Where does [beta]-mannosidase fit into the picture of tomato seed cell wall mobilization?
Tomato seed [beta]-mannosidase was purified to homogeneity and its cDNA ('LeMside 1') obtained by 3'-RACE PCR using oligonucleotide sequences based on four peptide sequences of the purified enzyme. [beta]-Mannosidase cDNA enabled the complete primary sequence of 491 amino acids in the mature enzyme to be deduced. The deduced N-terminal and three internal amino acid sequences were 94% identical to the four-peptide sequences obtained from the pure enzyme. The derived amino acid sequence of the tomato [beta]-mannosidase shows the enzyme has a very low sequence identity with that of [beta]-mannosidases from non-plant sources; no other plant sequence for the enzyme is known. There appears to be only one gene encoding [beta]-mannosidase in tomato, the sequence of which has been determined ('LeMSide 2'). Expression of the gene occurs first in the micropylar endosperm, and then declines after germination. This is followed by an increase in gene expression in the lateral endosperm. Expression of both endo-[beta]-mannanase and [beta]-mannosidase genes is mainly in the endosperm, but this requires signals from the embryo for induction. Six hours of contact with the embryo is sufficient to induce a maximum increase in activity of both enzymes in the endosperm. GA is able to partially replace the influence of the signal from the embryo. Since activity of both enzymes in the gibberellin-deficient 'gib'-1 mutant seeds is induced by gibberellin, the signal from the embryo may be this hormone.