Blood-feeding arthropods secrete special salivary proteins that suppress the defensive reaction they induce in their hosts. This is in contrast to herbivores, which are thought to be helpless victims of plant defences elicited by their oral secretions. On the basis of the finding that caterpillar regurgitant can reduce the amount of toxic nicotine released by the tobacco plant Nicotiana tabacum, we investigate here whether specific salivary components from the caterpillar Helicoverpa zea might be responsible for this suppression. We find that the enzyme glucose oxidase counteracts the production of nicotine induced by the caterpillar feeding on the plant.

Pre-inoculation of plants with a pathogen that induces necrosis leads to the development of systemic acquired resistance (SAR) to subsequent pathogen attack [1]. The phenylpropanoid-derived compound salicylic acid (SA) is necessary for the full expression of both local resistance and SAR [2] [3]. A separate signaling pathway involving jasmonic acid (JA) is involved in systemic responses to wounding and insect herbivory [4] [5]. There is evidence both supporting and opposing the idea of cross-protection against microbial pathogens and insect herbivores [6] [7]. This is a controversial area because pharmacological experiments point to negative cross-talk between responses to systemic pathogens and responses to wounding [8] [9] [10], although this has not been demonstrated functionally in vivo. Here, we report that reducing phenylpropanoid biosynthesis by silencing the expression of phenylalanine ammonialyase (PAL) reduces SAR to tobacco mosaic virus (TMV), whereas overexpression of PAL enhances SAR. Tobacco plants with reduced SAR exhibited more effective grazing-induced systemic resistance to larvae of Heliothis virescens, but larval resistance was reduced in plants with elevated phenylpropanoid levels. Furthermore, genetic modification of components involved in phenylpropanoid synthesis revealed an inverse relationship between SA and JA levels. These results demonstrate phenylpropanoid-mediated cross-talk in vivo between microbially induced and herbivore-induced pathways of systemic resistance.

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The impact of phenolics on a specialist herbivore, Manduca sexta, and a generalist herbivore, Heliothis virescens, was investigated using transgenic tobacco with differential expression of phenylalanine ammonia-lyase. Foliar phenolics such as chlorogenic acid, rutin, and total flavonoids differentially accumulated in the respective transgenic tobacco lines; the amount of chlorogenic acid ranged from 201 to 2202 μg g-1 of fresh leaf, that of total flavonoids from 211 to 500 μg g-1 of fresh leaf, and that of rutin from 73 to 172 μg g-1 of fresh leaf. However, the levels of the phenolics and larval growth of M. sexta or H. virescens were not significantly correlated. Likewise, phenolic levels were not correlated with larval survival of M. sexta. These results suggest that plant phenolics are not a determining factor for host plant resistance against these insects in this system. Keywords: Phenylalanine ammonia-lyase; phenolics; host plant resistance; Manduca sexta; Heliothis virescens

The possible involvement of membranes and water viscosity in the temperature effects on imbibition and solute leakage of radish (Raphauus sativa var. Early Scarlet Globe) seeds and excised sugar pine (Pinus Imbertiama Dougl.) embryos was evaluated. In these two seed materials, the temperature effect on initial rates of imbibition and solute leakage could be accounted for primarily by changes in water viscosity, the relationship being approximatel linear. It appears that membranes are involved both in water uptake and solute leakage. Heat-killed radish seeds and sugar pine embryos exhibited significantly higher rates of imbibition and solute leakage than did viable ones. In addtion, sugar pine embryos exhibited an abrupt change in rates of imbibition and solute leakage between 15 and 20°C, resulting in abnormally high water uptake and solute leakage above this temperature. logical nature, explained by the chemical activity of water expressed as the proportion of active "hydrone" molecules (2). Rate effects in peas, corn, and Xanthium, on the other hand (observed Qlo < 2.0), were suggested to be of a physical nature, with no "chemical" change of the water involved (12, 13). Rates of water uptake in wheat, pea, and rape seeds were suggested to be influenced strongly by the binding of water in the colloids and micellary structure of the seeds, but no temperature effect could be attributed to water viscosity (1). However, imbibition is more likely a complex phenomenon, depending both on properties of the water as well as the seed colloidal material (7). Similar temperature effects on imbibition of heat-killed seeds has been presented as evidence that water uptake of seeds is a physical, nonmetabolic process (7), and not appreciably effect by membranes (5).A clearer understanding of the temperature effects on seed imbibition appears to have been hampered by a variety of interfering features of the seeds themselves: partially impermeable seed Membranes of dry seeds are in a disorganized, possibly nonlamellar state and, as such, are inefficient barriers to movement of water and solutes (15). Conditions during the initial phase of seed imbibition, when the hydrating membranes are becoming reorganized into a continuous bilayer configuration exhibiting efficient semipermeable properties, are critical to survival and successful germination. Excessive rates ofwater uptake before reorganization is accomplished may lead to displacement of membrane components, deleterious mixing of cellular compounds, and loss of cellular components through leakage into the aqueous medium (15).Temperature is one of the more prominent environmental factors affecting seed germination. The majority ofnondormant seeds show an optimal temperature range for germination (commonly between 15 and 30°C), with drastic reductions in germination above and below this range (3,4,16). Both high and low temperature-related reductions in germinability seem to have a common cause, the inability of seed membranes to function as efficient semipermeable memb...

Fruit of four clingstone peach [Prunus persica (L.) Batsch.] seedling populations, 54 advanced selections, and the cultivars Allgold and Goldilocks were analyzed for sugar content, soluble solids concentration (SSC), and acidity. Sucrose, glucose, fructose, and sorbitol were identified and quantified by high pressure liquid chromatography (HPLC) analysis. Variation was found for all the sugars between the seedling populations. Comparison of the means for the advanced selections with `Goldilocks' indicates that progress has already been made toward selection for a sweeter peach. Glucose, fructose, sorbitol, SSC, and acidity (citric) differed significantly for two harvest years, while sucrose and total sugar concentrations showed no significant yearly variation. A fruit maturity study revealed no significant changes in SSC, percentages of glucose, fructose, and total sugar during ripening on the tree. Acidity and percent sorbitol decreased, while the sugar: acid ratio and percent sucrose increased with increasing maturity. Broad-sense heritability of SSC, acidity, and sugar: acid ratio bad values >0.72, while values for individual sugars and total sugars were much lower. Transgressive segregation for each sugar was found in seedling populations.