Molecular regulation of sink-source transition in rice leaf sheaths during the heading period
Date Issued
2009
Date
2009
Author(s)
Chen, Huai-Ju
Abstract
Upper leaf sheath of rice (Oryza sativa L.) serves as a temporary carbohydrate sink tissue before panicle heading. Starch pre-stored in upper leaf sheaths prior to heading would be remobilized to filling grains at post-heading stage. Thus, upper leaf sheaths could be converted to source tissues from sink tissue during heading period. The process of starch changes in leaf sheath is defined as the sink-source transition. The purpose of this project is to reveal the molecular mechanism of the sink-source transition in rice leaf sheaths during heading period. First, the expression profiles of genes involved in starch synthesis pathway were analyzed and compared to starch content in the second leaf sheath below the flag leaf. The results indicated the changes of ADP-glucose pyrophosphorylase large subunit 2 (AGP-L2), granule-bound starch synthase II (GBSSII), soluble starch synthase I (SSSI), starch branching enzyme (SBE) I, SBEIII, and SBEIV mRNA levels were highly correlated with starch content changes during the heading period in the leaf sheath, and these starch synthesis-related gene promoters shared several common hormone-responsive elements. In addition, the α-amylase2A and β-amylase were considered as major genes that regulated the starch degradation at the post-heading period. Of the five sucrose transporter (OsSUT) genes, OsSUT1 and OsSUT4 appeared to play an important role in sucrose loading into the phloem of source leaf sheaths. Besides, to reveal whether phytohormones were the fac-tors to control the starch metabolism-related enzyme activities and OsSUTs gene ex-pressions, the effects of GA、ABA and BAP on expressions of these enzymes and genes in detached leaf sheaths were examined. The results indicated that not only the expres-sions of OsSUT1 and 4 but the activities of α-amylase and β-amylase can be enhanced by ABA, suggested that ABA is one of the factors to regulate the carbohydrate metabolism in leaf sheaths. Since OsSUT4 gene was significantly up-regulated at post-heading stage, it was used as an indicator gene to identify the molecular mechanism of sink-source transition in leaf sheaths. In order to find the cis-acting elements on OsSUT4 promoter involved in controlling rice leaf sheaths sink-source transition during the heading period, we constructed nine various 5’-deletion OsSUT4 promoter fragments containing GUS reporter gene and analyzed the avtivities by particle bombardment assay. A 67-bp promoter fragment was identified, which might contain important regulatory elements involved in regulation of OsSUT4 gene up-expression in leaf sheath at post-heading stage. Moreover, microarray analysis was also applied to study the mechanism of sink-source transition in leaf sheaths during heading period. The data implied that the dominant processes associated with functional leaf sheath transition from sink to source were not only carbohydrate metabolism but also the translocation of the nitrogen sources and inorganic phosphate.
Subjects
sink-source transition
Type
thesis
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