The CaPRP1 gene encoding a putative proline-rich glycoprotein is highly expressed in rapidly elongating early roots and leaves in hot pepper (Capsicum annuum L. cv. Pukang)

The Cell Wall PAC (Proline-Rich, Arabinogalactan Proteins, Conserved Cysteines) Domain-Proteins Are Conserved in the Green Lineage

Plant cell wall proteins play major roles during plant development and in response to environmental cues. A bioinformatic search for functional domains has allowed identifying the PAC domain (Proline-rich, Arabinogalactan proteins, conserved Cysteines) in several proteins (PDPs) identified in cell wall proteomes. This domain is assumed to interact with pectic polysaccharides and O-glycans and to contribute to non-covalent molecular scaffolds facilitating the remodeling of polysaccharidic networks during rapid cell expansion. In this work, the characteristics of the PAC domain are described in detail, including six conserved Cys residues, their spacing, and the predicted secondary structures. Modeling has been performed based on the crystal structure of a Plantago lanceolata PAC domain. The presence of β-sheets is assumed to ensure the correct folding of the PAC domain as a β-barrel with loop regions. We show that PDPs are present in early divergent organisms from the green lineage and in all land plants. PAC domains are associated with other types of domains: Histidine-rich, extensin, Proline-rich, or yet uncharacterized. The earliest divergent organisms having PDPs are Bryophytes. Like the complexity of the cell walls, the number and complexity of PDPs steadily increase during the evolution of the green lineage. The association of PAC domains with other domains suggests a neo-functionalization and different types of interactions with cell wall polymers

Arabinogalactan protein 31 (AGP31), a putative network-forming protein in Arabidopsis thaliana cell walls?

BACKGROUND AND AIMS

Arabinogalactan protein 31 (AGP31) is a remarkable plant cell-wall protein displaying a multi-domain organization unique in Arabidopsis thaliana: it comprises a predicted signal peptide (SP), a short AGP domain of seven amino acids, a His-stretch, a Pro-rich domain and a PAC (PRP-AGP containing Cys) domain. AGP31 displays different O-glycosylation patterns with arabinogalactans on the AGP domain and Hyp-O-Gal/Ara-rich motifs on the Pro-rich domain. AGP31 has been identified as an abundant protein in cell walls of etiolated hypocotyls, but its function has not been investigated thus far. Literature data suggest that AGP31 may interact with cell-wall components. The purpose of the present study was to identify AGP31 partners to gain new insight into its function in cell walls.

METHODS

Nitrocellulose membranes were prepared by spotting different polysaccharides, which were either obtained commercially or extracted from cell walls of Arabidopsis thaliana and Brachypodium distachyon. After validation of the arrays, in vitro interaction assays were carried out by probing the membranes with purified native AGP31 or recombinant PAC-V5-6xHis. In addition, dynamic light scattering (DLS) analyses were carried out on an AGP31 purified fraction.

KEY RESULTS

It was demonstrated that AGP31 interacts through its PAC domain with galactans that are branches of rhamnogalacturonan I. This is the first experimental evidence that a PAC domain, also found as an entire protein or a domain of AGP31 homologues, can bind carbohydrates. AGP31 was also found to bind methylesterified polygalacturonic acid, possibly through its His-stretch. Finally, AGP31 was able to interact with itself in vitro through its PAC domain. DLS data showed that AGP31 forms aggregates in solution, corroborating the hypothesis of an auto-assembly.

CONCLUSIONS

These results allow the proposal of a model of interactions of AGP31 with different cell-wall components, in which AGP31 participates in complex supra-molecular scaffolds. Such scaffolds could contribute to the strengthening of cell walls of quickly growing organs such as etiolated hypocotyls.

Characterization of PhPRP1, a histidine domain arabinogalactan protein from Petunia hybrida pistils

An arabinogalactan protein, PhPRP1, was purified from Petunia hybrida pistils and shown to be orthologous to TTS-1 and TTS-2 from Nicotiana tabacum and NaTTS from Nicotiana alata. Sequence comparisons among these proteins, and CaPRP1 from Capsicum annuum, reveal a conserved histidine-rich domain and two hypervariable domains. Immunoblots show that TTS-1 and PhPRP1 are also expressed in vegetative tissues of tobacco and petunia respectively. In contrast to the molecular mass heterogeneity displayed by the pistil proteins, the different isoforms found in seedlings, roots, and leaves each has a discrete size (37, 80, 160, and 200 kDa) on SDS-PAGE gels. On the basis of their chemistry, distinctive domain architecture, and the unique pattern of expression, we have named this group of proteins HD-AGPs (histidine domain-arabinogalactan proteins).

Temporal and spatial expression analysis of PRGL in Gerbera hybrida

GASA-like genes form multigene families in diverse plant species and encode the proteins with a unique cysteine-rich domain (GASA domain). In our previously work, we cloned a GASA-like gene PRGL (Proline-rich GASA-like) from gerbera. Here we report the expression profiles of PRGL and the subcellular localization of PRGL protein. Multiple sequence alignment of the GASA domains indicates that PRGL shows the highest homology to AtPRGL (73.3% of amino acid identity) from Arabidopsis. Phylogenic analysis based on the full amino acid sequences indicates that PRGL and AtPRGL belong to a novel subfamily of GASA proteins. Northern blot assay showed that PRGL is highly expressed in young flower, young leaf and young root, whereas hardly detected when these organs became mature. Furthermore, in young inflorescence, PRGL transcript accumulation only occurred in the fast elongation organs such as scape, ray floret petal and disc floret petal. Western blot and immunolocalization assay revealed that PRGL protein is located in cell wall and high level accumulation of PRGL was found to correlate with the fast organ elongation in scape. Our results suggest that PRGL participates in the regulation of cell elongation during the development of Gerbera hybrida plant.

Heterologous expression, and biochemical and cellular characterization of CaPLA1 encoding a hot pepper phospholipase A1 homolog

Phospholipid signaling has been recently implicated in diverse cellular processes in higher plants. We identified a cDNA encoding the phospholipase A1 homolog (CaPLA1) from 5-day-old early roots of hot pepper. The deduced amino acid sequence showed that the lipase-specific catalytic triad is well conserved in CaPLA1. In vitro lipase assays and site-directed mutagenesis revealed that CaPLA1 possesses PLA1 activity, which catalyzes the hydrolysis of phospholipids at the sn-1 position. CaPLA1 was selectively expressed in young roots, at days 4-5 after germination, and rapidly declined thereafter, suggesting that the expression of CaPLA1 is subject to control by a development-specific mechanism in roots. Because transgenic work was extremely difficult in hot peppers, in this study we overexpressed CaPLA1 in Arabidopsis so as to provide cellular information on the function of this gene. CaPLA1 overexpressors had significantly longer roots, leaves and petioles, and grew more rapidly than the wild-type plants, leading to an early bolting phenotype with prolonged inflorescence. Microscopic analysis showed that the vegetative tissues of 35S:CaPLA1 plants contained an increased number of small-sized cells, which resulted in highly populated cell layers. In addition, mRNAs for cell cycle-controlled proteins and fatty acid catabolizing enzymes were coordinately upregulated in CaPLA1-overexpressing plants. These results suggest that CaPLA1 is functionally relevant in heterologous Arabidopsis cells, and hence might participate in a subset of positive control mechanisms of cell and tissue growth in transgenic lines. We discuss possible biochemical and cellular functions of CaPLA1 in relation to the phospholipid signaling pathway in hot pepper and transgenic Arabidopsis plants.

Constitutive expression of abiotic stress-inducible hot pepper CaXTH3, which encodes a xyloglucan endotransglucosylase/hydrolase homolog, improves drought and salt tolerance in transgenic Arabidopsis plants

Xyloglucan endotransglucosylase/hydrolase (XTH) has been recognized as a cell wall-modifying enzyme, participating in the diverse physiological roles. From water-stressed hot pepper plants, we isolated three different cDNA clones (pCaXTH1, pCaXTH2, and pCaXTH3) that encode XTH homologs. RT-PCR analysis showed that three CaXTH mRNAs were concomitantly induced by a broad spectrum of abiotic stresses, including drought, high salinity and cold temperature, and in response to stress hormone ethylene, suggesting their role in the early events in the abiotic-related defense response. Transgenic Arabidopsis plants that constitutively expressed the CaXTH3 gene under the control of the CaMV 35S promoter exhibited abnormal leaf morphology; the transgenic leaves showed variable degrees of twisting and bending along the edges, resulting in a severely wrinkled leaf shape. Microscopic analysis showed that 35S-CaXTH3 leaves had increased numbers of small-sized cells, resulting in disordered, highly populated mesophyll cells in each dorsoventral layer, and appeared to contain a limited amount of starch. In addition, the 35S-CaXTH3 transgenic plants displayed markedly improved tolerance to severe water deficit, and to lesser extent to high salinity in comparison with the wild-type plants. These results indicate that CaXTH3 is functional in heterologous Arabidopsis cells, thereby effectively altering cell growth and also the response to abiotic stresses. Although the physiological function of CaXTHs is not yet clear, there are several possibilities for their involvement in a subset of physiological responses to counteract dehydration and high salinity stresses in transgenic Arabidopsis plants.