Plant peroxidases : biochemistry and physiology

Resistant (Reba B50) and susceptible (Acala 44) cotton plants were investigated for intratissular growth of bacterial populations and peroxidase (POx) activity, after infection of cotyledons with races 18 or 20 from #Xanthomonas (#Axonopodis$) campestris$ pv. #malvacearum$. Considerable multiplication of the bacterial population was noticed in the compatible interaction (Acala 44 / Xcm race 18) ; it was much lower during the incompatible interaction when race 18 was infiltrated into cotyledons of Reba B50. An intermediate level of bacterial growth was obtained when Reba B50 was infiltrated with race known to overcome resistance of this line. High increase in POx activity occurred into the infected cotyledons during incompatible interaction, while the increase was much lower when the interactions were compatible. On leaves, a similar and significant difference in enzyme activity was also observed indicating that the "peroxidase response" was systemically induced in entire resistant plants. Five isoperoxidases were evidenced by IEF in both lines, whether they were infected or not. But only two of them accounted for the increase in activity in infected resistant cotyledons. Microscopy revealed that POx activity, detected at the infection sites two hours after infiltration of the resistant line was mainly located in cell walls and the middle lamella bordering intercellular spaces. Our data indicate that bacterial infection of cotton plants enhanced the activity of two of the preexistent isoperoxidases in resistant plants and suggest that stimulation of POx activity is associated with resistance mechanisms. (Résumé d'auteur

Flavonoids accumulate in cell walls, middle lamellae and callose-rich papillae during an incompatible interaction between Xanthomonas campestris pv. malvacearum and cotton

Interactions between cotton cotyledons and #Xanthomonas campestris pv. #malvacearum were examined. During an incompatible interaction, fluorescence microscopy revealed that flavonoid compounds accumulated within 10 h after inoculation. Electron micrographs showed ultrastructural modifications of cells that exhibited an intense fluorescence suggesting the presence of flavonoids. Phenol-like molecules were produced by cells of infection sites and were found in paramural areas within papillae enriched with callose and in host cell walls and middle lamellae. Histochemistry showed that peroxidase activity and terpenoids were detected in the infected resistant plants, 4 and 48 h after inoculation, respectively. In contrast, no changes in the deposits of lignin, suberin, and catechin were seen in either the infected susceptible or resistant lines. We suggest that early flavonoid accumulation is associated with the hypersensitive reaction of cotton cotyledons to #X. campestris pv. #malvacearum. The activity of wall-bound peroxidases may play a role in the incorporation of flavonoids in cell walls and paramural papillae. (Résumé d'auteur

Some Consequences of Thermosolutal Convection: The Grain Structure of Castings

The essential principles of thermosolutal convection are outlined, and how convection provides a transport mechanism between the mushy region of a casting and the open bulk liquid is illustrated. The convective flow patterns which develop assist in heat exchange and macroscopic solute segregation during solidification; they also provide a mechanism for the transport of dendritic fragments from the mushy region into the bulk liquid. Surviving fragments become nuclei for equiaxed grains and so lead to blocking of the parental columnar, dendritic growth front from which they originated. The physical steps in such a sequence are considered and some experimental data are provided to support the argument

Apoplastic peroxidase generates superoxide anions in cells of cotton cotyledons undergoing the hypersensitive reaction to Xanthomonas campestris pv. malvacearum race 18

Cotton cotyledons displayed a hypersensitive reaction (HR) in the resistant cultivar Reba B50 after infiltration with the avirulent race 18 of #Xanthomonas campestris pv. #malvacearum (Xcm). Generation of active oxygen species during the HR was studied biochemically and cytochemically. O2(-) was detected in cotyledon disks by the cytochrome c reduction assay 3 h after inoculation. This activity was inhibited by superoxide dismutase (SOD) and by the peroxidase inhibitors salicylhydroxamic acid (SHAM) and KCN but not by the NADPH oxidase inhibitor diphenyleneiodonium chloride (DPI). Strong NADH oxidation activity also was found 3 h after inoculation in crude extracts or in apoplastic washing fluid and was dramatically decreased after treatment with SHAM or KCN. NADH oxidation was activated by 2,4-dichlorophenol and MnCl2, indicating the involvement of a peroxidase. Activity of cationic peroxidase isoforms (pI 9 to 9.5) constitutively expressed in cotyledons was found to be enhanced 3 h after inoculation in the resistant cultivar. Activities of apoplastic peroxidase(s) and H2O2 accumulation were observed cytochemically, 3 and 4 h post inoculation, respectively. When digitonin, a O2(-) elicitor, was infiltrated into cotyledons of resistant and susceptible cultivars, generation of O2(-) radicals was shown to be reduced by SOD and inhibited by SHAM and KCN as observed after infection, and also by DPI. Our results strongly suggest that cotton cotyledons contain two O2(-) generating systems and that cells undergoing the HR in response to an avirulent race of Xcm produce O2(-) through the activation of an apoplastic peroxidase. (Résumé d'auteur