Arabidopsis is known to synthesize four GDP-linked sugars: GDP-L-fucose, GDP-L-galactose, GDP-D-glucose and GDP-D-mannose. GDP-mannose for the glycosylation of glycosylinositolphosphorylceramides is transported into the Golgi by GONST1 , whereas GDP-fucose is transported by GONST4, which has been renamed GDPfucose transporter1 . No Golgi-localized GDP-L-galactose transporters have been identified to date. GDP-L-galactose is synthesized from GDP-mannose in the cytosol by GDP-mannose epimerase . Most GDP-L-galactose is then converted, via L-galactose, into L-ascorbate , which is important for maintaining redox balance in the cell, particularly under abiotic or biotic stress . However, some GDP-Lgalactose is required for cell wall polysaccharide synthesis since L-galactose is present in side chain A of RG-II, in the side-chains of xyloglucan from a limited number of plant species and in corn bran glucuronoarabinoxylan . Here we provide evidence that GONST3 probably encodes a Golgi-localized GDP-L-galactose transporter, which we rename Golgi GDP-L-galactose transporter1 . We used RNA interference to suppress GGLT1 expression in Arabidopsis, since complete loss of GGLT1 is lethal. Plants with decreased GGLT1 expression have growth defects, which are rescued by increasing the amount of borate in their growth medium. Chemical analysis of the cell walls of GGLT1 knock-down plants revealed a substantial reduction in the L-galactose decoration of RG-II,vertical gardening in greenhouse which is correlated with a decrease in the proportion of RG-II dimer in the wall and a decrease in the stability of the cross link. Our results underscore the importance of RG-II to plant survival, and highlight an unexpectedly critical role for L-galactose in borate cross linking of this unusual pectic polysaccharide.
Publicly available gene expression data reveal that GGLT1 is a ubiquitously expressed gene, with a level of expression that is slightly lower than GONST1 and GFT1 . In an earlier study, the subcellular localization of GGLT1 was not determined because tagged GGLT1 could not be expressed in vivo . To overcome this issue, the full-length GGLT1 coding sequence tagged with a fluorescent protein was introduced into onion epidermal cells by biolistic transformation. Confocal imaging revealed that the fluorescently tagged GGLT1 gave a punctate signal that co-localized with a Golgi marker .No Arabidopsis lines carrying a T-DNA insertion in the GGLT1 open reading frame have been reported. A singleT-DNA line , with an insertion 841 bp upstream of the start of transcription was obtained, but we were unable to identify any plants homozygous for the TDNA insertion despite screening at least 30 different seedlings. Therefore, we took a targeted gene-knockdown approach and generated RNAi transgenic lines with a hairpin RNA construct, which specifically targeted GGLT1. Forty independent hpGGLT1 transformants were screened, and four were selected for characterization . These lines all had rosettes which were smaller than the empty vector control . Quantitative real-time PCR showed that in the rosette leaves of the hpRNAi lines 1–3 the levels of GGLT1 silencing were similar . These data, together with the lack of T-DNA lines, suggest that stronger suppression of GGLT1 or null mutants will produce plants that are not viable. The expression of GFT1, the closest homolog of GGLT1, was not affected in lines 3 and 4 but was decreased by up to 50% in lines 1 and 2 . The monosaccharide compositions of the walls, including fucose, were not significantly altered in any of the hpRNAi lines , indicating that their phenotypes do not result from altered fucosylation of cell wall glycans and are thus a consequence of GGLT1 silencing.
Moreover, the shortened petiole phenotype that is characteristic of silenced GFT1 plants as well as mur1 plants, which also have cell walls with reduced fucose , was not observed in our hpGGLT1 knock-down lines .L-Galactose replaces L-fucose in the xyloglucan formed by mur1 and GFT1-silenced plants where GDP-fucose synthesis or transport is perturbed . Since GGLT1 and GFT1 are closely related NSTs, we first determined if xyloglucan fucosylation is altered in hpGGLT1 . No differences were discernible in the matrix-assisted laser desorption–ionization time-of-flight mass spectra of the oligosaccharides generated by enzymatic fragmentation of the xyloglucan from hpGGLT1 and EV control lines . The presence of fucosylated side chains, together with no substantial increase in the abundance of galactosylated side-chains in the hpGGLT1 lines, supports our assertion that GDP-fucose transport is unaffected in the silenced plants. GGLT1 is in the same NST subclade as GONST1, which provides GDP-mannose specifically for GIPC glycosylation, as opposed to polysaccharide biosynthesis . Although glycosylation of GIPCs is still poorly understood, it is possible that other GDP-sugars, in addition to GDP-mannose are required. Therefore, we used thin layer chromatography and LC-MS to determine the GIPC glycan composition of hpGGLT1. No major differences were discernible between hpGGLT1 and EV GIPCs. The overall sphingolipidomic composition was also unchanged . Together, the combined results of these studies show that GGLT1 does not encode a Golgi-localized protein involved in the transport of GDP-L-fucose or GDP-D-mannose. Thus, we next investigated if the L-galactose content of the wall was altered in the GGLT1 suppressed lines.No significant differences were detected in the wall monosaccharide compositions of leaves from soil-grown EV and hpGGLT1 lines . This is not surprising since in primary cell walls D-galactose is far more abundant than L-galactose . Moreover, D-galactose and L-galactose are not separated when the monosaccharide composition of the cell wall is determined by high-performance anion exchange chromatography with pulsed amperometric detection . Rhamnogalacturonan-II is the only known L-galactose containing polysaccharide present in wild-type Arabidopsis cell walls, so we next determined whether the structure of RG-II differed in hpGGLT1 and EV plants. Material enriched in pectic polysaccharides, including RG-II, was obtained by extracting hpGGLT1 and EV leaf alcohol-insoluble residue with ammonium oxalate, a calcium chelator. This material was then treated with endopolygalacturonase and the products separated by size-exclusion chromatography . This separates RG-II from RG-I and oligogalacturonides, and also separates the RG-II monomer and dimer. In EV control plants the dimer accounts for 77% of the total RG-II isolated from the wall. Somewhat unexpectedly, the dimer accounts for only 49% of the hpGGLT1 RG-II, and makes up only 6% of the RG-II in mur1-1 .
This led us to suspect that the ability of hpGGLT1 RG-II to form dimers or the stability of those dimers had been altered.Under these conditions, in the absence of a chelating agent, the dimer accounted for 97% of the RG-II in the EV control plants , 87% of the RG-II in the hpGGLT1 lines and 70% of the mur1-1 . These results, together with data showing that calcium chelators partially convert the RG-II dimer to the monomer , strongly suggest that both the extent of formation and the stability of the borate cross-link in RG-II are affected in the hpGGLT1 lines. The differences in dimer abundance in the EPG and oxalate fractions were most pronounced with mur1-1 plants. This mutant produces RG-II that lacks L-galactose because its A side-chain is truncated , which led us to suspect that the L-galactose content of side-chain A of the RG-II from the hpGGLT1 lines may also be reduced. To determine if RG-II structure is indeed altered in the hpGGLT1 lines we isolated the total RG-II from the silenced and EV plants. Glycosyl residue composition analyses showed that D/L-galactose was reduced by about 35% in the most strongly affected hpGGLT1 lines . We then treated the RG-II with warm trifluoroacetic acid to release side chains A and B. The MALDI-TOF MS analysis showed that a substantial portion of side-chain A from hpGGLT1 RG-II existed as a heptasaccharide whereas virtually all the A chain from the EV control was present as an octasaccharide . The A side-chains produced by hpGGLT1 and EV plants differ in mass by 162 Da, corresponding to a hexose residue, which we consider likely to be L-galactose. The side-chain B of RG-II contains a D-galactose residue . However, no differences were discernible in the structures of this side-chain from RG-II of hpGGLT1 and EV plants . Our structural data provide compelling evidence that the abundance of terminal L-galactose present on the A side-chain of RG-II is specifically affected in hpGGLT1 plants. To confirm the identity of the missing hexose in sidechain A,greenhouse vertical farming the RG-II monomers generated from the hpGGLT1 and EV plants were treated with a recently identified a-Lgalactosidase 95 from Bacteroides thetaiotaomicron that specifically removes the terminal L-galactose from side-chain A of RGII . Galactose was the only monosaccharide detected by HPAEC-PAD following hydrolysis of EV control RG-II with the a-L-galactosidase . Less galactose was released from the RG-II of the hpGGLT1- silenced lines relative to the control . The MALDI-TOF MS analysis of side-chain A, released by mild TFA hydrolysis following a-L-galactosidase treatment of RG-II monomer, revealed that the predominant oligosaccharides in the EV control plants correspond to side-chain A lacking L-galactose . The L-galactose was almost completely removed as only low-intensity signals corresponding to L-galactosylated A side-chains were discernible . The mass spectra of side-chain A from both hpGGLT1 silenced lines are similar to that of the EV control , demonstrating that the mass difference of 162 Da between the EV control and hpGGLT1 lines in Figure 3 is due to the specific loss of L-galactose. It has been proposed that pectin domains may be linked covalently to each other or to other cell wall components . To investigate whether the altered RG-II structure in the hpGGLT1 silenced lines had affected other pectic domains, the oxalate cell wall fraction was used to perform immune dotblots with a panel of antibodies raided against different pectin epitopes .
However, no difference was observed between the EV control and the silenced lines. In combination with the monosaccharide composition data and the xyloglucan data we conclude that the reduction in GGLT1 expression does not affect non-RG-II polymers. These data provide strong evidence that silencing of GGLT1 leads to a reduction in the abundance of L-galactose on side-chain A of RG-II, and provides additional evidence that the absence of this sugar leads to a decrease in the ability of the RG-II monomer to self-assemble into a borate cross-linked dimer. Moreover, this L-galactosedepleted dimer is less stable in the presence of calcium chelators than its wild-type counterpart, a result consistent with the notion that interactions of borate and calcium with RG-II are important for plant growth .Several growth phenotypes, including the dwarf phenotype of mur1, that have been attributed to defects in RG-II structure and cross-linking have been reported to be rescued by supplementing the growth medium with additional borate . To further explore the observed growth phenotypes of the hpGGLT1 lines , plants were grown hydroponically to control the availability of all macro- and micro-nutrients, including borate. In low-borate media the hpGGLT1 lines are severely stressed, and their rosette diameter is about 70% smaller than that of EV control plants . However, this phenotype is not observed when the silenced plants are grown in high-borate media . The amount of borate in the growth medium did not affect GGLT1 expression, thereby excluding a potential effect of borate deficiency or supplementation on transgene expression and silencing strength . Therefore, we conclude that partial loss of the RG-II L-galactose decoration in hpGGLT1 reduces the rate of RG-II borate-dependent dimerization, directly affecting plant development.Since hpGGLT1 plants grown in the presence of 1 mM boric acid or no added boric acid had different phenotypes,we were curious to know if altering the structure and dimerization of RG-II in hpGGLT1 led to changes in other cell wall components. Therefore we determined the monosaccharide composition of destarched leaf AIR from plants grown under different borate concentrations . No significant visible differences were discernible in hpGGLT1 and EV plants grown with 1 mM borate . However, we saw increases in the abundance of several neutral monosaccharides, in particular glucose , in the walls of plants grown with no added borate. No differences in aniline blue staining of the walls of EV and hpGGLT1 lines were observed, suggesting that the increase in non-cellulosic glucose in plants which appear severely stressed is not due to callose deposition . Finally, we performed Saeman hydrolysis of the TFA-resistant AIR to determine the amount of glucose derived from crystalline cellulose. A substantial increase in cellulose-derived glucose was detected in the hpGGLT1 lines grown with no added borate but not in plants grown under high-borate conditions .