Abstract
Vascular tissue was recently shown to be capable of producing nitric
oxide (NO), but the production sites and sources were not precisely
determined. Here, NO synthesis was analysed in the phloem of Vicia faba
in response to stress- and pathogen defence-related compounds.
The chemical stimuli were added to shallow paradermal cortical cuts in
the main veins of leaves attached to intact plants. NO production in
the bare-lying phloem area was visualized by real-time confocal laser
scanning microscopy using the NO-specific fluorochrome
4,5-diaminofluorescein diacetate (DAF-2 DA).
Abundant NO generation in companion cells was induced by 500 mu M
salicylic acid (SA) and 10 mu M hydrogen peroxide (H2O2), but the
fungal elicitor chitooctaose was much less effective. Phloem NO
production was found to be dependent on Ca2+ and mitochondrial electron
transport and pharmacological approaches found evidence for activity of
a plant NO synthase but not a nitrate reductase. DAF fluorescence
increased most strongly in companion cells and was occasionally
observed in phloem parenchyma cells. Significantly, accumulation of NO
in sieve elements could be demonstrated.
These findings suggest that the phloem perceives and produces
stress-related signals and that one mechanism of distal signalling
involves the production and transport of NO in the phloem.
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