TOBACCO PLANTS EPIGENETICALLY SUPPRESSED IN PHENYLALANINE AMMONIA-LYASE EXPRESSION DO NOT DEVELOP SYSTEMIC ACQUIRED-RESISTANCE IN RESPONSE TO INFECTION BY TOBACCO MOSAIC-VIRUS

The response of tobacco (Nicotiana tabacum L. cv. Xanthinc) plants, ep igenetically suppressed for phenylalanine ammonia-lyase (PAL) activity , was studied following infection by tobacco mosaic virus (TMV). These plants contain a bean PALP transgene in the sense orientation, and ha ve reduced endogenous tobacco PAL mRNA and suppressed production of ph enylpropanoid products. Lesions induced by TMV infection of PAL-suppre ssed plants are markedly different in appearance from those induced on control plants that have lost the bean transgene through segregation, with a reduced deposition of phenolics. However, they develop at the same rate as on control tobacco, and pathogenesis-related (PR) protein s are induced normally upon primary infection. The levels of free sali cylic acid (SA) produced in primary inoculated leaves of PAL-suppresse d plants are approximately fourfold lower than in control plants after 84 h, and a similar reduction is observed in systemic leaves. PR prot eins are not induced in systemic leaves of PAL-suppressed plants, and secondary infection with TMV does not result in the restriction of les ion size and number seen in control plants undergoing systemic acquire d resistance (SAR). In grafting experiments between wild-type and PAL- suppressed tobacco, the SAR response can be transmitted from a PAL-sup pressed rootstock, but SAR is not observed if the scion is PAL-suppres sed. This indicates that, even if SA is the systemic signal for establ ishment of SAR, the amount of pre-existing phenylpropanoid compounds i n systemic leaves, or the ability to synthesize further phenylpropanoi ds in response to the systemic signal, may be important for the establ ishment of SAR. Treatment of PAL-suppressed plants with dichloro-isoni cotinic acid (INA) induces PR protein expression and SAR against subse quent TMV infection. However, treatment with SA, while inducing PR pro teins, only partially restores SAR, further suggesting that de novo sy nthesis of SA, and/or the presence or synthesis of other phenylpropano ids, is required for expression of resistance in systemic leaves.