Ultrastructural Modifications and Biochemical Changes during Senescence of Chloroplasts

Prasanna K. Mohanty (1934-2013): a great photosynthetiker and a wonderful human being who touched the hearts of many

Prasanna K. Mohanty, a great scientist, a great teacher and above all a great human being, left us more than a year ago (on March 9, 2013). He was a pioneer in the field of photosynthesis research; his contributions are many and wide-ranging. In the words of Jack Myers, he would be a "photosynthetiker" par excellence. He remained deeply engaged with research almost to the end of his life; we believe that generations of researchers still to come will benefit from his thorough and enormous work. We present here his life and some of his contributions to the field of Photosynthesis Research. The response to this tribute was overwhelming and we have included most of the tributes, which we received from all over the world. Prasanna Mohanty was a pioneer in the field of "Light Regulation of Photosynthesis", a loving and dedicated teacher-unpretentious, idealistic, and an honest human being.

Conditional senescence in Chlamydomonas reinhardtii (Chlorophyceae)

The mechanisms of microalgal senescence may play an important role in nutrient recycling and enhanced survival. However, the aging physiology of microalgae is an understudied phenomenon. To investigate the patterns of conditional senescence in Chlamydomonas reinhardtii P. A. Dangeard, we used a cell wall-less strain, transformed with a reporter gene to infer changes in photosynthetic gene expression. We examined plastid ultrastructure, photosynthetic function, and photoprotective mechanisms during aging in batch cultures. LHCII transcription levels decreased before the population entered stationary phase, and the characteristic transcriptional light-shift response was lost. A decline in photosynthetic proteins with a concomitant increase in the photoprotective protein, LHCSR, was observed over time. However, nonphotochemical quenching remained stable during growth and stationary phase, and then declined as alternative quenching mechanisms were up-regulated. Photosynthetic efficiency declined, while Fv/Fm remained stable until the death phases. As the culture progressed through stationary phase, disorganization of the chloroplast was observed along with an increase in cytoplasmic oil bodies. We also observed a partial recovery of function and proteins during the final death phase, and attribute this to the release of nutrients into the medium from cell lysis and/or active secretion while cells were senescing. Allowing open gas exchange resulted in high levels of sustained starch production and maintained maximum cell density, prolonging the stationary phase.

Damage of photosynthetic apparatus in the senescing basal leaf of Arabidopsis thaliana: a plausible mechanism of inactivation of reaction center II

Significant decline in oxygen evolution and DCPIP photoreduction and a marginal restoration of the later with DPC as an electron donor suggest the inactivation of reaction center of photosystem II. The declines in the height of thermoluminescence bands support the view and the damage of reaction center II could be central to the senescence process in Arabidopsis leaves. The enhancement in the number of reduced quinones, signifying a loss in redox homeostasis in the electron transport chain between photosystem II and I leads to the creation of an energy imbalance. The view is supported by the decline in actual quantum yield of photosystem II in the light adapted state and maximum quantum yield of primary photochemistry in the dark adapted state of chlorophyll fluorescence. An increase in chlorophyll a fluorescence polarization and decline in carotenoid to chlorophyll energy transfer efficiency suggest the perturbation in thylakoid structure. A plausible mechanism illustrating the senescence mediated inactivation of oxygen evolving complex has been proposed.

An alternative strategy of dismantling of the chloroplasts during leaf senescence observed in a high-yield variety of barley

Changes in function and composition of the photosynthetic apparatus as well as the ultrastructure of chloroplasts in mesophyll cells were analyzed in flag leaves of the high-yield barley (Hordeum vulgare) variety cv. Lomerit during senescence under field conditions in two successive years. In contrast to previous results obtained with the elder variety cv. Carina photosystem II efficiency measured by F(v)/F(m) was found to be rather stable until a very late stage of senescence. Chlorophyll a fluorescence and P700 absorbance measurements revealed that the activities of the two photosystems declined in parallel. An increase in the chlorophyll a/b ratio at a late stage of senescence was observed to coincide with a decline in the level of the Lhcb1 apoprotein of the light harvesting complex (LHC) and the level of the corresponding transcript. Ultrastructural investigations revealed the presence of gerontoplasts that have long, single or pairwise thylakoids and lack large grana stacks. It is hypothesized that the early degradation of grana thylakoids harboring the major LHC reduced the risk of photoinhibition and might be causally related to the high yield of the barley variety cv. Lomerit.

Senescence-induced loss in photosynthesis enhances cell wall beta-glucosidase activity

A link between senescence-induced decline in photosynthesis and activity of beta-glucosidase is examined in the leaves of Arabidopsis. The enzyme is purified and characterized. The molecular weight of the enzyme is 58 kDa. It shows maximum activity at pH 5.5 and at temperature of 50 degrees C. Photosynthetic measurements and activity of the enzyme are conducted at different developmental stages including senescence of leaves. Senescence causes a significant loss in total chlorophyll, stomatal conductance, rate of evaporation and in the ability of the leaves for carbon dioxide fixation. The process also brings about a decline in oxygen evolution, quantum yield of photosystem II (PS II) and quantum efficiency of PS II photochemistry of thylakoid membrane. The loss in photosynthesis is accompanied by a significant increase in the activity of the cell wall-bound beta-glucosidase that breaks down polysaccharides to soluble sugars. The loss in photosynthesis as a signal for the enhancement in the activity of the enzyme is confirmed from the observation that incubation of excised mature leaves in continuous dark or in light with a photosynthesis inhibitor 3-(3,4-dichlorophenyl)-1, 1-dimethylurea (DCMU) that leads to sugar starvation enhances the activity of the enzyme. The work suggests that in the background of photosynthetic decline, the polysaccharides bound to cell wall that remains intact even during late phase of senescence may be the last target of senescing leaves for a possible source of sugar for remobilization and completion of the energy-dependent senescence program.

Senescence progression in a single darkened cotyledon depends on the light status of the other cotyledon in Cucurbita pepo (zucchini) seedlings: potential involvement of cytokinins and cytokinin oxidase/dehydrogenase activity

Darkness mediates different senescence-related responses depending on the targeting of dark treatment (whole plants or individual leaves) and on the organs that perceive the signal (leaves or cotyledons). As no data are available on the potential role of darkness to promote senescence when applied to individual cotyledons, we have investigated how darkness affects the progression of senescence in either a single or both individually darkened cotyledons of young 10-day-old Cucurbita pepo (zucchini) seedlings. Strong acceleration of senescence was observed when both cotyledons were darkened as judged by the damage in their anatomical structure, deterioration of chloroplast ultrastructure in parallel with decreased photosynthetic rate and photochemical quantum efficiency of PSII. In addition, the endogenous levels of cytokinins (CKs) and IAA were strongly reduced. In a single individually darkened cotyledon, the structure and function of the photosynthetic apparatus as well as the contents of endogenous CKs and IAA were much less affected by darkness, thus suggesting inhibitory effect of the illuminated cotyledon on the senescence of the darkened one. Apparently, the effect of darkness to accelerate/delay senescence in a single darkened cotyledon depends on the light status of the other cotyledon from the pair. The close positive correlation between CK content and the activity of CK oxidase/dehydrogenase (CKX; EC 1.4.3.18/1.5.99.12) suggested that CKX was essentially involved in the mechanisms of downregulation of endogenous CK levels. Our results indicated that CKX-regulated CK signaling could be a possible regulatory mechanism controlling senescence in individually darkened cotyledons.

Biotic stress induced demolition of thylakoid structure and loss in photoelectron transport of chloroplasts in papaya leaves

Papaya mosaic virus (PMV) causes severe mosaic symptoms in the papaya (Carica papaya L.) leaves. The PMV-induced alterations in photosystem II (PS II) structure and photochemical functions were probed. An increase in chlorophyll a (Chl a) fluorescence polarization suggests pathogen-induced transformation of thylakoid membrane to a gel phase. This transformation in physical state of thylakoid membrane may result in alteration in topology of pigments on pigment-binding proteins as reflected in pathogen-induced loss in the efficiency of energy transfer from carotenoids to chlorophylls. The fast Chl a fluorescence induction kinetics of healthy and PMV-infected plants by F(O)-F(J)-F(I)-F(P) transients revealed pathogen-induced perturbation on PS II acceptor side electron transfer equilibrium between Q(A) and Q(B) and in the pool size of electron transport acceptors. Pathogen-induced loss in photosynthetic pigments, changes in thylakoid structure and decrease in the ratio of F(V)/F(M) (photochemical potential of PS II) further correlate with the loss in photoelectron transport of PS II as probed by 2,6-dichlorophenol indophenol (DCPIP)-Hill reaction. Restoration of the loss by 1,5-diphenyl carbazide (DPC), an exogenous electron donor, that donates electron directly to reaction centre II bypassing the oxygen evolving system (OES), leads towards the conclusion that OES is one of the major targets of biotic stress. Further, the data suggest that chlorophyll fluorescence could be used as a non-invasive handy tool to assess the loss in photosynthetic efficiency and symptom severity in infected green tissues vis-a-vis the healthy ones.

Cytokinin promotes catalase and ascorbate peroxidase activities and preserves the chloroplast integrity during dark-senescence

Increased oxidative stress displayed during dark-senescence of wheat leaves (Triticum aestivum L.) is caused not only by the increased levels of radicals but also by a loss of antioxidant capacity. Mature leaves were incubated in 6-benzylaminopurine (BAP 10(-4)M) or water (control) during 6d in the dark. The senescence-delaying effect of BAP was associated with the retention of the chloroplast structure, 60% of the initial content of chlorophyll (Chl) and 77% of the initial content of protein. BAP reduced the degradation of the light-harvesting chlorophyll a/b binding protein (LHCP-2), and the large (LSU) and small subunits (SSU) of Rubisco. Our results indicated that the presence of the NADPH:protochlorophyllide oxidoreductase (POR, EC.1.6.99.1) was not promoted by the cytokinin, leading to the conclusion that BAP maintains the level of Chl, preventing its degradation, rather than inducing Chl biosynthesis. The internal structure of chloroplasts was maintained in BAP-treated leaves for up to 6d, with well-organized grana thylakoids and small plastoglobuli; in contrast, chloroplasts of control leaves deteriorated rapidly from day 4 with disorganized internal membranes, and more and larger plastoglobuli. BAP increased the activities of catalase (CAT, EC 1.11.1.6) and ascorbate peroxidase (APX, EC 1.11.1.11) and reduced the level of H(2)O(2) in the delayed-senescence tissue. The present research indicates that BAP reduces levels of reactive oxygen species (ROS), and enhances the activity of antioxidant enzymes (CAT, APX). Our results suggest that BAP protects the cell membranes and the photosynthetic machinery from oxidative damage during delay of senescence in the dark.

Increased stability of LHCII by aggregate formation during dark-induced leaf senescence in the Arabidopsis mutant, ore10

Leaf senescence in a stay-green mutant of Arabidopsis thaliana, ore10, was investigated during dark-incubation of its detached leaves. During this dark-induced senescence (DIS), Chl loss was delayed in ore10 mutants, as compared with wild type, but the rate of decline in the photochemical efficiency of PSII was not delayed in mutant leaves. After 2 d of DIS, native green gel electrophoresis of ore 10 leaf proteins resulted in a significant amount of pigment remaining as aggregates on top of the stacking gel. In addition, the accumulation of aggregates coincided with the emergence of a new band near 700 nm (F(699)) in the 77 K fluorescence emission spectrum of the aggregates. At 4 d, F(699) became a major band, both in the isolated aggregates and in intact leaves. Prolonged treatment with detergents revealed that light-harvesting complex II (LHCII) remaining after 2 d was highly stable, and the accumulation of aggregates coincided with the appearance of truncated LHCII in senescing ore10 leaves. These results suggest that increased LHCII stability is due to the formation of aggregates of trimmed LHCII. Thus, the LHCII protein degradation step that follows proteolysis of its terminal peptides is a possible lesion site of the ore10 mutant.

Ultraviolet-A induced changes in photosystem II of thylakoids: effects of senescence and high growth temperature

Ultraviolet-A (UV-A) radiation induced changes in photosystem II (PS II) of senescing leaves of wheat seedlings were investigated. UV-A radiation did not show any significant effect on the level of photosynthetic pigments. However, the decline in F(v)/F(m) and oxygen evolution rate indicated the damaging effect of the radiation on primary photochemistry of PS II. Modification at the Q(B)-binding site was inferred from the observed downshift of peak temperature of thermoluminescence (TL) B-bands. The UV-A induced changes in PS II of chloroplasts from senescing leaves were found to be synergistically accelerated by high growth temperature.

Senescing leaves possess potential for stress adaptation: the developing leaves acclimated to high light exhibit increased tolerance to osmotic stress during senescence

Plants may experience environmental stress factors operating in nature either simultaneously or in sequence. In the study, we have acclimated the developing primary leaves of wheat seedlings to high light stress and examined their photosynthetic response to polyethylene glycol (PEG) mediated osmotic stress during different developmental phases including senescence. The high light acclimated leaves show higher level of total carotenoids as compared to their non-acclimated counterparts experiencing osmotic stress during senescence. They also exhibit greater membrane stability as indicated by the measurements of fluorescence polarisation and energy transfer efficiency in photosystem I (PSI) and Photosystem II (PSII). From the data of DCPIP photoreduction and pulse amplitude modulated (PAM) fluorimetry, a similar trend is observed for PSII photochemistry of the leaves experiencing osmotic stress during senescence. Our results may suggest that the stress adaptive potential induced by one stress during development is retained by the leaves and helps to mitigate another stress effect operating in sequence during another developmental phase, namely senescence.

Response of senescing cotyledons of clusterbean to water stress in moderate and low light: Possible photoprotective role of beta-carotene

Senescence of clusterbean (Cyamopsis tetragonoloba L.) cotyledons in moderate light (12 W m-2) brings about a loss in the pigments, enhanced lipid peroxidation and a decline in PS II photochemical activity without any loss either in Dl protein or in the level of beta-carotene. The senescence syndrome is aggravated in the cotyledons of water-stressed seedlings with an increase in thylakoid lipid peroxidation, a decline in the level of beta-carotene and a quantitative loss in the Dl protein. Loss of the protein, however, is arrested in the seedlings experiencing water stress at low light (3 W m-2) intensity that correlates with the stability in the level of beta-carotene and a slow rate of lipid peroxidation. Loss of the protein in moderate light is attributed to water-stress sensitized photoinhibitory damage. The data on changes in the components of xanthophyll cycle suggest the low activity of the cycle both during senescence and water stress. It is, therefore, concluded that beta-carotene may contribute to the assembly and stability of the Dl protein during senescence and water stress in clusterbean cotyledons.

Dark-inducible genes from Arabidopsis thaliana are associated with leaf senescence and repressed by sugars

We have isolated 5 cDNA clones (din2, din6, din9, din10 and din11) corresponding to genes, the transcripts of which accumulated in leaves of Arabidopsis thaliana kept in the dark. These cDNA clones encode proteins similar to beta-glucosidase (EC 3.2.1.21, din2), asparagine synthetase (EC 6.3.5.4, din6), phosphomannose isomerase (EC 5.3.1.8, din9), seed imbibition protein (din10) and 2-oxoacid-dependent dioxygenases (din11). Accumulation of the transcripts from din6 and din10 occurred within 3 h after plants were transferred to darkness. The transcripts from din2, din9 and din11 were only detected after 24 h of dark treatment. We also observed the accumulation of the din transcripts in senescing leaves. Application of a photosynthesis inhibitor, 3-(3,4-dichlorophenyl)-1-1-dimethyl-urea, induced the expression of the din genes under illumination. Application of sucrose to detached leaves suppressed the accumulation of the din transcripts in the dark. These results indicate that expression of these genes partly depends on cellular sugar level. The sugar-modulated expression of the din genes suggests that dark-induced expression of these genes might be related to sugar starvation occurring in leaf cells in the dark, when the photosynthesis is hindered.

Response of senescing wheat leaves to ultraviolet A light: changes in energy transfer efficiency and PS II photochemistry

Response of senescing leaves of wheat seedlings to ultraviolet A (UVA) radiation (365 nm) has been examined. The results indicate that senescence-induced disorganization of thylakoid membrane, decline in carotenoid-to-chlorophyll energy transfer, and enhancement of lipid peroxidation are furthered by radiation. The senescence-induced decline in photochemical activity of photosystem II further declines on irradiation. UVA does not specifically alter any site other than those damaged by senescence.