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Light is one of the environmental signals that regulate axillary shoot development .
However , little is known about molecular and physiological mechanisms regulating the
development of the axillary shoot in response to light signals . Molecular events
associated with the enhanced apical dominance of phytochrome B mutant sorghum
(Sorghum bicolor ) were analyzed to reveal processes mediating axillary shoot
development in response to light . The enhanced apical dominance of phyB -1 mutant
sorghum is due to inhibition of bud outgrowth and is accompanied by upregulation of
the dormancy -associated gene (SbDRM1 ) in the buds . Increased expression of the
Teosinte Branched1 (SbTB1 ) gene (encoding a putative transcription factor that
represses bud outgrowth ) suggests that the inhibition of bud outgrowth in phyB -1
sorghum is due to the absence of active phyB to repress SbTB1 . The results were
confirmed by growing wild type seedlings at high plant density or with supplemental farred
(FR ) light that induces enhanced apical dominance . However , the SbTB1 gene is not
involved in the inhibition of bud outgrowth induced by defoliation in wild type seedlings . The results indicate variations in molecular mechanisms among different
signals inhibiting branching . Increased expression of SbMAX2 (which encodes an F -box
protein that represses bud outgrowth ) in buds repressed by light and defoliation suggests
common mechanisms at the downstream end of pathways inhibiting branching .
The expression levels of several cell cycle -related genes including SbPCNA , SbHis4 ,
SbCycD2 , SbCycB and SbCDKB were down -regulated in the repressed buds of FRtreated
and defoliated seedlings indicating the suspension of cell division in those buds .
However , these cell cycle -related genes were continuously expressed in the repressed
buds of phyB -1 , suggesting that inhibition of bud outgrowth in phyB -1 is not associated
with down -regulation of cell cycle -related gene expression . The down -regulation of cell
cycle -related genes in the buds of FR -treated wild type seedlings indicates that other
sensors , in addition to phyB , regulate bud outgrowth in response to FR enrichment . The
approaches used and results achieved will provide direction for future research on this
important topic . |
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