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Programmed cell death (pcd) in plants

Programmed cell death (PCD) is an important physiological process in plants that involves the selective elimination of unwanted tissues through controlled cell destruction. There are two main types of PCD in plants - autolytic PCD, which involves rapid cytoplasm clearance after vacuole rupture, and non-autolytic PCD where death occurs prior to vacuole rupture. PCD plays essential roles in plant development and defense. The purpose of developmental PCD is to regulate cell division and shape tissues and organs. Defensive PCD helps control invading microbes. Biochemical changes involved in PCD regulation include the action of various proteases and the vacuole. PCD occurs in many developmental processes including reproduction, seed and root development, and senescence.

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Programmed Cell Death (PCD) in Plants FOUSIYA. A 2019508006
Programmed cell death • Physiological cell death process involved in selective elimination of unwanted tissues (Ellis et al., 1991) • Programmed cell death (PCD) has been defined as a sequence of (potentially interruptible) events that lead to the controlled and organized destruction of the cell. (Lockshin and Zakeri, 2004) . • The term apoptosis –A Greek word originally means that ‘’fall of the leaf or leaf falling’’ . (John et al., 1972) • Synonyms = Chromatolysis, Karyolysis, Karyohexis, Shrinkage necrosis, Programmed cell death, Cell suicide, Self destruction and Apoptosis
Types of PCD Autolytic PCD • Rapid cytoplasm clearance after tonoplast rupture & replaced by vacuolar volume • Developmental PCD • Similar to autophagy in animal • For example, PCD that occurs during the formation of the male and female zygotes, in embryonic structures, and during development of roots and shoots • Mild abiotic stress, such as lack of oxygen and drought Non – Autolytic PCD • Death occurs prior to tonoplast rupture or tonoplast rupture does not occur or tonoplast rupture is not followed by complete clearance of the cytoplasm • Hypersensitive response (HR)-related PCD, Necrotrophic PCD. • Endosperm in cereal seeds is an example (no tonoplast rupture) (Vandroon et al., 2011) 3
What is the purpose of this PCD ? • Developmental PCD Essential for successful development & growth of complex multicellular organisms. Regulates the rate of cell division. Shaping of cells, tissues & organs. • Defensive PCD Control of cell populations & defense against invading microbes.
Steps in apoptosis: 1. The decision to activate the pathway; 2. The actual "suicide" of the cell; 3. Engulfment of the cell remains by specialized immune cells called phagocytes; 4. Degradation of engulfed cell. The actual steps in cell death require: 1. condensing of the cell nucleus and breaking it into pieces 2. condensing and fragmenting of cytoplasm into membrane bound apoptotic bodies; 3. breaking chromosomes into fragments containing multiple number of nucleosomes (a nucleosome ladder)
BIOCHEMICAL CHANGES
Regulation of PCD in Plants • Plant Proteases – Metacaspases: A family of cysteine proteases in plants that are most similar to animal caspases. – Metacaspases divided into Type 1 Metacaspases (AtMC 1, AtMC 2) , Type 2 Metacaspases (AtMC 4, AtMC 9) • Subtilisin-like serine proteases. • VPE family of protease : vacuolar processing enzyme (VPE) family of cysteine proteases.
Localization of Protease in Plant Cell • Proteolytic enzymes that are involved in PCD are localized in different compartments of plant cells: – The cytoplasm (metacaspases), – The vacuoles (VPE), – The intercellular fluid (phytaspases)
Role of Vacuoles in PCD • During viral infection, the tonoplast is lysed with the release of lytic enzymes of vacuoles into the cytosol. • The vacuole tonoplast fuse with the plasmalemma to release vacuolar contents extracellularly, or it disintegrate to release its contents to the cytosol. • Induction of rapid cell death pathways may be an effective way of cleansing cytoplasm from viral growth. • Sudden release of vacuolar contents into the cytoplasm causes rapid cell death as was also noted for the HR response to TMV in tobacco.
Developmental PCD in plants Reproductive phase • Cell death in female germline – Non functional pollen grain – Nucellus – Antipodal and synergid cells • Cell death in male germline – Tapetum – Pollen self incompatibility – Transmitting tract – Pollen tube burst Vegetative phase • Seed Dev. & Germination – Embryonic suspensor – Endosperm – Seed coat • Vegetative Development – Xylem trachery vessel – Leaf morphogenesis – Lateral and adventitious root differentiation – Aerenchyma cells • Senescence • Leaf senescence (Vanhautegem et al., 2015) 10
Cell death associated with non functional megaspore • The selection of the functional megaspore is position-dependent and varies between species • In Arabidopsis (Arabidopsis thaliana), the arabinogalactan protein AGP18 is expressed during meiosis in the abaxial pole of the ovule • Callose deposition • The deletion of the nonfunctional megaspores is important for optimal seed development (Vanhautegem et al., 2015)6/10/2020 11
Tapetal cell death • PMC – 4 pollen grains • Each pollen grain mature to produce vegetative cells and 2 sperm cells with the support of tapetum layer • Tapetum differentiation & PCD onset via GA-regulated myeloblastosis (GAMYB) transcription factors in rice and their homologues MYB33 and MYB65 in Arabidopsis (Kurusu et al., 2017) 12
Prominent calcium ion (Ca2+) signature Drop in cytoplasmic pH in pollen tube Downstream of putative PCD signals Alterations of the cytoskeleton Caspase like protease activity, MAPK signaling - nuclear DNA fragmentation and PCD execution Peaks of ROS and nitric oxide (NO) levels Incompatible pollen pistil interaction Rapid inhibition of pollen-tube growth, followed by PCD - self-incompatibility induction Arabidopsis Poppy Incompatible reaction 6/10/2020 13 Steven G. Thomas and Veronica E. FranklinTong
• The suspensor fixes the embryo proper within the seed and contributes to the setup of polarity of the early embryo • Transfers nutrients and signals to the embryo • Only required in the first stages of embryogenesis, and subsequently degenerates (In tobacco Cathepsin H-like protease NtCP14 – Promotes PCD) Unequal division Embryonic suspensor PCD (Vanhautegem et al., 2015) 6/10/2020 14
Lace plant Aponogeton madagascariensis which is known as the Madagascar Lace Plant , decorative aquatic plant - skeleton-like appearance • PCD process starts in young leaves between leaf veins • Alterations in the actin cytoskeleton • Chloroplast accumulation around the nucleus • Organelles are taken up into the vacuole in membrane bound vesicles - autophagy – Loss of mitochondrial membrane potential, – DNA fragmentation, – Activation of caspase-like proteases – Before vacuolar collapse, plasma membrane rupture and cell wall degradation • Perforation PCD occurs between the leaf veins, creating the fenestrate pattern of mature lace plant leaves
Senescence • Regulated by diverse endogenous and environmental factors • ROS signalling - promote senescence and age-induced PCD via the induction of senescence associated genes (SAGs) • Metacaspases (MCs) - upregulated in senescent leaves and flowers ORESARA1 (ORE1) - Senescence Young leaves = ORE1 - suppressed by miR164 Ageing leaves = ORE1 degraded via EIN2 EIN2 activates EIN3, directly activates ORE1 or represses miR164 “Plant senescence is a genetically encoded program that ultimately leads to the death of plant organs or the entire plant” (Vanhautegem et al., 2015) 6/10/2020 16
Cell injury Vs Cell death
REFERENCES • Kabbage M. et. al. (2017): The Life and Death of a Plant Cell. Annual Review of Plant Biology. 2017. 68:375–404. • Martin B. et. al. (2013): Centrality of Host Cell Death in Plant-Microbe Interactions. Annual Review of Phytopathology. 2013. 51:543–70 • John M. et. al. (2003) Plant disease resistance genes: recent insights and Potential applications. Trends in Biotechnology Vol. 21 No.4 April 2013. 178-183. • Agrios G N; Plant Pathology. Elsevier Academic Press, Burlington, MA, 2005.