Induction of UDP-Glucose:Salicylic Acid Glucosyltransferase Activity in Tobacco Mosaic Virus-Inoculated Tobacco (Nicotiana tabacum) Leaves

Salicylic acid (SA) is a putative signal that activates plant resistance to pathogens. SA levels increase systemically following the hypersensitive response produced by tobacco mosaic virus (TMV) inoculation of tobacco (Nicotiana tabacum L. cv Xanthi-nc) leaves. The SA increase in the inoculated leaf coincided with the appearance of a beta]-glucosidase-hydrolyzable SA conjugate identified as beta]-O-D-glucosylsalicylic acid (GSA). SA and GSA accumulation in the TMV-inoculated leaf paralleled the increase in the activity of a UDP-glucose:salicylic acid 3-O-glucosyltransferase (EC 2.4.1.35) (beta]-GTase) capable of converting SA to GSA. Healthy tissues had constitutive beta]-GTase activity of 0.076 milliunits g-1 fresh weight. This activity started to increase 48 h after TMV inoculation, reaching its maximum (6.7-fold induction over the basal levels) 72 h after TMV inoculation. No significant GSA or elevated beta]-Gtase activity could be detected in the healthy leaf immediately above the TMV-inoculated leaf. The effect of TMV inoculation on the beta]-GTase and GSA accumulation could be duplicated by infiltrating tobacco leaf discs with SA at the levels naturally produced in TMV-inoculated leaves (2.7-27.0 mu]g g-1 fresh weight). Pretreatment of leaf discs with the protein synthesis inhibitor cycloheximide inhibited the induction of beta]-GTase by SA and prevented the formation of GSA. Of 12 analogs of SA tested, only 2,6-dihydroxybenzoic acid induced beta]-GTase activity.