Long-day up-regulation of a GAMYB gene during Lolium temulentum inflorescence formation

Long-day exposure of the grass Lolium temulentum may regulate flowering via changes in gibberellin (CA) levels. Therefore, we have examined both CA le vels and expression of a MYB transcription factor that is specific to the C A signal transduction pathway in monocots. This MYB gene from L. temulentum shows over 90% nucleotide identity with the barley and rice GAMYB genes, a nd, like them, gibberellic acid (CA,) up-regulates its expression in the se ed. Furthermore, cDNAs of both the barley and L. temulentum GAMYB show the same simple patterns of hybridization with digests of L. temulentum genomic DNA. Compared with vegetative shoot apices of L. temulentum, the in situ m RNA expression of LtGAMYB does not change during the earliest steps of "flo ral" initiation at the apex. However, by 100 h (the double-ridge stage of f lowering) its expression increased substantially and was highest in the ter minal and lateral spikelet sites. Thereafter, expression declined overall b ut then increased within stamen primordia. Prior to increased LtGAMYB expre ssion, long-day exposure sufficient to induce flowering led to increased (5 - to 20-fold) levels of CA, and CA, in the leaf. Thus, increases first in G A level in the leaf followed by increased expression of LtGAMYB in the apex suggest important signaling and/or response roles in flowering.