Studies on the expression patterns of the circadian rhythm regulated genes in mango

Mango, an important fruit crop of the tropical and subtropical regions shows alternate bearing in most varieties causing a financial loss to the farmer. Genetic reasons for this undesirable trait have not been studied so far. In our attempts to investigate the genetic reasons for alternate bearing we have initiated studies on genes associated with the induction, repression and regulation of flowering in mango. We have previously identified and characterized FLOWERING LOCUS T (FT) genes that induce flowering and two TERMINAL FLOWER1 (TFL1) genes that repress flowering. In this communication, we have explored the association of GI-FKF1-CDF1-CO module with the regulation of flowering in mango. The role of this module in regulating flowering has been well documented in photoperiod sensitive plants. We have characterized these genes and their expressions during flowering in Ratna variety as also their diurnal fluctuations and tissue specific expressions. The data taken together suggest that GI-FKF1-CDF1-CO module may also be employed by mango in regulating its flowering. Further, we suggest that the temperature dependent flowering in mango is probably associated with the presence of temperature sensitive elements present in the promoter region of one of the GIGANTEA genes that have been shown to be closely associated with floral induction.

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The online version contains supplementary material available at 10.1007/s12298-021-01053-8.

Cloning, expression and characterization of P1 and NIa proteases from banana bract mosaic virus (BBrMV)

Banana bract mosaic virus (BBrMV) causes the banana bract mosaic disease in banana. It belongs to the genus Potyvirus within the family Potyviridae. To the best of our knowledge apart from BBrMV coat protein gene, there are no reports on cloning, expression and characterization of any other genes from BBrMV. In this study, the BBrMV P1 and NIa protease genes were amplified from BBrMV infected banana plant cultivar Nendran and were cloned into the protein expression vector pET28b. Recombinant plasmids were transferred to BL21-CodonPlus (DE3)-RP cells and the IPTG (Isopropyl β-d-1-thiogalactopyranoside) induced BBrMV P1 and NIa proteins with molecular weights of 42 and 32 KDa respectively were purified on Ni-NTA resin column under denaturing conditions using 8 M urea. BBrMV P1 and NIa purified proteins were detected by Western blot using anti-histidine antibody. The activity of both P1 and NIa proteases in native form was analyzed through in-gel zymographic assay. The activities of both the proteases were strongly inhibited by PMSF, suggesting that both the proteases are the serine type proteases. Interestingly both the proteases showed a temperature optimum of 50 °C while the pH optimum was 8. Both proteases lost their activity when incubated at 70 °C for 1 h. This is the first report of expression, purification and characterization of BBrMV P1 and NIa proteases.

Indeterminate growth of the umbel inflorescence and bulb is associated with increased expression of the TFL1 homologue, AcTFL1, in onion

TERMINAL FLOWER1 (TFL1) is a key gene for maintenance of vegetative and inflorescence indeterminacy and architecture. In onion, flowering and bulbing are two distinct developmental phases, each under complex environmental regulatory control. We have identified two CEN/TFL1-like genes from onion designated as AcTFL1 and AcCEN1. AcTFL1 is expressed during bulbing and inflorescence development with expression increasing with indeterminate growth of the umbel and the bulb suggesting possible conservation of function. Increase in AcTFL1 expression during umbel growth is associated with a simultaneous reduction in expression of AcLFY. Expression of AcTFL1 within the bulb is lowest in the outermost layers and highest in the innermost (youngest) layers. Bulb storage at room temperature or in cold leads to a gradual reduction in AcTFL1 levels in the meristem-containing tissues, the decrease being faster in the variety not requiring vernalization. Constitutive expression of AcTFL1, but not AcCEN1 complements the Arabidopsis tfl1-14 mutant and delays flowering in wild type suggesting conservation of the AcTFL1 function even in the distantly related Arabidopsis. Taken together, AcTFL1 appears to be the functional counterpart of TFL1 and regulates indeterminate growth of the umbel inflorescence as well as bulb development in onion.

Independent and combined abiotic stresses affect the physiology and expression patterns of DREB genes differently in stress-susceptible and resistant genotypes of banana

In tropics, combined stresses of drought and heat often reduce crop productivity in plants like Musa acuminata L. We compared responses of two contrasting banana genotypes, namely the drought-sensitive Grand Nain (GN; AAA genome) and drought tolerant Hill banana (HB; AAB genome) to individual drought, heat and their combination under controlled and field conditions. Drought and combined drought and heat treatments caused greater reduction in leaf relative water content and greater increase in ion leakage and H₂ O₂ content in GN plants, especially in early stages, while the responses were more pronounced in HB at later stages. A combination of drought and heat increased the severity of responses. Real-time expression patterns of the A-1 and A-2 group DEHYDRATION-RESPONSIVE ELEMENT BINDING (DREB) genes revealed greater changes in expression in leaves of HB plants for both the individual stresses under controlled conditions compared to GN plants. A combination of heat and drought, however, activated most DREB genes in GN but surprisingly suppressed their expression in HB in controlled and field conditions. Its response seems correlated to a better stomatal control over transpiration in HB and a DREB-independent pathway for the more severe combined stresses unlike in GN. Most of the DREB genes had abscisic acid (ABA)-responsive elements in their promoters and were also activated by ABA suggesting at least partial dependence on ABA. This study provides valuable information on physiological and molecular responses of the two genotypes to individual and combined drought and heat stresses.

Characterization of two TERMINAL FLOWER1 homologs PgTFL1 and PgCENa from pomegranate (Punica granatum L.)

FLOWERING LOCUS T (FT) and TERMINAL FLOWER1/CENTRORADIALIS (TFL1/CEN) are the key regulators of flowering time in plants with FT promoting flowering and TFL1 repressing flowering. TFL1 also controls floral meristem identity and its maintenance. In this study we have characterized two pomegranate (Punica granatum L.) TFL1/CEN-like genes designated as PgTFL1 and PgCENa. The expression of PgTFL1 and PgCENa fluctuated through alternate pruning and flowering cycles, being highly expressed during the vegetative phase (immediately after pruning) and decreasing gradually in the months thereafter such that their lowest levels, especially for PgCENa coincided with the flowering phase. Both the genes are able to functionally suppress the Arabidopsis tfl1-14 mutant flowering defect. Their expression in Arabidopsis resulted in delayed flowering time, increased plant height and leaf number, branches and shoot buds as compared with wild type, suggesting that PgTFL1 and PgCENa are bonafide homologs of TFL1. However, both the genes show distinct expression patterns, being expressed differentially in vegetative shoot apex and floral bud samples. While PgTFL1 expression was low in vegetative shoot apex and high in flower bud, PgCENa expression showed the opposite trend. These results suggest that the two TFL1s in pomegranate may be utilized to control distinct developmental processes, namely repression of flowering by PgCENa and development and growth of the reproductive tissues by PgTFL1 via distinct temporal and developmental regulation of their expression.

Photoinhibition of photosystem I in a pea mutant with altered LHCII organization

Comparative analysis of in vivo chlorophyll fluorescence imaging revealed that photosystem II (PSII) photochemical efficiency (Fv/Fm) of leaves of the Costata 2/133 pea mutant with altered pigment composition and decreased level of oligomerization of the light harvesting chlorophyll a/b-protein complexes (LHCII) of PSII (Dobrikova et al., 2000; Ivanov et al., 2005) did not differ from that of WT. In contrast, photosystem I (PSI) activity of the Costata 2/133 mutant measured by the far-red (FR) light inducible P700 (P700(+)) signal exhibited 39% lower steady state level of P700(+), a 2.2-fold higher intersystem electron pool size (e(-)/P700) and higher rate of P700(+) re-reduction, which indicate an increased capacity for PSI cyclic electron transfer (CET) in the Costata 2/133 mutant than WT. The mutant also exhibited a limited capacity for state transitions. The lower level of oxidizable P700 (P700(+)) is consistent with a lower amount of PSI related chlorophyll protein complexes and lower abundance of the PsaA/PsaB heterodimer, PsaD and Lhca1 polypeptides in Costata 2/133 mutant. Exposure of WT and the Costata 2/133 mutant to high light stress resulted in a comparable photoinhibition of PSII measured in vivo, although the decrease of Fv/Fm was modestly higher in the mutant plants. However, under the same photoinhibitory conditions PSI photochemistry (P700(+)) measured as ΔA820-860 was inhibited to a greater extent (50%) in the Costata 2/133 mutant than in the WT (22%). This was accompanied by a 50% faster re-reduction rate of P700(+) in the dark indicating a higher capacity for CET around PSI in high light treated mutant leaves. The role of chloroplast thylakoid organization on the stability of the PSI complex and its susceptibility to high light stress is discussed.

Restricted capacity for PSI-dependent cyclic electron flow in ΔpetE mutant compromises the ability for acclimation to iron stress in Synechococcus sp. PCC 7942 cells

Exposure of wild type (WT) and plastocyanin coding petE gene deficient mutant (ΔpetE) of Synechococcus cells to low iron growth conditions was accompanied by similar iron-stress induced blue-shift of the main red Chl a absorption peak and a gradual decrease of the Phc/Chl ratio, although ΔpetE mutant was more sensitive when exposed to iron deficient conditions. Despite comparable iron stress induced phenotypic changes, the inactivation of petE gene expression was accompanied with a significant reduction of the growth rates compared to WT cells. To examine the photosynthetic electron fluxes in vivo, far-red light induced P700 redox state transients at 820nm of WT and ΔpetE mutant cells grown under iron sufficient and iron deficient conditions were compared. The extent of the absorbance change (ΔA(820)/A(820)) used for quantitative estimation of photooxidizable P700(+) indicated a 2-fold lower level of P700(+) in ΔpetE compared to WT cells under control conditions. This was accompanied by a 2-fold slower re-reduction rate of P700(+) in the ΔpetE indicating a lower capacity for cyclic electron flow around PSI in the cells lacking plastocyanin. Thermoluminescence (TL) measurements did not reveal significant differences in PSII photochemistry between control WT and ΔpetE cells. However, exposure to iron stress induced a 4.5 times lower level of P700(+), 2-fold faster re-reduction rate of P700(+) and a temperature shift of the TL peak corresponding to S(2)/S(3)Q(B)(-) charge recombination in WT cells. In contrast, the iron-stressed ΔpetE mutant exhibited only a 40% decrease of P700(+) and no significant temperature shift in S(2)/S(3)Q(B)(-) charge recombination. The role of mobile electron carriers in modulating the photosynthetic electron fluxes and physiological acclimation of cyanobacteria to low iron conditions is discussed. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.

Photosystem II reaction centre quenching: mechanisms and physiological role

Dissipation of excess absorbed light energy in eukaryotic photoautotrophs through zeaxanthin- and DeltapH-dependent photosystem II antenna quenching is considered the major mechanism for non-photochemical quenching and photoprotection. However, there is mounting evidence of a zeaxanthin-independent pathway for dissipation of excess light energy based within the PSII reaction centre that may also play a significant role in photoprotection. We summarize recent reports which indicate that this enigma can be explained, in part, by the fact that PSII reaction centres can be reversibly interconverted from photochemical energy transducers that convert light into ATP and NADPH to efficient, non-photochemical energy quenchers that protect the photosynthetic apparatus from photodamage. In our opinion, reaction centre quenching complements photoprotection through antenna quenching, and dynamic regulation of photosystem II reaction centre represents a general response to any environmental condition that predisposes the accumulation of reduced Q(A) in the photosystem II reaction centres of prokaryotic and eukaryotic photoautotrophs. Since the evolution of reaction centres preceded the evolution of light harvesting systems, reaction centre quenching may represent the oldest photoprotective mechanism.

Excitation pressure regulates the activation energy for recombination events in the photosystem II reaction centres of Chlamydomonas reinhardtii

Using in vivo thermoluminescence, we examined the effects of growth irradiance and growth temperature on charge recombination events in photosystem II reaction centres of the model green alga Chlamydomonas reinhardtii. We report that growth at increasing irradiance at either 29 or 15 degrees C resulted in comparable downward shifts in the temperature peak maxima (T(M)) for S2QB- charge pair recombination events, with minimal changes in S2QA- recombination events. This indicates that such growth conditions decrease the activation energy required for S2QB- charge pair recombination events with no concomitant change in the activation energy for S2QA- recombination events. This resulted in a decrease in the DeltaT(M) between S2QA- and S2QB- recombination events, which was reversible when shifting cells from low to high irradiance and back to low irradiance at 29 degrees C. We interpret these results to indicate that the redox potential of QB was modulated independently of QA, which consequently narrowed the redox potential gap between QA and QB in photosystem II reaction centres. Since a decrease in the DeltaT(M) between S2QA- and S2QB- recombination events correlated with growth at increasing excitation pressure, we conclude that acclimation to growth under high excitation pressure narrows the redox potential gap between QA and QB in photosystem II reaction centres, enhancing the probability for reaction center quenching in C. reinhardtii. We discuss the molecular basis for the modulation of the redox state of QB, and suggest that the potential for reaction center quenching complements antenna quenching via the xanthophyll cycle in the photoprotection of C. reinhardtii from excess light.

Acclimation to temperature and irradiance modulates PSII charge recombination

Acclimation of wild type and the chlorina F2 mutant of barley to either high light or low temperature results in a 2- to 3-fold increase in non-photochemical quenching which occurred independently of either energy-dependent quenching (qE), xanthophyll cycle-mediated antenna quenching or state transitions. Results of in vivo thermoluminescence measurements used to address this conundrum indicated that excitation pressure regulates the temperature gap for S(2)Q(B)(-) and S(2)Q(A)(-) charge recombinations within photosystem II reaction centers. This is discussed in terms of photoprotection through non-radiative charge recombination.

Thermoluminescence: a technique for probing photosystem II

The noninvasive technique of thermoluminescence (TL) monitors recombination reactions of oxidized donors and quinone acceptors of photosystem (PS)II leading to light emission. The technique is simple, inexpensive, and yet powerful in probing PSII using leaf discs, algal cells, thylakoids, or PSII preparations. The procedure for obtaining glow curves from photosynthetic material has been described. Interpretation of the obtained data requires a thorough understand-ing of the charge pairs responsible for generating different TL bands. This chapter discusses characteristics of different TL bands to facilitate interpretation. Thermoluminescence and delayed light emission (DLE) are related to each other and each TL band represents one component of delayed light. The technique has found application in monitoring several aspects of the PSII electron transport. Through the possibility of interpreting changes in the redox potentials of donors and acceptors on the basis of a shift in glow peak temperature, the technique should prove useful in understanding the process of adaptation, responses to abiotic and biotic stresses, and structural modifications in the PSII polypeptides.

A transient exchange of the photosystem II reaction center protein D1:1 with D1:2 during low temperature stress of Synechococcus sp. PCC 7942 in the light lowers the redox potential of QB

Upon exposure to low temperature under constant light conditions, the cyanobacterium Synechococcus sp. PCC 7942 exchanges the photosystem II reaction center D1 protein form 1 (D1:1) with D1 protein form 2 (D1:2). This exchange is only transient, and after acclimation to low temperature the cells revert back to D1:1, which is the preferred form in acclimated cells (Campbell, D., Zhou, G., Gustafsson, P., Oquist, G., and Clarke, A. K. (1995) EMBO J. 14, 5457-5466). In the present work we use thermoluminescence to study charge recombination events between the acceptor and donor sides of photosystem II in relation to D1 replacement. The data indicate that in cold-stressed cells exhibiting D1:2, the redox potential of Q(B) becomes lower approaching that of Q(A). This was confirmed by examining the Synechococcus sp. PCC 7942 inactivation mutants R2S2C3 and R2K1, which possess only D1:1 or D1:2, respectively. In contrast, the recombination of Q(A)(-) with the S(2) and S(3) states did not show any change in their redox characteristics upon the shift from D1:1 to D1:2. We suggest that the change in redox properties of Q(B) results in altered charge equilibrium in favor of Q(A). This would significantly increase the probability of Q(A)(-) and P680(+) recombination. The resulting non-radiative energy dissipation within the reaction center of PSII may serve as a highly effective protective mechanism against photodamage upon excessive excitation. The proposed reaction center quenching is an important protective mechanism because antenna and zeaxanthin cycle-dependent quenching are not present in cyanobacteria. We suggest that lowering the redox potential of Q(B) by exchanging D1:1 for D1:2 imparts the increased resistance to high excitation pressure induced by exposure to either low temperature or high light.

Seasonal responses of photosynthetic electron transport in Scots pine (Pinus sylvestris L.) studied by thermoluminescence

The potential of photosynthesis to recover from winter stress was studied by following the thermoluminescence (TL) and chlorophyll fluorescence changes of winter pine needles during the exposure to room temperature (20 degrees C) and an irradiance of 100 micromol m(-2) s(-1). TL measurements of photosystem II (PSII) revealed that the S(2)Q(B)(-) charge recombinations (the B-band) were shifted to lower temperatures in winter pine needles, while the S(2)Q(A)(-) recombinations (the Q-band) remained close to 0 degrees C. This was accompanied by a drastically reduced (65%) PSII photochemical efficiency measured as F(v)/ F(m,) and a 20-fold faster rate of the fluorescence transient from F(o) to F(m) as compared to summer pine. A strong positive correlation between the increase in the photochemical efficiency of PSII and the increase in the relative contribution of the B-band was found during the time course of the recovery process. The seasonal dynamics of TL in Scots pine needles studied under field conditions revealed that between November and April, the contribution of the Q- and B-bands to the overall TL emission was very low (less than 5%). During spring, the relative contribution of the Q- and B-bands, corresponding to charge recombination events between the acceptor and donor sides of PSII, rapidly increased, reaching maximal values in late July. A sharp decline of the B-band was observed in late summer, followed by a gradual decrease, reaching minimal values in November. Possible mechanisms of the seasonally induced changes in the redox properties of S(2)/S(3)Q(B)(-) recombinations are discussed. It is proposed that the lowered redox potential of Q(B) in winter needles increases the population of Q(A)(-), thus enhancing the probability for non-radiative P680(+)Q(A)(-) recombination. This is suggested to enhance the radiationless dissipation of excess light within the PSII reaction center during cold acclimation and during cold winter periods.

Photosynthetic electron transport adjustments in overwintering Scots pine (Pinus sylvestris L.)

As shown before [C. Ottander et al. (1995) Planta 197:176-183], there is a severe inhibition of the photosystem (PS) II photochemical efficiency of Scots pine (Pinus sylvestris L.) during the winter. In contrast, the in vivo PSI photochemistry is less inhibited during winter as shown by in vivo measurements of deltaA820/A820 (P700+). There was also an enhanced cyclic electron transfer around PSI in winter-stressed needles as indicated by 4-fold faster reduction kinetics of P700+. The differential functional stability of PSII and PSI was accompanied by a 3.7-fold higher intersystem electron pool size, and a 5-fold increase in the stromal electron pool available for P700+ reduction. There was also a strong reduction of the QB band in the thermoluminescence glow curve and markedly slower Q-A re-oxidation in needles of winter pine, indicating an inhibition of electron transfer between QA and QB. The data presented indicate that the plastoquinone pool is largely reduced in winter pine, and that this reduced state is likely to be of metabolic rather than photochemical origin. The retention of PSI photochemistry, and the suggested metabolic reduction of the plastoquinone pool in winter stressed needles of Scots pine are discussed in terms of the need for enhanced photoprotection of the needles during the winter and the role of metabolically supplied energy for the recovery of photosynthesis from winter stress in evergreens.

Diurnal regulation of plastid genes in Populus deltoides

Light regulates leaf and chloroplast development, together with overall chloroplast gene expression at various levels. Plants respond to diurnal and seasonal changes in light by changing expression of photosynthesis genes and metabolism. In Populus deltoides, a deciduous tree species, leaf development begins in the month of March and leaf maturation is attained by summer, which is subsequently followed by autumnal senescence and fall. In the present study, diurnal changes in the steady state transcript levels of plastid genes were examined in the fully developed leaves during summer season. Our results show that steady state level of the psaA/B, psbA, psbEFLJ and petA transcripts showed differential accumulation during diurnal cycle in summer. However, there was no significant change in the pigment composition during the day/night cycle. Our studies suggest that the diurnal regulation of steady state mRNA accumulation may play a crucial role during daily adjustments in plants life with rapidly changing light irradiance and temperature.

Plastid gene expression is not associated with midday depression in CO(2) assimilation and electron transport

To investigate the effect of diurnal variations on chloroplastic electron transport as well as accumulation of gene products associated with it, studies were carried out on Populus deltoides, a tree species. Electron transport studies showed two peak responses as a large diurnal change with pronounced midday depression in whole chain (H(2)O--> MV) as well as partial reactions for PSII (H(2)O-->PBQ) and PSI (DCPIP-->MV). The electron transport rates first increased from 05:00 h to a maximum at around 09:00 h and then showed a decrease at 13:00 h followed by recovery and further decrease. The pigments associated with electron transport chain did not show any change during the day. Surprisingly midday depression in the accumulation of transcripts and polypeptides related to electron transport was not observed. This suggests that chloroplastic gene expression is not associated with the midday depression observed for both CO(2) assimilation and electron transport. Studies on the transcripts of psbD/C operon during the day showed that there were differences in the processing pattern although the steady state levels of the processed transcripts of this operon did not show any variation.

Organization and post-transcriptional processing of the psb B operon from chloroplasts of Populus deltoides

Chloroplast genes are typically organized into polycistronic transcription units that give rise to complex sets of mono- and oligo-cistronic overlapping RNAs through a series of processing steps. The psbB operon contains genes for the PSII (psbB, psbT, psbH) and cytochrome b(6)f (petB and petD) complexes which are needed in different amounts during chloroplast biogenesis. The functional significance of gene organization in this polycistronic unit, containing information for two different complexes, is not known and is of interest. To determine the organization and expression of these complexes, studies have been carried out on crop plants by different groups, but not much information is known about trees. We present the nucleotide sequences of PSII genes and RNA profiles of the genes located in the psbB operon from Populus deltoides, a tree species. Although the gene organization of this operon in P. deltoides is similar to that in other species, a few variations have been observed in the processing scheme.

Essential histidyl residues at the active site(s) of sucrose-phosphate synthase from Prosopis juliflora

Chemical modification of sucrose-phosphate synthase (EC 2.4.1.14) from Prosopis juliflora by diethyl pyrocarbonate (DEP) and photo-oxidation in the presence of rose bengal (RB) which modify the histidyl residues of the protein resulted in the inactivation of the enzyme activity. This inactivation was dependent on the concentration of the modifying reagent and the time of incubation and followed pseudo-first order kinetics. For both the reagents, the inactivation was maximum at pH 7.5, which is consistent with the involvement and presence of histidine residues at the active site of the enzyme. Substrates, UDPG and F6P protected the enzyme against the inactivation by the modifying reagents suggesting that the histidine residues may be involved in the binding of these substrates and are essential for the catalytic activity. Specificity of DEP was indicated by an increase in absorbance at 240 nm along with concomitant inactivation of the enzyme and reactivation of the modified enzyme by hydroxylamine. These results strongly suggest the presence of histidine residue(s) at or near the active site of the enzyme.

RAPD profile variation amongst provenances of neem

Neem, described as a tree for solving global problems, is an evergreen, long-lived, multipurpose tree of the tropics with a wide distribution range in India. It is believed to be highly cross-pollinated. Inter-provenance variations have been reported in neem in case of morphological and physiological characters. Yet no reports about the genetic determinism for these variations are available to our knowledge. In order to have an idea about the extent and/or nature of genetic (DNA) variation in neem, the powerful RAPD technique has been employed. RAPD profiles of 34 accessions/provenances of neem were generated with 200 decamer random primers, of which the data from the 49 primers, that resulted in reproducible amplification products, were considered for analysis. Based on the presence/absence of bands, a similarity matrix was computed. Dendrogram was constructed by UPGMA method based on the pairwise similarities amongst the RAPD profiles. The similarities in RAPD profiles amongst the different DNAs was more than that expected due to the cross-pollinated nature of the tree and furthermore, these more-than-expected similarities were not due to random chance. These results suggest that neem may have a narrow genetic base.

Nucleotide sequence and intergeminiviral homologies of the DNA-A of papaya leaf curl geminivirus from India

Coat protein gene, rep protein gene and intergenic region of the genome of a whitefly transmitted geminivirus (WTG) causing severe leaf curl in papaya plants were PCR amplified, cloned and sequenced. Comparison of the amino acid sequence of the putative coat protein product of papaya leaf curl virus (PLCV) with some other mono and bipartite WTGs revealed a maximum of 89.8% homology with Indian cassava mosaic virus. The genomic organization of PLCV-India is similar to other WTGs with bipartite genomes. Comparison of the coat protein N-terminal 70 amino acid sequence (and other biological features) of PLCV with other geminiviruses shows that PLCV is a distinct geminivirus from India and is related to WTGs from the old world.

Subunit structure of lysine sensitive aspartate kinase from spinach leaves

The lysine-sensitive isoenzyme of aspartate kinase was purified to homogeneity from spinach leaves and its subunit composition was studied. The purified preparation had an apparent molecular mass of 280,000 and separated into two subunits- a large subunit with molecular mass of 53,000 and smaller subunit with molecular mass of 17,000 by urea treatment and SDS PAGE. The enzyme molecule has subunit composition of 4 large and 4 small subunits. The activity of the large subunit was stimulated more than two fold by the addition of small subunit and the stimulated activity was inhibited by EGTA. This inhibition could be reversed by Ca++. Further characteristics of the smaller subunit such as heat stability, behavior on ion exchange chromatography, elctrophoretic mobility on polyacrylamide gels, amino acid composition and pattern, presence of trimethyl lysine, its ability to activate other calmodulin stimulated enzymes and its calmodulin-like nature in RIA tests suggested that this subunit is identical to calmodulin.

Leaf Curl Disease of Carica papaya from India May Be Caused by a Bipartite Geminivirus

Papaya has considerable economic importance to agriculture in India. Papaya leaf curl disease was first reported in 1939 by Thomas and Krishnaswamy (3). This disease is of moderate incidence and widely distributed in India. Recent observations of papaya fields in India indicated that there has been a continued increase in the incidence of papaya leaf curl disease (as shown by symptoms), resulting in severe economic losses. The disease is characterized by downward curling and cupping of leaves followed by vein clearing and thickening. Enations develop in the form of frills on green veins. The affected leaves become leathery and brittle and the petioles become twisted in a zig-zag manner. Diseased plants may bear a few small fruits, which are distorted in shape and tend to fall prematurely. The disease could be transmitted by the whitefly Bemisia tabaci Genn. Therefore, possible involvement of a geminivirus was suspected. Three different cloned geminiviral DNAs, Indian tomato leaf curl virus (ITLCV) (2), tomato yellow leaf curl virus from Sardinia (TYLCV Sar), and tomato golden mosaic virus (TGMV), were used as probes (with radioactive labeling) to detect the presence of geminiviral DNA from infected papaya tissue in both slot-blot and Southern blot hybridization studies with high stringency washes. These DNA probes gave strong signals with DNA isolated from infected papaya tissue whereas they did not give any signals with DNA from healthy tissue. Further, successful polymerase chain reaction (PCR)-based amplification of fragments from both DNA-A and DNA-B components with geminivirus degenerate primers (1) was accomplished only from the DNA of infected papaya plants. The PCR-amplified DNA fragments gave positive signals in Southern blot hybridization with the three geminiviral DNA probes. These results suggest that the causal agent of papaya leaf curl disease is a bipartite geminivirus that may be provisionally called papaya leaf curl virus (PLCV). References: (1) M. R. Rojas et al. Plant Dis. 77:340, 1993. (2) K. M. Srivastava et al. J. Virol. Methods 51:297, 1995. (3) K. M. Thomas and C. S. Krishnaswamy. Curr. Sci. 8:316, 1939.

Expression of chloroplastic genes during autumnal senescence in a deciduous tree Populus deltiodes

In Populus deltoides, a deciduous tree, the development on new leaves starts in the month of March, the leaves reach maturity by October and fall by December. Changes in the composition and function of the photosynthetic apparatus were analysed during autumnal senescence. With the progress of senescence, there was an initial increase followed by a decrease in the steady state levels of psbA, psbD/C and psaA/B gene transcripts. Decrease in the steady state level of D1 protein was faster than that of Cytochrome f. The decline in LHCP level was seen only during late senescence. Although the leaves continue to look green and healthy till late November, the electron transport driven by individual photosystems started declining by October end suggesting the onset of senescence.

Sucrose-phosphate synthase in tree species: light/dark regulation involves a component of protein turnover in Prosopis juliflora (SW DC)

Light dependent modulation of sucrose-phosphate synthase activity (SPS; EC 2.4.1.14) was studied in a tree species, namely Prosopis juliflora. In this paper we demonstrate that cycloheximide, an inhibitor of cytoplasmic protein synthesis, when fed to detached leaves of P. juliflora through transpiration stream in the dark or in light completely prevents in vivo light activation of Vlim and Vmax activities of SPS. In case of spinach, however, cycloheximide feeding affects only Vlim activity while Vmax activity remained unchanged. In contrast, chloramphenicol, an inhibitor of protein synthesis in chloroplast has no effect on the light activation of SPS in Prosopis. The treatment with cycloheximide showed slight reduction in the rate of O2 evolution indicating that cycloheximide had very little effect on overall photosynthesis. These results indicate that short term protein turnover of the SPS protein and some other essential component(s) (e.g., a putative protein that modifies SPS activity) is one of the primary steps in a complex and unique regulatory cascade effecting the reversible light activation of SPS.

Characterization of the orf31-petG gene cluster from the plastid genome of Populus deltoides

The orf31-petG gene cluster is located approximately 1.2 kb away from the psbEFLJ operon in the chloroplast genome of Populus deltoides. The orf31 (ycf7) encodes an unidentified polypeptide while the petG gene encodes subunit V of an important component, cytochrome b6/f complex, involved in photosynthetic electron transport. We have determined the nucleotide sequence of the orf31-petG gene cluster from the plastid genome of a tree, Populus deltoides. Our sequence analysis suggests that these genes possess high homology with the published sequences of these genes from other plants. Northern analysis suggests development dependent transcription of the orf31-petG cluster in leaves.

Reactivation of photosynthesis in the photoinhibited green alga Chlamydomonas reinhardtii: Role of dark respiration and of light

Effect of quality, quantity and minimum duration of light on the process of recovery was investigated in the photoinhibited cells of the green alga Chlamydomonas reinhardtii. Complete and rapid reactivation of photosynthesis took place in diffuse white light of 25 μmol m(-2) s(-1). The recovery was partial (< 10%) in the dark. Far red (725 nm), red (660 nm) and blue light (480 nm) in the range of 10 to 75 μmol m(-2) s(-1) did not enhance the process of reactivation. Photoinhibited cells incubated in dark for 15 min when exposed for 5 min to diffuse light (25 μmol m(-2) s(-1)) showed complete reactivation. Even exposure of 15 min dark incubated photoinhibited cells to photoinhibitory light (2500 μmol m(-2) s(-1)) for 5 s fully regained the photosynthesis. The study indicated a very precise and triggering effect of light in the process of reactivation. The dark respiratory inhibitor KCN and uncouplers FCCP and CCCP increased the susceptibility of C. reinhardtii to photoinhibition and also prevented photoinhibited cells to reactivate fully even after longer period of incubation under suitable reactivating conditions. Of the various possibilities envisaged to assign the role of dark respiration in recovery process, supply of ATP by mitochondrial respiration appeared sound and pertinent.

Association of a cucumber mosaic virus strain with mosaic disease of banana, Musa paradisiaca--an evidence using immuno/nucleic acid probe

Virus causing severe chlorosis/mosaic disease of banana was identified as a strain of cucumber mosaic virus (CMV). Association of CMV with the disease was established by Western immunoblot using polyclonal antibodies to CMV-T and slot blot hybridization with nucleic acid probe of CMV-P genome.

Purification and characterization of homoserine dehydrogenase from spinach leaves

Homoserine dehydrogenase (HSDH) has been purified to homogeneity from spinach leaves using ammonium sulphate fractionation followed by ion exchange chromatography, gel filtration and FPLC techniques. The purified enzyme has a relative molecular mass of 220,000 and subunit molecular mass of 55,000 and probably occurs as a tetramer. The enzyme was found to be sensitive to threonine and also exhibited aspartate kinase (AK) activity, which was also sensitive to threonine suggesting that it is a bifunctional protein. The enzyme protein also gave a positive cross reaction with antibodies raised against purified AK isoenzymes. Both HSDH and AK activities were stimulated by calcium and calmodulin.

Molecular cloning of Indian tomato leaf curl virus genome following a simple method of concentrating the supercoiled replicative form of viral DNA

DNA-A and DNA-B components of the genome of a whitefly transmitted virus causing yellowing and leaf curl in tomato (ITLCV) were cloned following a simple procedure for isolation of the double stranded replicative form of viral DNA from infected tomato plants. The method is based on extraction of total DNA from infected plants followed by concentration of the double stranded replicative form of viral DNA by an alkaline denaturation procedure identical to that used for isolation of plasmid DNA from Escherichia coli. The attempted cloning of DNA showed that 95% of the transformants contained plasmids with an insert of either DNA-A (2.75 kb) or DNA-B (2.55 kb). Cloned DNA-A and DNA-B when used as probes could detect DNA-A/DNA-B in total nucleic acid obtained from fresh diseased tissue. Both DNA-A and DNA-B are needed for infection and they have a common region of 166 bases with about 94% nucleotide sequence homology, a characteristic of all bipartite geminiviruses. Comparison of the amino acid sequence of the putative coat protein product of ITLCV with some other mono- and bipartite geminiviruses revealed a maximum of 86% homology with Indian cassava mosaic virus.

Translocation of adenine nucleotides in the mitochondria of male sterile and male fertile sorghum

The translocation of ATP from the inside to the outside of the mitochondrial membrane has been studied in a male sterile (2219 A) and a male fertile (2219 B) line of sorghum. The translocation of ATP was found to be substantially lower in case of mitochondria from the male sterile line. The affinity of adenine nucleotides to the translocator proteins of the mitochondrial membrane was found to be almost one half in case of 2219 A as compared to 2219 B when the Km for ATP-ADP was determined by two different assays. It is proposed that the inadequate supply of ATP in the cytosol resulting from its inefficient translocation may contribute to the male sterility in this line.

Lysine- and threonine-sensitive aspartokinase isoenzymes from spinach leaves share common antigenic determinants

The lysine- and threonine-sensitive isoenzymes of aspartate kinase were purified to homogeneity from spinach leaves and polyclonal antibodies were raised in rabbits. The antibodies were characterized by various immunological tests like Ouchterlonys-double-diffusion, titrations of the inhibition of enzyme activity and ELISA. The antibodies against the lysine-sensitive isoenzyme could recognise as little as 50 ng of the pure antigen protein and that against the threonine-sensitive form could recognise 200 ng of the protein in the ELISA tests. The immunological tests have also shown that the lysine and threonine sensitive isoenzymes of aspartate kinase share some common antigenic determinants and differ in others.

Inactivation of leucine aminotransferase with diethylpyrocarbonate and rose bengal: evidence for an active site histidine residue

Modification of leucine aminotransferase by diethylpyrocarbonate or rose bengal-sensitized photo-oxidation caused rapid inactivation of the enzyme. The inactivation of leucine aminotransferase depended on the concentration of the reagent, the time of incubation and exhibited pseudo-first order kinetics. Rose bengal-sensitized photo-oxidation was maximum at pH 6.5 and 9. Substrates leucine and alpha-ketoglutarate protected the enzyme against inactivation by these reagents, thus suggesting participation of histidine residue at the substrate binding site.

Detection of aspartate kinase in rat liver and its activation by Ca++ and calmodulin

The enzyme aspartate kinase (EC 2.7.2.4) has been detected in rat liver (animal tissue) for the first time. This enzyme, like the aspartate kinase from bacteria and plants is inhibited by lysine and threonine. Further, the activity of the enzyme is stimulated over two fold by Ca++ and calmodulin and inhibited by EGTA, a Ca++ chelator and trifluoperazine, an anti-calmodulin compound.

The oxygen-dependent deactivation and reactivation of spinach ribulose-1,5-bisphosphate carboxylase

Ribulose-1,5-bisphosphate carboxylase-oxygenase (3-phospho-D-glycerate carboxy-lyase (dimerizing), EC 4.1.1.39) is deactivated by the removal of oxygen, and reversibly reactivated by its readdition to the enzyme solution. A short pulse of oxygen to the anaerobic enzyme solution is sufficient to trigger the reactivation process; the Ka value for this reaction was estimated as 0.12 mM oxygen. The enzyme could not be reactivated under anaerobic conditions by an organic oxidant (benzoylperoxide) or by sulfhydryl group reducing reagents (dithiothreitol or beta-mercaptoethanol), suggesting that the process of reactivation was oxygen specific. Furthermore, the inhibition of the reactivation by superoxide anion scavengers such as Tiron (1,2-dihydroxybenzene-3,5-disulfonic acid), copper penicillamine, hydroxylamine, nitroblue tetrazolium, and ascorbate, indicated that the monovalent reduced oxygen was involved as the reacting species in this process. The deactivation of the enzyme associated with the removal of oxygen was also sensitive to the presence of scavengers of O2(-), suggesting that superoxide anion was also involved in the deactivation process. Both the carboxylase and the oxygenase activities were similarly affected under all the experimental conditions studied. On the basis of these results it is argued that the enzyme molecules are able to reduce oxygen and that superoxide anion causes the deactivation or reactivation of the enzyme.

The activation of ribulose-1,5-bisphosphate carboxylase-oxygenase from spinach by oxygen

The effect of oxygen on ribulose-1,5-bisphosphate carboxylase-oxygenase from spinach was investigated. Both activities were deactivated by removal of oxygen and reversibly reactivated by oxygenation of the enzyme solution. The change in enzyme activities was accompanied by conformational changes as studied by the use of intrinsic and extrinsic fluorescent probes. The analysis of cysteine sulfhydryl groups accessible to 5,5'-dithiobis-(2-nitrobenzoic acid) revealed that the number of these groups changed with the oxygen concentration. The kinetic of the exposure of eight cysteine residues was similar to the loss of enzyme activities. The modification of these groups with 5,5'-dithiobis-(2-nitrobenzoic acid) caused almost complete loss of both the activities. The enzyme isolated from a photolithotrophic organism, Chromatium vinosum, was not affected by oxygen removal. During air--argon transitions, neither the enzyme conformation nor the number of accessible sulfhydryl groups changed.

Inhibition of the nitrate reductase complex by dibromothymoquinone

The plastoquinone antagonist 2,5-dibromothymoquinone was found to inhibit NO-3 reduction from NADH by the nitrate reductase complex from wheat. It accepts electrons from NADH through the NADH dehydrogenase activity of the nitrate reductase. However, it does not inhibit the reduction of 2,6-dichlorophenol-indophenol by the enzyme. This suggests that the two compounds may be accepting electrons at different places from the enzyme. Further it was observed that reduced DCIP could be oxidized by DBMIB in the absence of NADH indicating that the electron flow in the nitrate reductase complex may take place in a unidirectional way.

Characterization of the low temperature thermoluminescence band Zv in leaf: an explanation for its variable nature

Out of the six thermoluminescence bands reported for a mature leaf, one band (Zv) appearing at the lowest temperatures is dependent on the temperature of illumination. The characteristics of this band in fresh leaf are compared with those is a leaf heated to 60 degrees C for 5 min. It is concluded here that this band, following illumination at temperatures lower than 173 K, is part of Arnold and Azzi's Z band (Arnold, W. and Azzi, J.R. (1971) Photochem. Photobiol. 14, 233--240). However, it is part of peak I when observed subsequent to illumination beyond 173 K. An explanation for the appearance of this band at different temperatures is proposed.