Circadian oscillations of a transcript encoding a germin-like protein that is associated with cell walls in young leaves of the long-day plant Sinapis alba L.

As part of an attempt to analyze rhythmic phenomena in the long-day plant Sinapis alba L. at the molecular level, we have searched for mRNAs whose concentration varies as a function of time of day. Differential screening of a cDNA library established from mRNAs expressed at the end of the daily light phase with probes representing transcripts expressed predominantly in the morning or evening has identified one major transcript. The cDNA, Saglp, encodes a predicted 22-kD protein with an N-terminal signal sequence. The protein shows homology to germin, a protein expressed in wheat embryos after onset of germination. The Saglp mRNA level undergoes circadian oscillations in light/dark cycles with maxima between 8 and 12 PM (zeitgeber time [zt]12-zt16) and minima around 8 PM (zt0). In plants grown from seed in constant light, transcript levels are constitutive. In constant light regular temperature shifts function as an alternative "zeitgeber" to initiate Saglp transcript oscillations. At the cellular level, Saglp transcripts are expressed in the epidermis and spongy parenchyma of young leaves, and in distinct regions of the epidermis and the cortex in stems and petioles. Strong signals are observed in these tissues around zt12, whereas little expression is found around zt20, suggesting that the underlying oscillatory mechanism(s) operate(s) synchronously in different plant organs. The SaGLP steady-state protein concentration remains constant over light/dark cycles. Immunogold labeling shows that the SaGLP protein is associated with primary cell walls.