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 the
β-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).
