Programmed cell death (PCD) is a suicide mechanism adopted by multicellular organisms that is essential for development and resistance to different forms of stress. In plants, PCD is involved from embryogenesis to death of the whole plant. PCD is genetically regulated and the molecular pathways involved in different forms of this process in animals are relatively more understood, less is known in plants. At the morphological level, apoptosis, one of the forms of PCD in animals, and plant PCD have some similarities such as cell shrinkage, shrinkage of the nucleus, and DNA fragmentation. Since morphological characteristics are a product of the genetically encoded PCD mechanism, it is of interest to figure out how much of the apoptotic pathway is shared with plant PCD in terms of the genes involved. Evidence of some level of similarities has been gathered in the last decade; including conservation during signaling, regulation and execution of plant PCD and apoptosis. A continued search into the genomes of plants has provided insights about homologues of apoptosis genes present in plants and functional analysis are providing evidence about which genes are carrying out similar roles during apoptosis and plant PCD. This review is aimed at updating on the progress of plant PCD mechanism research and highlighting some of the similarities and differences between plant and mammalian PCD mechanisms, with special focus on the commonalities.

Programmed cell death (PCD) plays an important role in several plant developmental processes. The phytohormone ethylene has been implicated in PCD signalling in many plant systems, but it is also important in developmental processes such as seed germination, flowering, and climacteric fruit ripening. Lace plant (Aponogeton madagascariensis (Mirbel) H. Bruggen) is an aquatic monocot that develops perforated leaves via the deletion of cells through developmentally regulated PCD. The plant is ideal for studying PCD; however, little is known about the regulation of cellular death involved in this system. The current study examines ethylene as a potential signalling molecule in lace plant PCD and investigates climacteric-like behaviour during lace plant leaf development. Whole plants were treated with the ethylene biosynthesis inhibitor aminoethoxyvinylglycine (AVG), the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC), or a combination of both. Subsequently, ethylene levels were monitored, and leaf development was analyzed. The results indicate that ethylene is involved in lace plant PCD signalling. AVG-treated plants had significantly lower ethylene outputs and a significant reduction in perforation formation. The inhibitory effect of AVG was recovered when AVG and ACC were applied simultaneously. The data presented here show for the first time, to our knowledge, climacteric-like behaviour during the remodelling of leaves.

The lace plant, Aponogeton madagascariensis, is an aquatic monocot that forms perforations in its leaves as part of normal leaf development. Perforation formation occurs through developmentally regulated programmed cell death (PCD). The molecular basis of PCD regulation in the lace plant is unknown, however ethylene has been shown to play a significant role. In this study, we examined the role of ethylene receptors during perforation formation. We isolated three lace plant ethylene receptors AmERS1a, AmERS1b and AmERS1c. Using quantitative PCR, we examined their transcript levels at seven stages of leaf development. Through laser-capture microscopy, transcript levels were also determined in cells undergoing PCD and cells not undergoing PCD (NPCD cells). AmERS1a transcript levels were significantly lower in window stage leaves (in which perforation formation and PCD are occurring) as compared to all other leaf developmental stages. AmERS1a and AmERS1c (the most abundant among the three receptors) had the highest transcript levels in mature stage leaves, where PCD is not occurring. Their transcript levels decreased significantly during senescence-associated PCD. AmERS1c had significantly higher transcript levels in NPCD compared to PCD cells. Despite being significantly low in window stage leaves, AmERS1a transcripts were not differentially expressed between PCD and NPCD cells. The results suggested that ethylene receptors negatively regulate ethylene-controlled PCD in the lace plant. A combination of ethylene and receptor levels determines cell fate during perforation formation and leaf senescence. A new model for ethylene emission and receptor expression during lace plant perforation formation and senescence is proposed.

Perforation formation in Aponogeton madagascariensis (Mirb.) H.Bruggen (lace plant) is an excellent model for studying developmentally regulated programmed cell death (PCD). In this study, we isolated and identified two lace plant vacuolar processing enzymes (VPEs) and investigated their involvement in PCD and throughout leaf development. Lace plant VPE transcript levels were determined during seven different stages of leaf development. PCD and non-PCD cells from “window” stage leaves (in which perforations are forming) were separated through laser-capture microscopy and their transcript levels were also determined. VPE activity was also studied between the cell types, through a VPE activity-based probe JOPD1. Additionally, VPE transcript levels were studied in plants treated with an ethylene biosynthesis inhibitor, aminoethoxyvinylglycine (AVG). The two isolated VPEs, AmVPE1 and AmVPE2, are vegetative type VPEs. AmVPE1 had higher transcript levels during a pre-perforation developmental stage, immediately prior to visible signs of PCD. AmVPE2 transcript levels were higher later during window and late window stages. Both VPEs had higher transcript and activity levels in PCD compared with the non-PCD cells. AVG treatment inhibited PCD and associated increases in VPE transcript levels. Our results suggested that VPEs are involved in the execution of the ethylene-related PCD in the lace plant.

Editor: John L. Bowman Premise of research. The lace plant is an excellent and unique model for studying developmentally regulated programmed cell death (PCD) in plants. Perforations form in highly predictable and easily accessible and distinguishable areas in lace plant leaves. However, little is known about the genes involved in regulation of this PCD or leaf development. In this study, for the first time, a general gene expression profile for lace plant leaf development was investigated.Methodology. A cDNA-amplified fragment length polymorphism involving 64 primer combinations was used for a half-genome analysis of 4666 transcripts. Two hundred and thirty differentially expressed transcriptderived fragments (TDFs) were sequenced. A partial expressed sequence tag (EST) database for window-stage (in which PCD is occurring) leaves was also established. Through a reverse transcription polymerase chain reaction, the possible role of ubiquitin in lace plant PCD was investigated.Pivotal results. Seventy-nine TDFs were successfully annotated. The isolated TDFs and ESTs encoded genes involved in processes such as photosynthesis, biosynthesis pathways, gene regulation, stress responses, defense against pathogens, and PCD, among others. Indirect evidence through ubiquitin transcript levels suggests involvement of proteasome machinery in lace plant development and PCD. This study provides a foundation for selective studies on regulation of lace plant leaf development and PCD.