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, epigenetically suppressed for phenylalanine ammonia‐lyase (PAL) activity, was studied following infection by tobacco mosaic virus (TMV). These plants contain a bean PAL2 transgene in the sense orientation, and have reduced endogenous tobacco PAL mRNA and suppressed production of phenylpropanoid products. Lesions induced by TMV infection of PAL‐suppressed 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 phenofics. However, they develop at the same rate as on control tobacco, and pathogenesis‐related (PR) proteins are induced normally upon primary infection. The levels of free salicylic acid (SA) produced in primary inoculated leaves of PAL‐suppressed plants are approximately fourfold lower than in control plants after 84 h, and a similar reduction is observed in systemic leaves. PR proteins are not induced in systemic leaves of PAL‐suppressed plants, and secondary infection with TMV does not result in the restriction of lesion size and number seen in control plants undergoing systemic acquired resistance (SAR). In grafting experiments between wild‐type and PAL‐suppressed tobacco, the SAR response can be transmitted from a PAL‐suppressed root‐stock, but SAR is not observed if the scion is PAL‐suppressed. This indicates that, even if SA is the systemic signal for establishment of SAR, the amount of pre‐existing phenylpropanoid compounds in systemic leaves, or the ability to synthesize further phenylpropanoids in response to the systemic signal, may be important for the establishment of SAR. Treatment of PAL‐suppressed plants with dichloro‐isonicotinic acid (INA) induces PR protein expression and SAR against subsequent TMV infection. However, treatment with SA, while inducing PR proteins, only partially restores SAR, further suggesting that de novo synthesis of SA, and/or the presence or synthesis of other phenylpropanoids, is required for expression of resistance in systemic leaves.