The root-hypocotyl of Arabidopsis produces a relatively large amount of secondary vascular tissue when senescence is delayed by the removal of inflorescences, and plants are grown at low population density. Peptidase zymograms prepared from isolated xylem and phloem revealed the existence of distinct proteolytic enzyme profiles within these tissues. cDNA libraries were constructed from isolated xylem and bark of the root-hypocotyl and screened for cDNAs coding for cysteine, serine, and aspartic peptidases. Three cDNAs, two putative papain-type cysteine peptidases (XCP1 and XCP2) and one putative subtilisin-type serine peptidase (XSP1), were identified from the xylem library for further analysis. Using RNA gel blots it was determined that these peptidases were expressed in the xylem and not in the bark. Quantitative reverse transcriptase-polymerase chain reaction confirmed the RNA gel-blot results and revealed high levels of XCP1 and XCP2 mRNA in stems and flowers of the infloresence. A poly-histidine-tagged version of XCP1 was purified from Escherichia coli by denaturing metal-chelate chromatography. Following renaturation, the 40-kD recombinant XCP1 was not proteolytically active. Activation was achieved by incubation of recombinant XCP1 at pH 5.5 and was dependent on proteolytic processing of the 40-kD inactive polypeptide to a 26-kD active peptidase.

XCP1 is a xylem-specific papain-like cysteine peptidase in Arabidopsis. To determine whether XCP1 could be involved in tracheary element autolysis, promoter activity and localization of XCP1 were investigated using XCP1 promoter-␤-glucuronidase fusions and immunofluorescence confocal microscopy. A tracheary element expression pattern was detected for XCP1. Results from confocal microscopy and biochemical subcellular fractionation indicated that XCP1 was localized in the vacuole. Ectopic expression of XCP1 resulted in a reduction in plant size in some lines and early leaf senescence, as indicated by early loss of leaf chlorophyll. Reduced plant size was correlated with higher levels of XCP1, as shown by immunoblot and peptidase activity gel analyses. The XCP1 prodomain exhibits exceptionally high similarity (greater than 80%) to the prodomains of papain and other papain-like enzymes isolated from papaya (Carica papaya) laticifers when compared with all other reported papain-like enzymes. The potential for XCP1 and papain to perform common functions as catalysts of autolytic processing following cell death due to programmed suicide or to wounding is discussed.In plants, increased peptidase gene expression is associated with remobilization of nitrogen from senescing source tissues to storage or reproductive sinks and from seed protein reserves in support of germination and seedling growth (for review, see Granell et al., 1998; Beers et al., 2000). Increased peptidase activity is also linked to the carbon starvation response (Moriyasu and Ohsumi, 1996; Brouquisse et al., 1998). A detailed understanding of how peptidases function as necessary effectors of nitrogen remobilization during these processes is lacking.Genetic evidence has contributed to an expanded view of plant peptidases that includes important roles as regulators of responses to environmental cues and plant hormones (for review, see Callis and Vierstra, 2000; Estelle, 2001). The ubiquitinproteasome pathway is important in light-and auxin-mediated signaling pathways. For example, the level of Hy5, a transcription factor important in photomorphogenesis, is likely to be determined by COP1-mediated targeting to the proteasome (Osterlund et al., 2000). Turnover of IAA/AUX proteins may also be proteasome dependent (Worley et al., 2000; Estelle, 2001). Ser and Asp peptidases are important to cellular differentiation and plant-pathogen interactions. A subtilisin-like Ser peptidase gene, SDD1, is required for regulation of stomatal density and distribution (Berger and Altmann, 2000). In addition, activation tagging of a secreted Asp peptidase in Arabidopsis led to enhanced resistance to virulent strains of Pseudomonas syringae pv. tomato and P. syringae pv. Maculicola, and was correlated with constitutive expression of defense-related genes and elevated salicylic acid levels (R. Dixon and C. Lamb, personal communication).Experimental models for studying tracheary element (TE) differentiation provide opportunities to investigate the potential of peptidases to a...

Abstract-The present study is aimed at investigating the effects of zinc oxide nanoparticles (nano-ZnO) and titanium dioxide nanoparticles (nano-TiO2) on rice (Oryza sativa L.) roots. Three parameters are examined in this study: seed germination percentage, root length, and number of roots. The results show that there is no reduction in the percent seed germination from both nanoparticles, however nano-ZnO is observed to have detrimental effects on rice roots at early seedling stage. Nano-ZnO is found to stunt roots length and reduce number of roots. Whereas nano-TiO 2 has no effect on root length. This study shows that direct exposure to specific types of nanoparticles causes significant phytotoxicity, emphasizes the need for ecologically responsible disposal of wastes containing nanoparticles and also highlights the necessity for further study on the impacts of nanoparticles on agricultural and environmental systems.

The aim of this study was to assess the temperature response of photosynthesis in rubber trees (Hevea brasiliensis Müll. Arg.) to provide data for process-based growth modeling, and to test whether photosynthetic capacity and temperature response of photosynthesis acclimates to changes in ambient temperature. Net CO 2 assimilation rate (A) was measured in rubber saplings grown in a nursery or in growth chambers at 18 and 28°C. The temperature response of A was measured from 9 to 45°C and the data were fitted to an empirical model. Photosynthetic capacity (maximal carboxylation rate, V cmax , and maximal light driven electron flux, J max ) of plants acclimated to 18 and 28°C were estimated by fitting a biochemical photosynthesis model to the CO 2 response curves (A-C i curves) at six temperatures: 15, 22, 28, 32, 36 and 40°C. The optimal temperature for A (T opt ) was much lower in plants grown at 18°C compared to 28°C and nursery. Net CO 2 assimilation rate at optimal temperature (A opt ), V cmax and J max at a reference temperature of 25°C (V cmax25 and J max25 ) as well as activation energy of V cmax and J max (E aV and E aJ ) decreased in individuals acclimated to 18°C. The optimal temperature for V cmax and J max could not be clearly defined from our response curves, as they always were above 36°C and not far from 40°C. The ratio J max25 /V cmax25 was larger in plants acclimated to 18°C. Less nitrogen was present and photosynthetic nitrogen use efficiency (V cmax25 /N a ) was smaller in leaves acclimated to 18°C. These results indicate that rubber saplings acclimated their photosynthetic characteristics in response to growth temperature, and that higher temperatures resulted in an enhanced photosynthetic capacity in the leaves, as well as larger activation energy for photosynthesis.