Galactosidases in opening, senescing and water-stressed Sandersonia aurantiaca flowers

Three glycosyl hydrolase family 35 β-galactosidase-encoding cDNAs, SaGAL1 (full-length), SaGAL2 and SaGA3L (both partial), have been isolated from Sandersonia aurantiaca (Hook.) SaGAL1 protein was functionally expressed in E. coli and β-galactosidase identity confirmed by activity assay. All three clones are primarily expressed in tepal tissues of senescing sandersonia flowers. In order to identify relationships between tepal texture and galactose metabolism, cut sandersonia flowers were treated with sucrose, periods of dryness or PEG and parameters associated with galactose metabolism and firmness were monitored. Sucrose supplementation, known to increase tepal firmness, delayed expression of SaGAL1 and SaGAL3 in opening (stage 5) flowers, whereas the response to periods of dryness followed by rehydration depended on the maturity of the flower. These treatments also tended to hasten the onset of processes associated with programmed cell death, monitored by PRT5 (a senescence-associated protease) expression. Galactosidase activity and cell wall galactose content were also affected but in an inconsistent manner. PEG supplied to opening flowers for 1 d followed by water, induced a long period of wilt, and intensive PRT5 expression. However, β-galactosidase gene expression and activity was delayed in these flowers, and cell-wall galactose content changed apparently independently of galactosidase activity. We have not been able to demonstrate a causal connection between the change in petal texture and concurrent induction of galactose mobilisation in sandersonia during normal development and senescence.