Molecular Structure and Function of Protective Plant Polymers

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The leaf and fruit cuticles of higher plants function principally as barriers, controlling bacterial and fungal attack as well as the diffusion of water and chemicals from the outside environment. Their major chemical constituents are waxes that provide waterproofing and either of two insoluble structural support polymers, cutin (for aerial organs) and suberin (at internal locations and in wound tissue). This project aims to understand how the monomer units of the biopolymers are covalently linked together and to cell-wall polysaccharides, how the mechanical properties of the cuticular surface change in response to stress conditions, and how suberin or related materials are involved in textural hardening of potato tissues. Several biophysical approaches will be taken to these problems. (1) Oligomeric fragments of lime fruit cutin and suberized potato wound periderm will be produced by chemical and enzymatic means, separated chromatographically, and identified by nuclear magnetic resonance (NMR) and mass spectrometry (MS). (2) NMR experiments on solvent-swelled samples will be used to develop and apply methodologies to identify polymer chain structures, cross-links, and cell-wall linkages in lime fruit cutin, potato wound periderm, suberized green cotton, and hardened potato tissues. (3) The impact of abrasion, chilling injury, and foliar delivery of pesticides on cuticular mechanical properties and molecular flexibility will be assessed using rheometry, atomic force microscopy (AFM), and solid-state NMR. (4) The biosynthesis of hard polymeric substances deposited in potato tubers that have 'hard-to-cook syndrome' will be studied using texture analysis and NMR spectroscopy. The overall objectives of this project include understanding how the monomer units of the protective cutin and suberin biopolymers are linked together and to supporting cell-wall matrices, how environmental stresses such as wind abrasion and temperature shock alter the mechanical properties of the cuticular membrane, how fruit cuticles interact with aqueous detergents used in application of agrochemicals, and how suberin or related polyphenols are synthesized during the hardening process that degrades potato texture. Ultimately, this research should have both agricultural and economic impact, aiding in the design of essential crop protection strategies. More broadly, the microstructural and molecular insights developed from this work may assist the development of synthetic waterproofing materials for industrial or cosmetic use. Finally, this project will serve to introduce methods such as solid-state NMR and AFM to the community of plant scientists.