Differential Expression of Isopentenyl Transferase and Cytokinin Oxidase/Dehydrogenase During Pod and Seed Development in Brassica

摘要

Consistency of yield and quality of seed are traits not yet optimised by the brassica seed industry inNew Zealand. As of 2008, seed producers in Canterbury, New Zealand, exported approximately $18mof brassica seed. However, there is a need to increase both seed quantity and/or quality. The planthormone group, the cytokinins, regulates many stages of plant growth and development, includingcell division and enhancement of sink strength, both of which are important processes in seeddevelopment and embryonic growth.The two gene families targeted in this project play a key role in maintaining cytokinin homeostasis.Isopentenyl transferase (IPT) catalyzes the rate limiting step in the formation of cytokinins, andcytokinin oxidase/dehydrogenase (CKX) irreversibly inactivates cytokinins. The aim of this projectwas to identify those cytokinin gene family members expressing specifically during the early phasesof pod and seed development. Initially this study used a rapid-cycling Brassica rapa (RCBr) becauseof its rapid life cycle, then, as the project developed, a commercial crop of forage brassica (B.napus) was studied.Reverse transcriptase PCR (RT-PCR) and BLAST analysis was used to identify putative IPT andCKX genes from RCBr and B. napus. Quantitative reverse transcriptase polymerase chain reaction(qRT-PCR) was used to measure the expression of individual gene family members during leaf,flower, pod and seed development. BrIPT1, -3, and -5, and BrCKX1, -2, -3, and -5/7 and were shownto express differentially both temporally and spatially within RCBr root, stem, leaf, seed, and podtissues. BnIPT1, 3 and 7 and BnCKX1, 2, 5 and 7 were also differentially expressed. Particularlystrong expression was shown by BrIPT3, BrIPT5 and BrCKX2 in developing seeds. Both BrandBnIPT3 expressed strongly in maturing leaves.In normal plant growth and development, biosynthesis and metabolism of cytokinin is tightlyregulated by the plant. Increasing the levels of cytokinins during seed development, either by overexpressing IPT3 or IPT5, or decreasing the expression of CKX2, or both, could potentially increaseboth seed yield and seed vigour.