DARK AND CIRCADIAN REGULATION OF MESSENGER-RNA ACCUMULATION IN THE SHORT-DAY PLANT PHARBITIS-NIL

The developmental transition of the meristem from vegetative to reprod uctive growth is controlled by the cyclic alternation of light and dar kness in photoperiodic plants. Photoperiod is perceived in the leaves or cotyledons, where a flower-inducing signal is produced and transmit ted to the apex. To begin to understand the molecular basis of the pho toperiodic induction of flowering, we investigated changes in gene exp ression at the level of mRNA abundance that occur in association with dark induction of flowering in the short-day species Pharbitis nil. Se veral cDNAs were isolated that corresponded to mRNAs whose abundance i s altered after the transition to darkness. The pattern of increase in mRNA levels corresponding to one cDNA clone, PN1, showed a dark-induc ed maximum at 8 h of darkness, whereas a second clone, PN9, showed a d ark-induced accumulation of mRNA with peak levels at 12 to 16 h of dar kness. When plants were held in continuous darkness, both PN1 and PN9 exhibited rhythmic patterns of mRNA accumulation with an approximate c ircadian periodicity, suggesting that their expression is under the co ntrol of an endogenous crock. The observed pattern of expression of PN 1: and PN9 in cotyledon tissue was unusual in that darkness rather tha n light promoted mRNA accumulation, which is a temporal pattern of exp ression distinct from that of several other Pharbitis genes, including Cab, PsaG, and actin, whose mRNAs were most prevalent or equally prev alent in the light. Brief illumination of an inductive dark period by a red right night break strongly inhibited the accumulation of both PN 1 and PN9 mRNA. The expression of both PN1 and PN9 was spatially regul ated in that mRNA transcripts were detected in the cotyledons and stem s, but not the roots, of photoperiodically competent seedlings. Both P N1 and PN9 appeared to be present as single-copy genes in the Pharbiti s genome. Sequence analysis has not determined the identity of these g enes. Overall, the accumulation of mRNAs corresponding to both PN1 and PN9 closely paralleled the process of photoperiodic floral induction in P. nil, but a clear involvement with this process cannot be establi shed from our findings because of the difficulty of separating photope riodic events from other light-regulated processes, especially those i nvolved in photosynthesis, such as Cab gene expression. These results identify the products of circadian-regulated genes in photoreceptive t issue of P. nil and support the concept that circadian-regulated gene expression interacting with darkness may be involved in the regulation of photoperiodically controlled physiological processes, including fl ower induction.