Programmed Cell Death During The Development Of Tubular Leaf In Allium

Some species in Allium of Liliaceae show tubular shape leaves, in this study we studied the development of leaf in Allium fistulosum and Allium cepa to prove up the character of this shape.In this study the development of leaf in Allium fistulosum was observed with light and electron microscopy (TEM), and the morphological change of nucleus and the change of DNA were analyzed through DAPI (4',6-diamidino-2-phenylindole) assay, TUNEL (terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling) assay and DNA agarose gel electrophoresis. In light microscopy, the ultrastructure of Allium fistulosum leaf gradually changed from solid to hollow. With the extension of initial intercellular spaces formed by dispersion of cells in the central site 2-3mm from leaf apex and the shrink and structural disorganization of cells in the centrifugal direction, the central cavity came into being, while the neighboring cells around which remained normal. In TEM, the condensed chromatin, the deformity and degradation of nuclei also shown in DAPI assay were observed, remnants of organelles and cell components were degraded in cytoplasm. The TUNEL assay suggested internucleosomal cleavage of DNA, and the "laddering" from DNA agarose gel electrophoresis showed DNA fragments of 180bp. These results revealed that:the development of tubular leaf in Alliums fistulosum is a phenomenon of programmed cell death (PCD).Besides, the development of leaf in Allium cepo was observed with light microscopy, the morphological change of nucleus and the change of DNA were analyzed through DAPI assay and DNA agarose gel electrophoresis.The observation of Allium cepa revealed that the development of its lamina was similar to which of Allium fistulosum, undergoing the process from solid to hollow as well, and its tubular shaped leaf was also developed from the disorganization of parenchyma in the central mesophyll cells. And fragments of DNA in the 180bp to 700bp were observed in DNA agarose gel electrophoresis. Besides, the nuclei of dying cells in Dapi assay were observed smeared and misshapen. So that the tubular shaped leaf in Allium cepa resulted from the operation of PCD as well.Programmed cell death plays a key role in organ remodeling in plants, which is universal, and elaborately reported in the leaf remodeling of lace plant and Monstera deliciosa [Gunawardena et al.,2004,2005]. The development of tubular shaped leaf in Allium was first studied in this research and provided valuable evidence in the leaf remodeling resulted by PCD.