Saez-Aguayo, S. | bioRxiv Science

bioRxiv2022

Transcriptomic reprogramming in a susceptible Phaseolus vulgaris L. variety during Pseudomonas syringae attack: The key role of homogalacturonan methylation

The susceptibility of common bean varieties to Pseudomonas syringae pv. phaseolicola (Pph) has been well-documented. However, the molecular mechanism that drives this susceptibility has not been clarified yet. In an attempt to understand this process, 15-day-old common bean plants, variety rinon, were infected with Pph to analyze the transcriptomic changes during the first steps of the infection (at 2 and 9 h). RNA-seq analysis showed an upregulation of defense-and signaling-related genes at 2h, most of them being downregulated at 9h, suggesting that Pph would inhibit the transcriptomic reprogramming of the plant. This trend was also observed in the modulation of 101 cell wall (CW) related genes, suggesting that Pph could produce/induce changes in the CW. However, the changes in CW composition at early stages of Pph infection were related to homogalacturonan (HG) methylation and the formation of HG egg boxes. From all HG-related genes modulated by the infection, a common bean pectin methylesterase inhibitor 3 (PvPMEI3) gene - closely related to AtPMEI3 -- was detected. In addition, PMEI3 protein was located in the apoplast and its PME inhibitory activity was demonstrated. Therefore, PvPMEI3 seems to be a good candidate to play a key role in Pph infection. This premise was supported by the analysis of Arabidopsis pmei3 mutant, which showed susceptibility to Pph, in contrast to resistant Col-0 control plants. All these changes could be an attempt to reinforce the CW structure and thus, hinder the attack of the bacterium. However, these transcriptional and CW-remodeling processes are neither maintained during the necessary time, nor are deep enough to block the action of the pathogen, facilitating the well-known susceptibility of rinon variety to Pph.

plant biology↗

bioRxiv2021

Discovery of putative Golgi S-Adenosyl methionine transporters reveals the importance of plant cell wall polysaccharide methylation

Polysaccharide methylation, especially that of pectin, is a common and important feature of land plant cell walls. Polysaccharide methylation takes place in the Golgi apparatus and therefore relies on the import of S-adenosyl methionine (SAM) from the cytosol into the Golgi. However, to date, no Golgi SAM transporter has been identified in plants. In this work, we studied major facilitator superfamily members in Arabidopsis that we identified as putative Golgi SAM transporters (GoSAMTs). Knock-out of the two most highly expressed GoSAMTs led to a strong reduction in Golgi-synthesised polysaccharide methylation. Furthermore, solid-state NMR experiments revealed that reduced methylation changed cell wall polysaccharide conformations, interactions and mobilities. Notably, the NMR revealed the existence of pectin egg-box structures in intact cell walls, and showed that their formation is enhanced by reduced methyl-esterification. These changes in wall architecture were linked to substantial growth and developmental phenotypes. In particular, anisotropic growth was strongly impaired in the double mutant. The identification of putative transporters that import SAM into the Golgi lumen in plants provides new insights into the paramount importance of polysaccharide methylation for plant cell wall structure and function.

plant biology↗