Polymer mobility in cell walls of transgenic tomatoes with reduced polygalacturonase activity

Cell walls were prepared from unripe and red-ripe tomato fruit, cv. Ailsa Craig, and from ripe transgenic fruit carrying antisense genes downregulating the ripening-related polygalacturonase activity. The cell walls were examined by C-13 NMR using a cross-polarization/magic-angle spinning experiment with variable contact time to estimate the proton magnetic relaxation parameter T-1 rho, which is sensitive to molecular motions on the kHz timescale and hence to the rigidity of the polymer network of the cell walls; and the cross-polarization time constant T-CH which is sensitive to similar, and more local, motional effects. The proton T-1 rho values for the C-13 resonances from each cell wall preparation were separated into two groups. One of these showed slow relaxation, implying low mobility, and included the C-13 resonances characteristic of cellulose. The other, corresponding to resonances characteristic of pectins only, showed two-component relaxation behaviour with one component relaxing much faster than the previous group, and hence corresponding to the pectic polymers in the middle lamella and perhaps also between the microfibrils. The T-1 rho values for the first group shortened on ripening as polymer mobility increased within the cell wall. This was still evident, but less so, in the PG-antisense material, showing that the antisense genes reduced the softening of the cell wail, but did not do so as much as might be expected from the almost complete loss of polygalacturonase activity. The T-CH was separated into very fast and slower components corresponding to the times required for local and larger-scale magnetization transfer.