promoter entry clones were a gift from George Coupland , and CAT1 promoter was a gift from Gerhard Leubner (Leubner-Metzger, 2005).…”