New insights on the expression patterns of specific Arabinogalactan proteins in reproductive tissues of Arabidopsis thaliana

Eight hydroxyproline-O-galactosyltransferases play essential roles in female reproductive development

In angiosperms, ovules give rise to seeds upon fertilization. Thus, seed formation is dependent on both successful ovule development and tightly controlled communication between female and male gametophytes. During establishment of these interactions, cell walls play a pivotal role, especially arabinogalactan-proteins (AGPs). AGPs are highly glycosylated proteins decorated by arabinogalactan side chains, representing 90 % of the AGP molecule. AGP glycosylation is initiated by a reaction catalysed by hydroxyproline-O-galactosyltransferases (Hyp-GALTs), specifically eight of them (GALT2-9), which add the first galactose to Hyp residues. Five Hyp-GALTs (GALT2, 5, 7, 8 and 9) were previously described as essential for AGP functions in pollen and ovule development, pollen-pistil interactions, and seed morphology. In the present work, a higher order Hyp-GALT mutant (23456789) was studied, with a high degree of under-glycosylated AGPs, to gain deeper insight into the crucial roles of these eight enzymes in female reproductive tissues. Notably, the 23456789 mutant demonstrated a high quantity of unfertilized ovules, displaying abnormal callose accumulation both at the micropylar region and, sometimes, throughout the entire embryo sac. Additionally, this mutant displayed ovules with abnormal embryo sacs, had a disrupted spatiotemporal distribution of AGPs in female reproductive tissues, and showed abnormal seed and embryo development, concomitant with a reduction in AGP-GlcA levels. This study revealed that at least three more enzymes exhibit Hyp-O-GALT activity in Arabidopsis (GALT3, 4 and 6), and reinforces the crucial importance of AGP carbohydrates in carrying out the biological functions of AGPs during plant reproduction.

AGPs as molecular determinants of reproductive development

BACKGROUND AND AIMS

Morphogenesis occurs through accurate interaction between essential players to generate highly specialized plant organs. Fruit structure and function are triggered by a neat transcriptional control involving distinct regulator genes encoding transcription factors (TFs) or signalling proteins, such as the C2H2/C2HC zinc-finger NO TRANSMITTING TRACT (NTT) or the MADS-box protein SEEDSTICK (STK), which are important in setting plant reproductive competence, feasibly by affecting cell wall polysaccharide and lipid distribution. Arabinogalactan proteins (AGPs) are major components of the cell wall and are thought to be involved in the reproductive process as important players in specific stages of development. The detection of AGPs epitopes in reproductive tissues of NTT and other fruit development-related TFs, such as MADS-box proteins including SHATTERPROOF1 (SHP1), SHP2 and STK, was the focus of this study.

METHODS

We used fluorescence microscopy to perform immunolocalization analyses on stk and ntt single mutants, on the ntt stk double mutant and on the stk shp1 shp2 triple mutant using specific anti-AGP monoclonal antibodies. In these mutants, the expression levels of selected AGP genes were also measured by quantitative real-time PCR and compared with the respective expression in wild-type (WT) plants.

KEY RESULTS

The present immunolocalization study collects information on the distribution patterns of specific AGPs in Arabidopsis female reproductive tissues, complemented by the quantification of AGP expression levels, comparing WT, stk and ntt single mutants, the ntt stk double mutant and the stk shp1 shp2 triple mutant.

CONCLUSIONS

These findings reveal distinct AGP distribution patterns in different developmental mutants related to the female reproductive unit in Arabidopsis. The value of the immunofluorescence labelling technique is highlighted in this study as an invaluable tool to dissect the remodelling nature of the cell wall in developmental processes.

Type II arabinogalactans initiated by hydroxyproline-O-galactosyltransferases play important roles in pollen-pistil interactions

Arabinogalactan-proteins (AGPs) are hydroxyproline-rich glycoproteins containing a high sugar content and are widely distributed in the plant kingdom. AGPs have long been suggested to play important roles in sexual plant reproduction. The synthesis of their complex carbohydrates is initiated by a family of hydroxyproline galactosyltransferase (Hyp-GALT) enzymes which add the first galactose to Hyp residues in the protein backbone. Eight Hyp-GALT enzymes have been identified so far, and in the present work a mutant affecting five of these enzymes (galt2galt5galt7galt8galt9) was analyzed regarding the reproductive process. The galt25789 mutant presented a low seed set, and reciprocal crosses indicated a significant female gametophytic contribution to this mutant phenotype. Mutant ovules revealed abnormal callose accumulation inside the embryo sac and integument defects at the micropylar region culminating in defects in pollen tube reception. In addition, immunolocalization and biochemical analyses allowed the detection of a reduction in the amount of glucuronic acid in mutant ovary AGPs. Dramatically low amounts of high-molecular-weight Hyp-O-glycosides obtained following size exclusion chromatography of base-hydrolyzed mutant AGPs compared to the wild type indicated the presence of underglycosylated AGPs in the galt25789 mutant, while the monosaccharide composition of these Hyp-O-glycosides displayed no significant changes compared to the wild-type Hyp-O-glycosides. The present work demonstrates the functional importance of the carbohydrate moieties of AGPs in ovule development and pollen-pistil interactions.