β1, 4 endo-glucanases
Because hemicellulose, and xyloglucans specifically, are disassembled in
ripening fruit substantial research has focused on β-4 endo-glucanasess as a
class of enzymes because they have the capacity to cleave theβ-4-glucan linkages of xyloglucan. At least two β1, 4 endo-glucanases are expressed in
ripening tomato fruit, and one of them, Cel1, is also expressed in fruit abscission
zones (Lashbrook et al., 1994). Analysis of expression patterns demonstrated
that the mRNA corresponding to a second β1, 4 endo-glucanase, Cel2, is more
abundant in ripening fruit. However, the expression of both Cel1 and Cel2 during ripening suggested that the two β1, 4 endo-glucanases could act
synergistically on their substrates in the softening cell wall. Transgenic plants
in which Cel1 expression was suppressed by an antisense gene construct
produced fruit that softened normally, and abscission was partially reduced
(Lashbrook et al., 1998). Transgenic plants engineered for suppression of the Cel2 gene also exhibited no changes in fruit ripening or softening but also were
altered in abscission zones, requiring greater force for the abscission zone
breakage (Brummell et al., 1999a).
