Altered middle lamella homogalacturonan and disrupted deposition of (1 -> 5)-alpha-L-arabinan in the pericarp of Cnr, a ripening mutant of tomato

Cnr (colorless non-ripening) is a pleiotropic tomato (Lycopersicon esculent um) fruit ripening mutant with altered tissue properties including weaker c ell-to-cell contacts in the pericarp (A.J. Thompson, M. Tor, C.S. Barry, J. Vrebalov, C. Orfila, M.C. Jarvis, J.J. Giovannoni, D. Grierson, G.B. Seymo ur [1999] Plant Physiol 120: 383-390). Whereas the genetic basis of the Cnr mutation is being identified by molecular analyses, here we report the ide ntification of cell biological factors underlying the Cnr texture phenotype . In comparison with wild type, ripe-stage Cnr fruits have stronger, non-sw ollen cell walls (CW) throughout the pericarp and extensive intercellular s pace in the inner pericarp. Using electron energy loss spectroscopy imaging of calcium-binding capacity and anti-homogalacturonan (HG) antibody probes (PAM1 and JIM5) we demonstrate that maturation processes involving middle lamella HG are altered in Cnr fruit, resulting in the absence or a low leve l of HG-/calcium-based cell adhesion. We also demonstrate that the depositi on of (1 -->5)-alpha -L-arabinan is disrupted in Cnr pericarp CW and that t his disruption occurs prior to fruit ripening. The relationship between the disruption of (1 -->5)-alpha -L-arabinan deposition in pericarp CW and the Cnr phenotype is discussed.