Diurnal control of the drought-inducible putative histone H1 gene in tomato (Lycopersicon esculentum Mill L)

The mRNA of genes le20, lcyP2, lhcII, and asr1 was quantified in leaves and roots of tomato plants (Lycopersicon esculentum Mill. L.) during three day/night cycles and 48 h constant illumination. lcyp2 and lhcII are known to exhibit diurnal or circadian rhythms in leaf tissue while asr1 has shown no evidence of diurnal fluctuations, Previously reported diurnal fluctuations of le20 mRNA in leaves could have been due to either changes in plant water status and abscisic acid concentration (le20 is a drought- and ABA-inducible gene) or changes in climate variables. Plants were grown hydroponically and at constant temperature (20.6+/-0.5 degrees C) and humidity (66+/-1%) such that no changes in plant water status or tissue ABA concentration were detectable, le20, lcyP2 and lhcII mRNAs all fluctuated diurnally in leaf tissue and all reached a maximum during the light period, Surprisingly, le20 and lcyP2 mRNA showed diurnal cycles in root tissue. There was no evidence for diurnal trends in asr1 mRNA, but levels increased steadily during constant light in both leaves and roots. le20, lcyP2 and lhcII mRNA showed only one cycle during 48 h illumination and while carbon assimilation remained high and constant during this period, stomatal conductance decreased after 6 h light and then remained low. Photosystem II efficiency decreased during illumination, recovered during dark periods and showed a weak rhythm during constant light. It was concluded that le20 and lcyP2 have a diurnal component controlling their expression in leaves and roots, responding to light/dark cycles independently of water status or ABA concentration.