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Systemic acquired resistance (SAR) is a broad-spectrum, systemic defense response that is activated in many plant species after pathogen infection. We have previously described Arabidopsis mutants that constitutively express SAR and concomitantly develop lesions simulating disease (lsd). Here, we describe two new mutants, lsd6 and lsd7, that develop spontaneous necrotic lesions and possess elevated levels of salicylic acid (SA) as well as heightened disease resistance, similar to the previously characterized lsd and accelerated cell death (acd2) mutants. Genetic analysis of lsd6 and lsd7 showed that the mutant phenotypes segregated as simple dominant traits. When crossed with transgenic Arabidopsis plants containing the SA-degrading enzyme salicylate hydroxylase, the F1 progeny showed suppression of both SAR gene expression and resistance. In addition, salicylate hydroxylase suppressed lesion formation in the F1 progeny, suggesting that SA or some SA-dependent process may have a role in pathogen-associated cell death. Surprisingly, lesions were restored in the lsd6 F1 progeny after the application of either 2,6-dichloroisonicotinic acid or SA. Lesions were not restored by treatment with either compound in the lsd7 F1 plants. Our findings demonstrate that steps early in the signal transduction pathway leading to SAR and disease resistance are potentiated by later events, suggesting feedback control of lesion formation.

Salicylic acid (SA) is a signal in systemic acquired resistance and an inducer of the alternative oxidase protein in tobacco (Nicofiana tabacum cv Xanthi nc) cell suspensions and during thermogenesis i n aroid spadices. The effects of SA on the levels of alternative oxidase protein and the pathogenesis-related l a mRNA (a marker for systemic acquired resistance), and on the partitioning of electrons between the Cyt and alternative pathways were investigated in tobacco. Leaves were treated with 1.0 mM SA and mitochondria isolated at times between 1 h and 3 d after treatment. Alternative oxidase protein increased 2.5-fold within 5 h, reached a maximum (9-fold) after 12 h, and remained at twice the level of control plants after 3 d. Measurements of isotope fractionation of '*O by intact leaf tissue gave a value of 23% at all times, identical to that of control plants, indicating a constant 27 to 30% of electron-flow partitioning to the alternative oxidase independent of treatment with SA. Transgenic NahC tobacco p!ants that express bacterial salicylate hydroxylase and possess very low levels of SA gave a fractionation of 23% and showed control levels of alternative oxidase protein, suggesting that steady-state alternative oxidase accumulates i n an SA-independent manner. lnfection of plants with tobacco mosaic virus resulted in an increase in alternative oxidase protein in both infected and systemic leaves, but no increase was observed in comparably infected NahC plants. Total respiration rate and partitioning of electrons to the alternative pathway in virus-infected plants was comparable to that in uninfected controls.

The effect of 0.5 millimolar O-acetyl-L-serine added to the nutrient solution on sulfate assimilation of Lemna minor L., cultivated in the light or in the dark, or transferred from light to the dark, was examined. During 24 hours after transfer from light to the dark the extractable activity of adenosine 5'-phosphosulfate sulfotransferase, a key enzyme of sulfate assimilation, decreased to 10% of the light control. Nitrate reductase (EC 1.7.7.1.) activity, measured for comparison, decreased to 40%. Adenosine 5'-triphosphate (ATP) sulfurylase (EC 2.7.7.4.) and O-acetyl-L-serine sulfhydrylase (EC 4.2.99.8.) activities were not affected by the transfer. When O-acetyl-L-serine was added to the nutrient solution at the time of transfer to the dark, adenosine 5'-phosphosulfate sulfotransferase activity was still at 50% of the light control after 24 hours, ATP sulfurylase and O-acetyl-L-serine sulfhydrylase activity were again not affected, and nitrate reductase activity decreased as before. Addition of O-acetyl-L-serine at the time of the transfer caused a 100% increase in acid-soluble SH compounds after 24 hours in the dark. In continuous light the corresponding increase was 200%. During 24 hours after transfer to the dark the assimilation of 35S042-into organic compounds decreased by 80% without O-acetyl-L-serine but was comparable to light controls in its presence. The addition of O-acetyl-L-serine to Lemna minor precultivated in the dark for 24 hours induced an increase in adenosine 5'-phosphosulfate sulfotransferase activity so that a constant level of 50% of the light control was reached after an additional 9 hours. Cycloheximide as well as 6-methylpurine inhibited this effect. In the same type of experiment 0-acetyl-L-serine induced a 100-fold increase in the incorporation of label from 35S042-into cysteine after additional 24 hours in the dark. Taken together, these results show that exogenous 0-acetyl-L-serine has a regulatory effect on assimilatory sulfate reduction of L. minor in light and darkness. They are in agreement with the idea that this compound is a limiting factor for sulfate assimilation and seem to be in contrast to the proposed strict light control of sulfate assimilation. Light (3,21,26) and the rate of nitrate assimilation or the availability of amino acids (4,16,17,25)

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