Diurnal cycle of chlorophyll fluorescence parameters was done in Colocasia esculenta L. (swamp taro) grown in marshy land under sun or under shade. The sun leaves maintained higher electron transport rate (ETR) and steady state to initial fluorescence ratio (F_s/F₀) than shade leaves. In spite of lower ETR, higher photochemical quenching (PQ), and effective quantum yield of photosystem 2 (Φ_PS2) was evident in shade plants compared to plants exposed to higher irradiance. ETR increased linearly with increase in irradiance more under low irradiance (r ² = 0.84) compared to higher irradiance (r ² = 0.62). The maximum quantum yield of PS 2 (F_v/F_m) did not differ much in sun and shade leaves with the exception of midday when excess of light energy absorbed by plants under sun was thermally dissipated. Hence swamp taro plants adopted different strategies to utilize radiation under different irradiances. At higher irradiance, there was faster decline in proportion of open PS 2 centers (PQ) and excess light energy was dissipated through non-photochemical quenching (NPQ). Under shade, absorbed energy was effectively utilized resulting in higher Φ_PS2.

Maize (Zea mays L.) is generally considered to be a plant with aerenchyma formation inducible by environmental conditions. In our study, young maize plants, cultivated in various ways in order to minimise the stressing effect of hypoxia, flooding, mechanical impedance or nutrient starvation, were examined for the presence of aerenchyma in their primary roots. The area of aerenchyma in the root cortex was correlated with the root length. Although 12 different maize accessions were used, no plants without aerenchyma were acquired until an ethylene synthesis inhibitor was employed. Using an ACC-synthase inhibitor, it was confirmed that the aerenchyma formation is ethylene-regulated and dependent on irradiance. The presence of TUNEL-positive nuclei and ultrastructural changes in cortical cells suggest a connection between ethylene-dependent aerenchyma formation and programmed cell death. Position of cells with TUNEL-positive nuclei in relation to aerenchyma-channels was described.

We followed the diurnal cycles of isoprenoid emissions from Quercus ilex seedlings under drought and after re-watering. We found that Quercus ilex, generally considered a non-isoprene emitter, also emitted isoprene although at low rates. The emission rates of isoprene reached 0.37 ± 0.02 nmol m^-2 s^-1 in controls, 0.15 ± 0.03 nmol m^-2 s^-1 under drought and 0.35 ± 0.04 nmol m^-2 s^-1 after re-watering, while emission rates of monoterpenes reached 11.0 ± 3.0, 7.0 ± 1.0 and 23.0 ± 5.0 nmol m^-2 s^-1, respectively. Emission rates recovered faster after re-watering than photosynthetic rate and followed diurnal changes in irradiance in controls and under drought, but in leaf temperature after re-watering.

To study the effect of sucrose on the sink-source relationship in in vitro-grown plants, Cistus incanus seedlings and plantlets were grown horizontally in a two-compartment Petri dish (split dish), with the root system in one compartment and the shoot in the other. Shoots and roots were exposed to different sucrose concentrations (0-30 g dm^-3), two irradiance levels (25 and 160 µmol m^-2s^-1) and the presence or absence of a minimum medium containing minerals and vitamins (M medium). Root and shoot biomass of the seedlings was enhanced by an increase in irradiance when the growth medium was not supplemented with sucrose indicating the role of photosynthesis in biomass production. When sucrose was added to either organ growth was enhanced as well. In the presence of sucrose in the root compartment, sucrose applied to the shoot compartment enhanced growth of both organs under low irradiance, while under high irradiance, sucrose had no further additive effect. In the absence of sucrose in the root compartment, the enhancement of root biomass by sucrose added to the shoot compartment was lower under high irradiance than under low irradiance. The response of Cistus plantlets to sucrose and irradiance differed from that of seedlings, probably reflecting a greater susceptibility of the plantlets to sucrose feedback inhibition on photosynthesis and biomass accumulation. The decrease in root and shoot growth when M medium was added to the shoot compartment and the relatively better growth of these organs when the roots were supplied with minerals and the shoot with sucrose, indicate that growth of the two organs in our experimental set-up was regulated by opposing fluxes of C and nutrients.

Tobacco (Nicotiana tabacum L.) plantlets were grown on Murashige and Skoog medium in ventilated Magenta boxes and for the last subculture 10 µM ABA was added to the medium. After three weeks plantlets were transferred into pots with Perlite moistened with water and grown in controlled conditions (16-h photoperiod, day/night temperature 25/20 °C, air humidity about 45 %) either under low or high irradiance of 150 (LI) and 700 (HI) µmol m^-2 s^-1, respectively. Content of endogenous ABA was 271.7 pmol g^-1(f.m.) in ABA treated plantlets, while in control plantlets it was only 53.3 pmol g^-1(f.m.). After ex vitro transfer, stomatal conductance and transpiration rate decreased considerably in comparison with in vitro grown plantlets and remained lower also 7 d after ex vitro transfer, especially in ABA-treated plants and so wilting of plants was practically eliminated. Net photosynthetic rate also decreased 1 d after ex vitro transfer but after 7 d it was mostly higher than that of in vitro grown plantlets. Water use efficiency significantly increased in ABA-treated plants. Chlorophyll a+b content did not change immediately after ex vitro transfer, nevertheless, after 7 d chlorophyll content was higher in ABA-treated plants. Pool of xanthophyll cycle pigments (XCP) and the degree of their deepoxidation (DEPS), which are connected with harmless dissipation of light energy, increased under high irradiance. Contents of XCP and ABA precursors (neoxanthin and violaxanthin) were lower in ABA-treated plants than in control plants indicating less stress in these plants. Most chlorophyll a fluorescence parameters did not change considerably after ex vitro transfer and so the photoinhibition was not observed even under HI. Slight increase in non-photochemical quenching under HI in ABA-treated plants suggested their better photoprotection. Thus application of ABA to the last subculture can improve acclimatization of in vitro grown plants to ex vitro conditions

Biofortification of foods with essential micronutrients and phytoremediation of the contaminated sites are the two sides of the same coin for metals like zinc. In the present study, Zn accumulation potential, growth and antioxidant status of Hydrilla verticillata (L.f.) Royle plants were studied upon supplementation of Zn (0-5 000 µM) for 2 and 7 d. At 5000 µM Zn, plants accumulated about 7.60 and 18.07 mg(Zn) g^-1(d.m.) after 2 and 7 d, respectively. Plants exposed to Zn concentrations up to 500 µM showed significantly increased contents of low molecular mass antioxidants and activities of antioxidant enzymes in comparison with controls. Only upon exposure of plants to 5 000 µM Zn, toxicity was observed after 7 d. Therefore, owing to their high Zn accumulation capacity, Hydrilla plants may be used both as a Zn source (via culturing in ca. 100 µM Zn supplemented nutrient medium) or as a phytoremediator.

Nicotiana is a small and well characterized genus of Solanaceae and in this study we have used polymorphisms in phytochrome A coding sequence (phyA) and promoter to asses the phylogenetic relationships among species representative of all the sections of the genus. Allopolyploid species kept the two copies of the gene derived from each of the progenitors as resulted from the analyses of the coding region and promoter. Moreover, both copies of phyA present in tetraploids are transcribed, indicating that are properly regulated and do not undergo silencing.

This study aims to understand the changes in the transcriptome of durum wheat (Tricitum durum cv. Balcali-85) upon exposure to varying Cd concentrations using mRNA differential display (mRNA DD) technique. Sequence analyses of the two heavily induced genes upon exposure to Cd showed high homology to NADH dehydrogenase subunit 1 (EC907725) and PsaC gene encoding a photosystem 1 (PS 1) 9 kDa subunit protein (EC907731). Additionally, three differentially expressed genes (EC907726, EC907729 and EC907730) were identified. Their sequence analyses revealed no significant homologies to known genes. The expressions of NADH dehydrogenase subunit 1 and PsaC genes were confirmed by Northern blot analysis and quantified by real time PCR. This is the first report for the induction of NADH dehydrogenase subunit 1 gene during Cd stress in wheat.

The relative water content (RWC), cell membrane integrity, protein pattern and the expression of late embryogenesis abundant proteins (LEA; group 1, 2, 3 and 4) under different levels of salt stress (0, 1.0, 1.5 and 2.0 % NaCl) were investigated in mulberry (Morus alba L.) cultivars (S1 and ATP) with contrasting salt tolerance. RWC and membrane integrity decreased with increase in NaCl concentration more in cv. ATP than in cv. S1. SDS-PAGE protein profile of mulberry leaves after the NaCl treatments showed a significant increase in 35, 41, 45 and 70 kDa proteins and significant decrease in 14.3, 18, 23, 28, 30, 42, 47 and 65 kDa proteins. Exposure of plants to NaCl resulted in higher accumulation of LEA proteins in S1 than ATP. The maximum content of LEA (group 3 and 4) was detected in S1 at 2.0 % NaCl, which correlates with its salt tolerance.

The leaf water potential, gas exchange and chlorophyll fluorescence were evaluated in five common bean (Phaseolus vulgaris) genotypes A222, A320, BAT477, Carioca and Ouro Negro subjected to moderate water deficit. At the maximum water deficit (10 d of water withholding), the leaf water potential of genotypes A320 and A222 was higher (-0.35 and -0.50 MPa) when compared to the other genotypes (-0.67 to -0.77 MPa). The stomatal conductance and net photosynthetic rate were significantly reduced in all genotypes due to the water deficit. The greater reduction in stomatal conductance of A320 under drought resulted in high intrinsic water use efficiency. Mild water deficit affected the photochemical apparatus in bean genotypes probably by down-regulation since plants did not show photoinhibition. The photochemical apparatus of A222 and A320 genotypes was more sensitive to drought stress, showing reduced apparent electron transport even after the recovery of plant water status. On the other hand, even after 10 d of water withholding, the maximum efficiency of photosystem 2 was not affected, what suggest efficiency of the photoprotection mechanisms.

In vitro organogenesis of Citrus volkameriana and C. aurantium was studied considering three explant types: epicotyl segment, internodal segment, and hypocotyl segment with attached cotyledon fragment. The explants were cultured in medium according to Grosser and Gmitter (EME) supplemented with 0, 0.5, 1.0, 1.5, and 2.0 mg dm^{- 3} 6-benzyl-aminopurine (BAP), incubated firstly in darkness for 4 weeks, and then transferred to 16-h photoperiod for 2 weeks. Comparing epicotyl and internodal segments, a higher percentage of responsive explants and a higher number of shoots per explant were obtained with epicotyl segments, regardless of the BAP concentration. For C. volkameriana the highest percentage of responsive epicotyl segments (42 %) was obtained in EME with 1.0 mg dm^-3 BAP, while for C. aurantium (59 %) in EME with 0.5 mg dm^-3 BAP. The organogenesis efficiency was the best with the use of the hypocotyl segment with attached cotyledon fragment (77 % for C. volkameriana and to 75 % for C. aurantium). With this explant the morphogenesis occurred only in the hypocotyl region. The in vitro organogenesis was characterized by histological analyses showing that the morphogenic process started in the cambium region near the explant cut end.

The lengths of open reading frame (ORF)100 and ORF29-TrnC^GCA, the intronic sequence of rps16 and the transcribed spacer of TrnT^UGU-TrnL^UAA in chloroplast from different lines of cytoplasmic male sterility (CMS) rice were studied using indica types, japonica types and common wild rice as controls. The results show that the lengths of ORF100 and ORF29-TrnC^GCA in CMS lines are similar to those of typical indica. The sequences of the rps16 intron and the TrnT^UGU-TrnL^UAA spacer in sporophyte sterile types (wild-abortive type, Yinshui type and K type) are almost the same, and they also share a molecular marker of GTTGAG at nucleotide positions 220-225 in the rps16 intron. Therefore, it is speculated that the source of these three types is the same. In contrast, a gametophyte sterile type, Yuetai A does not contain such a GTTGAG sequence in the rps16 intron and has a unique G at position 595, which may works as a molecular marker distinguishing the sporophyte sterile type from the gametophyte sterile type. Based on the observation that CMS rice has much lower cytoplasmic polymorphism than indica, japonica and wild rice, it is concluded that CMS rice lack cytoplasm diversity. Therefore, it is important to introduce new sources of cytoplasm into hybrid rice.

We studied changes in physiological parameters of whole leaves and in antioxidant protection of chloroplasts during ageing and senescence of tobacco (Nicotiana tabacum L. cv. Samsun NN) leaves with enhanced cytokinin oxidase/dehydrogenase activity (CKX) or without it (WT). Old leaves of CKX plants maintained higher pigment content and photosystem 2 activity compared to WT leaves of the same age. Chloroplasts of old CKX plants showed better antioxidant capacity represented by higher superoxide dismutase, dehydroascorbate reductase and glutathione reductase activities.

Na⁺ accumulation was investigated in the roots of 11-d-old cowpea [Vigna unguiculata (L.) Walp.] plants. The relative contribution of different membrane transporters on Na⁺ uptake was estimated by applying Ca²⁺, K⁺, NH₄ ⁺, and pharmacological inhibitors. Na⁺ accumulation into the root symplast was decreased by half in the presence of 1 mM Ca²⁺ and it was almost abolished by 100 mM K⁺. The inhibitory effect of external NH⁴⁺ on Na⁺ accumulation was more pronounced in the roots of NH₄ ⁺-free growing plants. Na⁺ accumulation was reduced about 73 % by 0.1 mM flufenamate and it was almost blocked by 2 mM quinine. In addition, 20 mM tetraethylammonium and 1.0 mM Cs⁺ decreased Na⁺ accumulation by 28 and 30 %, respectively. These results evidenced that low-affinity Na⁺ uptake by cowpea roots depends on Ca²⁺-sensitive and Ca²⁺-insensitive pathways. The Ca²⁺-sensitive pathway is probably mediated by nonselective cation channels and the Ca²⁺-insensitive one may involve K⁺ channels and to a lesser extent NH₄ ⁺-sensitive K⁺ transporters.

In this study, seven cDNA fragments of genes, differentially expressed in ovaries after pollination in Cymbidium hybridium, were identified and characterized by mRNA differential display reverse transcription polymerase chain reaction (DDRT-PCR). Four (CDD-313, CDD-272, CDD-265, CDD-243) among these seven cDNA fragments showed no significant homology with ESTs or genes in the databases of NCBI; another three (CDD-193, CDD-218, CDD-470) showed significant homologies with sequences encoding components of an ABC-type transporter, a GTPase and 40S ribosomal S3 proteins (RPS3), respectively. The differential expression patterns of them were confirmed by reverse Northern dot blot analysis. More interestingly, CDD-470 appeared to be present and highly expressed in the pollinated ovaries and encoded a new factor of RPS3 participating in cell growth and proliferation. We deduced that this 40s ribosomal S3 like protein was involved in ovary development of orchids.

The effect of different oxygen availability on the nitrate reductase (NR, EC 1.6.6.1) activity in cucumber roots was studied. NR activity measured in the presence of Mg²⁺ (actual NR activity) as well as activity measured with EDTA (maximum NR activity) increased distinctly after 30 min of root incubation in a medium flushed with N₂ (anaerobic conditions). In contrast, aeration of roots (aerobic conditions) decreased both enzyme activities. Such inactivation of NR was rapidly reversed after transferring the roots to anaerobic conditions. An air-induced decrease of the actual enzyme activity was prevented by staurosporin, a protein kinase inhibitor; whereas microcistin LR, an inhibitor of protein phosphatases, completely eliminated the reactivation of NR actual activity under limited oxygen availability. An increase of the NR actual activity in roots incubated in a nitrogen-flushed buffer was correlated with a lower content of ATP in root tissues. These data suggest that reversible protein phosphorylation is involved in the regulation of NR activity under limited oxygen. On the other hand, feeding roots with inhibitors of protein kinases as well as phosphatases did not affect the maximal activity of NR indicating that other modification(s) of enzyme activity could also function in cucumber roots. Since the changes in the expression level of gene encoding nitrate reductase (CsNR) under different oxygen availability were not correlated with the enzyme activity, the transcription level of oxygen action was excluded. On the other hand, it was demonstrated that oxygen-induced alteration of NR was dependent on the ratio of oxidized/reduced pyridine nucleotides in tissues. In aerobic conditions, when maximal NR activity was inhibited, a drop of the NAD(P)H level was also observed. These data point to hysteretic modifications of NR protein induced by NAD(P)H as the target of reversible and rapid changes in maximal enzyme activity under different oxygen availability.

We used a random amplified polymorphic DNA (RAPD) amplification method to identify molecular markers associated with high quercetin accumulation in the leaves of Psidium guajava L. trees, selected from four different Mexican agronomic regions. We identified six polymorphic RAPD fragments of 620, 590, 370, 690, 480 and 460 bp among individuals of P. guajava. Genetic linkage disequilibrium analysis revealed that three RAPD profiles considered as DNA markers (620/590 bp, 370 bp and 480/460 bp) had a positive, direct association with quercetin content. These informative molecular markers can be used for selective identification of plants with the highest accumulation of flavonoids.

Effects of zinc [0 and 5.0 mg Zn kg^-1 (soil)] on photosynthetic rate (P_N), and chlorophyll fluorescence in leaves of maize (Zea mays L.) cv. Zhongdan 9409 seedlings grown under different soil moisture regimes (40-45 % and 70-75 % of soil saturated water content) were studied. Zn application did not enhance maize plant adaptation to drought stress. The relative water content and the water potential of leaves were not affected by Zn treatment. Moreover, The P_N of drought-stressed plants was not improved by Zn supply. The increases of plant biomass, stomatal conductance and quantum yield of photosystem 2 due to Zn addition were notable in well-watered plants.

Somatic embryos differentiated from hypocotyl explant in cotton (Gossypium hirsutum L.) exhibited very divergent morphologies. Six different types of somatic embryos based on cotyledon development were observed. The growth hormones (2,4-dichlorophenoxyacetic acid and kinetin) used in induction and maintenance media did not affect embryo rooting and germination. The 95 % conversion of normal embryos (with two cotyledons) was achieved, while an overall conversion was only 38 %. Horn shaped embryos failed to exhibit shoot growth. Poorly developed apical meristems were responsible for lower conversion percentages in some of embryo classes. However, regenerated plants phenotypically resembled to seed grown control plants regardless of somatic embryo morphology.

This paper compares the changes in water content, chlorophyll a fluorescence and leaf ultrastructure during dehydration and rehydration in two desiccation tolerant plants Xerophyta viscosa and X. retinervis. Both species showed decreasing quantum efficiency of photosystem 2 (F_v/F_m) with decreasing water content. Extreme water loss observed after 25 d of dehydration resulted in considerable damage of leaf tissue ultrastructure. After rehydration, both species need several days to reconstitute their photosynthetic machinery.

Pollen mother cell (PMC) development in the apomictic Boechera species B. holboellii, B. gunnisoniana and B. divaricarpa were investigated by various cytological methods. In prophase I, in triploid species B. holboellii and B. gunnisoniana the individual chromosomes condensed into long strands within the nucleus. Then, in metaphase I, each PMC formed a restitutional nucleus thereby bypassing the rest of the first meiotic division. This is interpreted as representing apomeiosis. Subsequently, the restitution nuclei underwent a single cytokinesis as evidenced by the production of dyads. The cells within each dyad were separated by a callose wall. Most of the PMC in B. holboellii and B. gunnisoniana produced dyads, but a small proportion generated conspicuous tetrads. In contrast, diploid apomict B. divaricarpa produced only tetrads by simultaneous cytokinesis.

A rapid in vitro propagation of Holarrhena antidysenterica has been developed. Seedling cotyledonary nodes on Murashige and Skoog medium (MS) containing 2 mg dm^-3 N⁶-benzyladenine (BA) produced highest number of multiple shoots. The shoot numbers were increased further upon subculture on MS medium supplemented with 0.5 mg dm^-3 BA. By repeated subculture of derived shoots, a high multiplication rate was established. The excised shoots were rooted on MS basal medium without growth regulators. The in vitro formed shoots were also rooted ex vitro by dipping them in 2 mg dm^-3 of indole-3-butyric acid (IBA) solution for 2 min before transferring them onto the hardening medium. Successful hardening and further establishment (survival 90 %) of micropropagated plants under natural conditions was observed.

Oryza sativa L. F₂ population and F_2:3 derived from a cross between salt tolerance cv. Tarommahali and salt sensitive cv. Khazar were used in this study. A linkage map based on F₂ population was constructed (74 SSR markers on 192 individuals), which covered a total of 1231.50 cM with an average two locus interval of 19.83 cM. Two QTLs related to Na⁺/K⁺ ratio were found on chromosome 3 and 6. qDM-3 and qDM-8 (for dry mass of shoot) are major QTLs with very large effects explained 20.90 and 17.72 % of the total phenotypic variance, respectively. Major locus for DM (qDM-3) was bracketed by RM1022 - RM6283 spread over 13.6 cM on chromosome 3. Major part of the variability for standard tolerance ranking (STR) was explained by the qSTR-6 flanked by RM3727 - RM340 on chromosome 6, which exhibited phenotypic variance of 17.25 % and peak likelihood ratio (LR) of 17.51. The length of this QTL is 8.8 cM and identification of any tightly linked markers in this region will serve as a candidate gene for fine-mapping. qSTR-3 overlapped with qNA-3 and qNAK-3. The qSTR-3 may contain a new major gene for salt stress tolerance at seedling stage in rice. Major QTLs identified in this paper, after fine-mapping, could be used for marker assisted selection.

DNA variations of forty-eight Eucalyptus globulus plants, regenerated by successive culture from seven different explants were assessed by AFLP analysis using 18 primer combinations. At least one variation showed 66.7 % of the analyzed plants, and the numbers of polymorphic bands per plant ranged from 1 to 22. The proportion of polymorphic fragments did not correlate with the numbers of the regenerated plants. However, the more times of successive culture were done the more of polymorphic bands were found within the groups. On average, between 97.39 and 99.88 % of all fragments were shared within the same group. AMOVA analysis showed 39.33 % of the variation was found among the accessions that originated from different calli while 60.67 % was from same calli.

The growth and the content of endogenous cytokinins (CKs) of current-year-old shoots from juniper plants (Juniperus communis L.) growing over and off ore site were compared. The juniper shoots from ore site (M plants) had higher metal content and exhibited delayed growth. Less bases and nucleosides of Z- and iP- type CK and more iP-conjugates were present in the M shoots. These changes were probably due to inhibited CK export from the roots and/or altered CK metabolism forming less biologically active CKs.

Two different genotypes of Lycopersicon esculentum Mill. (cv. Cuor di Bue, O₃-sensitive and line 93.1033/1, O₃-resistant) were treated with a single dose of ozone (150 mm³ m^-3 for 3 h). The PS 2 activity was examined by measurements of chlorophyll a fluorescence on symptomatic and asymptomatic leaves. Symptoms were evident on the 4^th leaves from the bottom, in both genotypes, while the 2^nd leaves of the line 93.1033/1 were asymptomatic. In these leaves, the net photosynthetic rate (P_N) did not change even if the F_v/F_m ratio significantly decreased. A strong reduction in P_N, mostly due to the stomatal closure, was observed in Cuor di Bue. The non photochemical quenching coefficient (q_NP) and the degree of PS 2 reaction centres closure (1-q_P) were higher, while the quantum efficiency of PS 2 photochemistry (Φ_PS2) and quantum efficiency of excitation energy capture (Φ_exc.) were lower in O₃ treated leaves of both genotypes. The limitation of photosynthesis was shown also by a decrease in the parameter %P, which diminished compared to controls in both genotypes. The response of the two genotypes for the energy fraction dissipated as thermal energy in the PS 2 antennae (%D) was similar. The fraction of %P remained lower during the recovery in symptomatic leaves of the resistant line as compared to the controls, whereas %X, which represents the amount of light energy that is not utilized in photochemistry or dissipated in the PS 2 antennae, significantly rose in the asymptomatic leaves of this line and in both the leaves of Cuor di Bue. From data obtained we concluded that ozone affected the plants independently on the appearance of visible symptoms of injury because the leaves without visible symptoms of both the genotypes were negatively influenced.

Immunocytochemical analysis using antibody raised against human H2AX histones phosphorylated at serine 139 (γ-H2AX) demonstrates that root meristem cells of Vicia faba exposed to UV-radiation or incubated with hydroxyurea (HU) reveal discrete foci at the border of the nucleolus and perinucleolar chromatin or scattered over the whole area of cell nucleus. Western blots detected only one protein band at the position expected for the phosphorylated form of H2AX. The dose-effect relationship was demonstrated following treatment with 2.5 and 10 mM HU. Proteins extracted from root meristems incubated for 2 h either with HU and caffeine or with HU and sodium metavanadate showed unchanged amounts of bound γ-H2AX antibodies, as compared to root meristems treated with 2.5 mM HU. Higher quantities of phosphorylated H2AX histones were detected in proteins extracted from roots treated with HU and 2-aminopurine. All treatments were effective in producing evident aberrations of premature mitosis: broken and lagging chromatids, acentric fragments, chromosomal bridges and micronuclei. Our results show that phosphorylation of H2AX at the carboxy-terminal Ser-Gln-Glu sequence is among the earliest responses to double-strand breaks and, presumably, one of the key ATM/ATR-dependent signals indispensable for the repair of spontaneous and induced DNA damage in plant cells.

Transgenic papayas carrying the papaya ringspot virus coat protein gene were inoculated with Papaya ringspot virus. Infection was monitored by evaluating symptoms and by reverse transcription polymerase chain reaction (RT-PCR). Among eight tested transgenic lines, clone G2 was found highly resistant to virus infection during 3 years of testing. Further analysis of this clone revealed complex multicopy transgene insertion with aberrant copies. The suspected post transcriptional gene silencing was confirmed by siRNA detection. While the R0 generation of G2 transgenic papaya was found to be fully resistant to the infection, Papaya ringspot virus was able to break this resistance in subsequent generations by suppressing post-transcriptional gene silencing (PTGS).