PRPs localized to the middle lamellae are required for cortical tissue integrity in Medicago truncatula roots
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A family of repetitive proline-rich proteins interact with acidic pectins and play distinct roles in legume root cell walls affecting cortical and vascular structure.
A proline-rich protein (PRP) family, composed of tandemly repeated Pro-Hyp-Val-X-Lys pentapeptide motifs, is found primarily in the Leguminosae. Four distinct size classes within this family are encoded by seven tightly linked genes: MtPRP1, MtPRP2 and MtPRP3, and four nearly identical MtPRP4 genes. Promoter fusions to β-glucuronidase showed strong expression in the stele of hairy roots for all 4 PRP genes tested, with additional expression in the cortex for PRP1, PRP2 and PRP4. All except MtPRP4 are strongly expressed in non-tumorous roots, and secreted and ionically bound to root cell walls. These PRPs are absent from root epidermal cell walls, and PRP accumulation is highly localized within the walls of root cortical and vascular tissues. Within xylem tissue, PRPs are deposited in secondary thickenings where it is spatially exclusive to lignin. In newly differentiating xylem, PRPs are deposited in the regularly spaced paired-pits and pit membranes that hydraulically connect neighboring xylem elements. Hairpin-RNA knock-down constructs reducing PRP expression in Medicago truncatula hairy root tumors disrupted cortical and vascular patterning. Immunoblots showed that the knockdown tumors had potentially compensating increases in the non-targeted PRPs, all of which cross-react with the anti-PRP antibodies. However, PRP3 knockdown differed from knockdown of PRP1 and PRP2 in that it greatly reduced viability of hairy root tumors. We hypothesize that repetitive PRPs interact with acidic pectins to form block-copolymer gels that can play distinct roles in legume root cell walls.
KeywordsProline-rich proteins (PRPs) Medicago Root cell walls Pectins Root structure
Portions of this work were supported by the Department of Energy Biosciences program (DE-FG03-95ER20192) to JBC, and by a UC Biotechnology Training Grant. We gratefully thank Dr. Mary Tierney (University of Vermont) for a gift of anti-soybean PRP2 antibodies, Dr. Monte Radeke for help with peptide synthesis and advice on producing anti-peptide antibodies, Dr. Brian Matsumoto and Dr. Mary Raven for advice and training on the confocal microscope and for use of a vibratome, and Laura Polacco, Lucas Hanscom, Scott Spivack, Kim O'Keefe, Julia Dubiel, Mario Guzman, and Emily Garcia for general laboratory assistance.
JBC conceived and obtained funding for the project. JBC and RRF conceived and supervised experiments. BJE, NCS, NH, CW and MAS conceived and performed experiments. All authors contributed to writing or providing feedback on the manuscript.
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