Plant disease and the regulation of enzymes involved in lignification : Effects of osmotic pressure on phenylpropanoid enzymes and on the hypersensitive response of tobacco to tobacco mosaic virus.

Tobacco varieties carrying the N gene from Nicotiana glutinosa respond to infection by Tobacco Mosaic Virus (TMV) by forming necrotic local lesions (hypersensitive reaction), thereby localizing the infection. In this study, infected mesophyll leaf tissue of N. tabacum Samsun NN was treated with the non-permeating, non-metabolizable carbohydrate mannitol. The local lesions developed under iso-osmotic conditions (0.28 M mannitol), though with a slight delay and with a reduced rate of growth, as compared to those on attached leaves. At increasing plasmolysing concentrations of mannitol, necrotization was progressively inhibited, but not completely suppressed. The leaf tissue produced tiny translucent zones, with a delay that increased between the virus inoculation and application of the plasmolytica. Activities of phenylalanine ammonia-lyase (PAL, EC 4.3.1.5) and O-methyltransferase (OMT, EC 2.1.1.6) are strongly stimulated in hypersensitively reacting tobacco and were used as biochemical markers in the present study. This study was done to determine whether the inhibitory effect of plasmolysis on the elicitation of the hypersensitive response is due to a decrease in virus spread, resulting from the rupture of plasmodesmata or, at least in part, to metabolic alterations of the host cell exposed to osmotic stress. Since necrotization is normally preceded by intense virus multiplication, the inhibitory effects found for early applications (i.e., before local lesion appearance) of plasmolytica could easily be related to an inhibition of virus spread which also occurred in similarly treated leaf tissue of the systemically reacting variety Samsun. The most meaningful data were obtained from mannitol treatments performed on leaf tissue already carrying local lesions, i.e., in which the elicitor(s) and/or the factor(s) of necrotization were already operating. Under iso-osmotic conditions, we found the stimulated PAL and OMT activities characteristic of the hypersensitive response. At plasmolysing concentrations of mannitol, we observed the counteracting effects of two different mechanisms controlling the phenylpropanoid enzymes. Floating the leaf material on the liquid medium induced an ageing-like effect with a continuous increase in enzyme activities that was independent on osmotic pressure and sensitive to cycloheximide. At the same time, the stimulated enzyme activities related to hypersensitivity decreased at a rate related to osmotic pressure. Since PAL and OMT of tobacco leaves are long-lived enzymes, it is likely that the increased de novo synthesis of the enzymes was suppressed by plasmolysis while their degradation and/or inactivation was maintained or even increased. From these results it is concluded that the apparent inhibition of the hypersensitive response by plasmolysis is due to both a decrease in virus spead (artificially caused by the rupture of connections between cells) and to drastic metabolic alterations of the host cell exposed to high osmotic pressure.