[Sex determination in cucurbits]

Sex determination in plants leads to the development of unisexual flowers from an originally bisexual floral meristem. Cucurbits are not only species of agronomic interest but they also represent model species for the study of plant sex determination, because of their ability to harbor different sexual types. Such sexual forms are controlled by the identity of the alleles at the following loci: andromonoecious (a) and gynoecious (g) in melon, or androecious (a), Female (F), and Monoecious (M) in cucumber. We firstly showed that the andromonoecious a gene in melon encodes for an ACC synthase (CmACS7) and demonstrated that andromonoecy results from a mutation in the active site of the enzyme. Expression of the active enzyme inhibits the development of the male organs and is not required for carpel development. Because the a gene in melon and M gene in cucumber control the same sexual transition, monoecy to andromonoecy, we isolated the andromonoecy M gene in cucumber using a candidate gene approach in combination with genetic and biochemical analysis. We demonstrated the co-segregation of CsACS2, a close ortholog of CmACS7, with the M locus, and showed that the cucumber andromonoecious phenotype is also due to a loss of ACS enzymatic activity. CsACS2 is expressed specifically in carpel primordia of female flowers and should play a similar role to that of CmACS7 in melon in the inhibition of stamina development. Finally, we also showed that the transition from male to female flowers in the gynoecious lines results from epigenetic changes in the promoter of a C(2)H (2) zinc-finger transcription factor, CmWIP1. This epigenetic change is elicited by the insertion of a DNA transposon, which causes the spreading of DNA methylation to the CmWIP1 promoter. Expression of CmWIP1 leads to carpel abortion, resulting in the development of unisexual male flowers. From all these results, we built a model in which CmACS7 and CmWIP1 interact to control the development of male, female and hermaphrodite flowers in melon.

Identification and characterization of tomato mutants affected in the Rx-mediated resistance to PVX isolates

Five tomato mutants affected in the Rx-mediated resistance against Potato virus X (PVX) were identified by screening a mutagenized population derived from a transgenic, Rx1-expressing 'Micro-Tom' line. Contrary to their parental line, they failed to develop lethal systemic necrosis upon infection with the virulent PVX-KH2 isolate. Sequence analysis and quantitative reverse-transcription polymerase chain reaction experiments indicated that the mutants are not affected in the Rx1 transgene or in the Hsp90, RanGap1 and RanGap2, Rar1 and Sgt1 genes. Inoculation with the PVX-CP4 avirulent isolate demonstrated that the Rx1 resistance was still effective in the mutants. In contrast, the virulent PVX-KH2 isolate accumulation was readily detectable in all mutants, which could further be separated in two groups depending on their ability to restrict the accumulation of PVX-RR, a mutant affected at two key positions for Rx1 elicitor activity. Finally, transient expression of the viral capsid protein elicitor indicated that the various mutants have retained the ability to mount an Rx1-mediated hypersensitive response. Taken together, the results obtained are consistent with a modification of the specificity or intensity of the Rx1-mediated response. The five Micro-Tom mutants should provide very valuable resources for the identification of novel tomato genes affecting the functioning of the Rx gene.

Towards a better understanding of the generation of fructan structure diversity in plants: molecular and functional characterization of a sucrose:fructan 6-fructosyltransferase (6-SFT) cDNA from perennial ryegrass (Lolium perenne)

The main storage compounds in Lolium perenne are fructans with prevailing β(2-6) linkages. A cDNA library of L. perenne was screened using Poa secunda sucrose:fructan 6-fructosyltransferase (6-SFT) as a probe. A full-length Lp6-SFT clone was isolated as shown by heterologous expression in Pichia pastoris. High levels of Lp6-SFT transcription were found in the growth zone of elongating leaves and in mature leaf sheaths where fructans are synthesized. Upon fructan synthesis induction, Lp6-SFT transcription was high in mature leaf blades but with no concomitant accumulation of fructans. In vitro studies with the recombinant Lp6-SFT protein showed that both 1-kestotriose and 6G-kestotriose acted as fructosyl acceptors, producing 1- and 6-kestotetraose (bifurcose) and 6G,6-kestotetraose, respectively. Interestingly, bifurcose formation ceased and 6G,6-kestotetraose was formed instead, when recombinant fructan:fructan 6G-fructosyltransferase (6G-FFT) of L. perenne was introduced in the enzyme assay with sucrose and 1-kestotriose as substrates. The remarkable absence of bifurcose in L. perenne tissues might be explained by a higher affinity of 6G-FFT, as compared with 6-SFT, for 1-kestotriose, which is the first fructan formed. Surprisingly, recombinant 6-SFT from Hordeum vulgare, a plant devoid of fructans with internal glucosyl residues, also produced 6G,6-kestotetraose from sucrose and 6G-kestotriose. In the presence of recombinant L. perenne 6G-FFT, it produced 6G,6-kestotetraose from 1-kestotriose and sucrose, like L. perenne 6-SFT. Thus, we demonstrate that the two 6-SFTs have close catalytic properties and that the distinct fructans formed in L. perenne and H. vulgare can be explained by the presence of 6G-FFT activity in L. perenne and its absence in H. vulgare.

Transforming a fructan:fructan 6G-fructosyltransferase from perennial ryegrass into a sucrose:sucrose 1-fructosyltransferase

Fructosyltransferases (FTs) synthesize fructans, fructose polymers accumulating in economically important cool-season grasses and cereals. FTs might be crucial for plant survival under stress conditions in species in which fructans represent the major form of reserve carbohydrate, such as perennial ryegrass (Lolium perenne). Two FT types can be distinguished: those using sucrose (S-type enzymes: sucrose:sucrose 1-fructosyltransferase [1-SST], sucrose:fructan 6-fructosyltransferase) and those using fructans (F-type enzymes: fructan:fructan 1-fructosyltransferase [1-FFT], fructan:fructan 6G-fructosyltransferase [6G-FFT]) as preferential donor substrate. Here, we report, to our knowledge for the first time, the transformation of an F-type enzyme (6G-FFT/1-FFT) into an S-type enzyme (1-SST) using perennial ryegrass 6G-FFT/1-FFT (Lp6G-FFT/1-FFT) and 1-SST (Lp1-SST) as model enzymes. This transformation was accomplished by mutating three amino acids (N340D, W343R, and S415N) in the vicinity of the active site of Lp6G-FFT/1-FFT. In addition, effects of each amino acid mutation alone or in combination have been studied. Our results strongly suggest that the amino acid at position 343 (tryptophan or arginine) can greatly determine the donor substrate characteristics by influencing the position of the amino acid at position 340. Moreover, the presence of arginine-343 negatively affects the formation of neofructan-type linkages. The results are compared with recent findings on donor substrate selectivity within the group of plant cell wall invertases and fructan exohydrolases. Taken together, these insights contribute to our knowledge of structure/function relationships within plant family 32 glycosyl hydrolases and open the way to the production of tailor-made fructans on a larger scale.

Impact of defoliation frequency on regrowth and carbohydrate metabolism in contrasting varieties of Lolium perenne

The aims of the study were to gain a better understanding of fructan metabolism regulation during regrowth of Lolium perenne, and to evaluate the role of fructans of remaining tissues as well as carbon assimilation of new leaf tissues in refoliation. Two varieties that contrast for carbohydrate metabolism, Aurora and Perma, were subject to severe and frequent or infrequent defoliations before regrowth. Aurora, which had a greater content of fructans in leaf sheaths than Perma before defoliation, produced more leaf biomass within the 4 days following the first cut. At the end of the regrowth period, Aurora produced more leaf biomass than Perma. Photosynthetic parameters, which were barely affected by defoliation frequency, could not explain these differences. Fructan synthesising activities [sucrose:sucrose 1-fructosyltransferase (1-SST) and fructan:fructan 6G-fructosyltransferase (6G-FFT)], declined after defoliation. In elongating leaf bases, corresponding transcript levels did not decline concomitantly, suggesting a post-transcriptional regulation of expression, while in leaf sheaths the gene expression pattern mostly followed the time-course of the enzyme activities. Regulation of Lp1-SST and Lp6G-FFT gene expression depends, therefore, on the sink-source status of the tissue after defoliation. During the phase of reserve accumulation, fructosyltransferase activities together with corresponding transcripts increased more in frequently defoliated plants than in infrequently defoliated plants.

Cloning, gene mapping, and functional analysis of a fructan 1-exohydrolase (1-FEH) from Lolium perenne implicated in fructan synthesis rather than in fructan mobilization

Fructans, which are beta-(2,1) and/or beta-(2,6) linked polymers of fructose, are important storage carbohydrates in many plants. They are mobilized via fructan exohydrolases (FEHs). The cloning, mapping, and functional analysis of the first 1-FEH (EC 3.2.1.153) from Lolium perenne L. var. Bravo is described here. By screening a perennial ryegrass cDNA library, a 1-FEH cDNA named Lp1-FEHa was cloned. The Lp1-FEHa deduced protein has a low iso-electric point (5.22) and it groups together with plant FEHs and cell-wall type invertases. The deduced amino acid sequence shows 75% identity to wheat 1-FEH w2. The Lp1-FEHa gene was mapped at a distal position on the linkage group 3 (LG3). Functional characterization of the recombinant protein in Pichia pastoris demonstrated that it had high FEH activity towards 1-kestotriose, 1,1-kestotetraose, and inulin, but low activity against 6-kestotriose and levan. Like other fructan-plant FEHs, no hydrolase activity could be detected towards sucrose, convincingly demonstrating that the enzyme is not a classic invertase. The expression pattern analysis of Lp1-FEHa revealed transcript accumulation in leaf tissues accumulating fructans while transcript level was low in the photosynthetic tissues. The high expression level of this 1-FEH in conditions of active fructan synthesis, together with its low expression level when fructan contents are low, suggest that it might play a role as a beta-(2,1) trimming enzyme acting during fructan synthesis in concert with fructan synthesis enzymes.

Molecular and functional characterization of a cDNA encoding fructan:fructan 6G-fructosyltransferase (6G-FFT)/fructan:fructan 1-fructosyltransferase (1-FFT) from perennial ryegrass (Lolium perenne L.)

Fructans are the main storage compound in Lolium perenne. To account for the prevailing neokestose-based fructan synthesis in this species, a cDNA library of L. perenne was screened by using the onion (Allium cepa) fructan:fructan 6G-fructosyltransferase (6G-FFT) as a probe. A full length Lp6G-FFT clone was isolated with significant homologies to vacuolar type fructosyltransferases and invertases. The functionality of the cDNA was tested by heterologous expression in Pichia pastoris. The recombinant protein demonstrated both 6G-FFT and fructan:fructan 1-fructosyltransferase activities (1-FFT) with a maximum 6G-FFT/1-FFT ratio of two. The activity of 6G-FFT was investigated with respect to developmental stage, tissue distribution, and alterations in carbohydrate status expression and compared to sucrose:sucrose 1-fructosyltransferase (1-SST). Lp6G-FFT and Lp1-SST were predominantly expressed in the basal part of elongating leaves and leaf sheaths. Expression of both genes declined along the leaf axis, in parallel with the spatial occurrence of fructan and fructosyltransferase activities. Surprisingly, Lp6G-FFT was highly expressed in photosynthetically active tissues where very low extractable fructosyltransferase activity and fructan amounts were detected, suggesting a post-transcriptional regulation of expression. Lp6G-FFT gene expression increased only in elongating leaves following similar increases of sucrose content in blades, sheaths, and elongating leaf bases. Regulation of Lp6G-FFT gene expression depends on the tissue according to its sink-source status.