COMPROMISING EARLY SALICYLIC-ACID ACCUMULATION DELAYS THE HYPERSENSITIVE RESPONSE AND INCREASES VIRAL DISPERSAL DURING LESION ESTABLISHMENTIN TMV-INFECTED TOBACCO

To investigate the role of salicylic acid (SA) in the hypersensitive r esponse (HR) its accumulation was compromised during different phases of lesion development by differential expression of a salicylate hydro xylase gene (SH-L). Constitutive suppression of SA accumulation was ac hieved by expression of a gene fusion between the CaMV35S promoter (35 S) and SH-L. Using the H2O2-responsive AoPR1 promoter to drive SH-L SA accumulation could be compromised at an early stage, on lesion format ion and possibly prior to visible necrosis, whilst use of the salicyla te-responsive PR1a promoter reduced SA accumulation at a later stage a s lesions expand. TMV infection of 35S-SH-L and AoPR1-SH-L, but not PR 1a-SH-L, tobacco resulted in significantly greater rates of lesion gro wth than in wild-type tobacco. TMV was detected in asymptomatic tissue surrounding lesions only in 35S-SH-L and AoPR1-SH-L lines; subsequent ly these transgenic lines exhibited a 'spreading-necrosis' originating from the lesion which entered the stem and eventually other leaves, a phenotype which could be correlated with the presence of TMV particle s. Analysis of TMV-infected and 'temperature-shifted' tobacco indicate d that both 35S-SH-L and AoPR1-SH-L, but not PR1a-SH-L, transgenics ex hibited delayed cell-death compared to wild-type infections. We propos e that the SH-L phenotypes indicate that early SA accumulation is a ma jor factor in preventing viral escape, via mechanism(s) which may incl ude influencing the rate of host-cell death and, possibly, an effect o n viral function.