Long-Day Up-Regulation of a GAMYB Gene duringLolium temulentum Inflorescence Formation

Abstract

Abstract Long-day exposure of the grassLolium temulentum may regulate flowering via changes in gibberellin (GA) levels. Therefore, we have examined both GA levels and expression of a MYB transcription factor that is specific to the GA signal transduction pathway in monocots. ThisMYB gene from L. temulentum shows over 90% nucleotide identity with the barley and rice GAMYBgenes, and, like them, gibberellic acid (GA3) up-regulates its expression in the seed. Furthermore, cDNAs of both the barley andL. 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 mRNA expression ofLtGAMYB does not change during the earliest steps of “floral” initiation at the apex. However, by 100 h (the double-ridge stage of flowering) its expression increased substantially and was highest in the terminal and lateral spikelet sites. Thereafter, expression declined overall but then increased within stamen primordia. Prior to increased LtGAMYBexpression, long-day exposure sufficient to induce flowering led to increased (5- to 20-fold) levels of GA1 and GA4in the leaf. Thus, increases first in GA level in the leaf followed by increased expression of LtGAMYB in the apex suggest important signaling and/or response roles in flowering.