Effect of Soil Salinity on the Expression of Betaine Aldehyde Dehydrogenase in Leaves: Investigation of Hydraulic, Ionic and Biochemical Signals.

Betaine aldehyde dehydrogenase (BADH) catalyses the last step in glycine betaine synthesis. The levels of BADH enzyme and BADH mRNA have previously been shown to be increased several-fold by salt stress. To characterise this induction more thoroughly, BADH mRNA levels and enzyme activities were analysed in leaves of sugar beet plants (Beta vulgaris L.) subjected to different salinisation regimes. Following a salt shock (transfer from 0 to 400 mM NaCI) BADH enzyme activity rose slowly for several days. In contrast, BADH mRNA level first decreased for several hours, and then increased. When salt was leached from the rooting medium of salinised plants, BADH enzyme activity declined, with a half-life of more than 4 days. However, the level of BADH mRNA declined sharply with an apparent half-life of 2 h showing that transcription of the BADH gene or the stability of BADH mRNA in leaves can respond very dynamically to salinity changes around the root. In plants which had been gradually salinised and then held at various NaCl concentrations, the steady state level of enzyme rose continuously between 0 and 500 mM NaCl, whereas that of BADH mRNA reached a plateau at 100 mM NaCl. In general, the observed BADH mRNA fluctuations could not be satisfactorily explained by assuming them to be responses to hydraulic signals. This suggests the participation of a non-hydraulic signal or signals coming from the root. The non-hydraulic signal is unlikely to be NaCl, because leaf disks exposed to salt concentrations typical of the apoplast of salinised leaves did not accumulate BADH mRNA. A biochemical messenger is thus implied. Although abscisic acid application to leaf disks elicited significant increases in BADH mRNA level, these were several-fold smaller than those observed in leaves of intact salinised plants, suggesting the involvement of some other substance.