Submitted to: Annals Of Botany
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/2/2010
Publication Date: 12/17/2010
Citation: Rijavec, T., Jain, M., Dermastia, M., Chourey, P.S. 2010. Spatial and temporal profiles of cytokinin biosynthesis and accumulation in developing caryopsis of maize. Annals Of Botany. 107:1235–1245. Interpretive Summary: Cytokinin (CK) is a major plant hormone that was first isolated and described in developing seeds of maize nearly half a century ago. Yet, very little is known on biosynthesis and cell-specific pattern of localization in various parts of the seed during seed development in maize. Here in this report, based on a collaborative study of scientists at CMAVE, ARS, SAA, Gainesville, the University of Ljubljana and the National Institute of Biology, Ljubljana, Slovenia, we developed a method to detect CKs in tissue sections for microscopic visualization. A successful use of the method has enabled us to show for the first time in maize that there is a steep rise in the CK levels specifically in certain maternal cell types soon after fertilization that gives rise to the seed. Based on several lines of evidence, we have proposed that high levels of the maternal CKs control a critical developmental process called programmed cell death, presumably essential for rapid transport of nutrients from mother plant to the seed. Future studies are aimed to test alternative roles for the maternal CK that may also be transported to the seed to control cell division that leads to increased sink surface in the seed, an ultimate unit of crop yields.
Technical Abstract: Developing seeds of maize have high levels of cytokinins (CKs); however, little is known on their spatial and temporal distribution. Biochemical, cellular and molecular approaches were used to describe the overall CK profiles, and several gene expression assays were used for two critical genes to assess cell-specific biosynthesis and conversion to biologically inactive form. The key observations during 0 – 28 days after pollination (DAP) are: (i) larger quantities of the CKs were seen in the maternal pedicel region relative to the filial tissues during the early stages after fertilization; (ii) unpollinated ovules did not show such massive quantitative increases; (iii) maternal nucellar region showed little or no CK signal; (iv) at 12 DAP, filial endosperm and embryo showed the highest CK levels, the most predominant forms were zeatin riboside and zeatin-9-glucoside, respectively; (v) high levels of CK immuno-signal was seen in certain specific cell types in pedicel, and in endosperm and embryo. Collectively, the CKs of developing maize caryopsis may originate from both local syntheses as well as by transport. High levels of fertilization-dependent CKs in pedicel suggest filial control on metabolism in the maternal tissue; and it may also trigger developmental PCD in the pedicel.