Molecular and biochemical studies of the <em>Craterostigma plantagineum</em> cell wall during dehydration and rehydration

Abstract

<em>Craterostigma plantagineum</em> belongs to the desiccation tolerant angiosperm plants. Upon dehydration leaves fold and the cells shrink which is reversed during rehydration. To understand this process changes in cell wall pectin composition, and the role of the apoplastic glycine-rich protein1 (CpGRP1) were analysed. Cellular microstructural changes in hydrated, desiccated and rehydrated leaf sections were analysed using scanning electron microscopy. These studies visualised the folding and unfolding of cell walls upon dehydration and rehydration. Pectin composition in different cell wall fractions was analysed with monoclonal antibodies against homogalacturonan, rhamnogalacturonan-I, rhamnogalacturonan-II and hemicellulose epitopes. The data demonstrate changes in pectin composition during dehydration/rehydration which is suggested to affect cell wall properties. Homogalacturonan was less methylesterified upon desiccation and changes were also demonstrated in the detection of rhamno-galacturonan-I, rhamnogalacturonan-II and hemicelluloses. CpGRP1 seems to have a central role in cellular adaptations to water deficit, as it interacts with pectin through a cluster of arginine residues, and de-methylesterified pectin presents more binding sites for the protein-pectin interaction than pectin from hydrated leaves. CpGRP1 can also bind phosphatidic acid and cardiolipin. The binding of CpGRP1 to pectin appears to be dependent on the pectin methylesterification status and it has a higher affinity to pectin than its binding partner CpWAK1. It is hypothesised that changes in pectin composition are sensed by the CpGRP1-CpWAK1 complex thus leading to the activation of dehydration-related responses and leaf folding (Figure 1). Phosphatidic acid might participate in the modulation of CpGRP1 activity. <br /> To investigate the process of pectin de-methylesterification upon desiccation transcriptome data for pectinmethylesterases and pectinmethylesterase inhibitor proteins were analysed. One enzyme was selected as an interesting candidate because it was the only pectinmethylesterase which was upregulated upon desiccation. This pectinmethylesterase had similarities to the pectinmethylesterase31 from <em>Arabidopsis thaliana</em> (AtPME31). The transcript was highly abundant in the vegetative tissue of <em>C. plantagineum</em> upon desiccation, but in <em>A. thaliana</em> the transcript is highly abundant in dry seeds. This pectinmethylesterase might be an interesting candidate to understand cell wall remodelling processes in <em>C. plantagineum</em>.