Abstract
Ca2+-binding proteins in the watery saliva of Megoura viciae counteract Ca2+-dependent occlusion of sieve plates in Vicia faba and so prevent the shut-down of food supply in response to stylet penetration. The question arises whether this interaction between aphid saliva and sieve-element proteins is a universal phenomenon as inferred by the coincidence between sieve-tube occlusion and salivation. For this purpose, leaf tips were burnt in a number of plant species from four different families to induce remote sieve-plate occlusion. Resultant sieve-plate occlusion in these plant species was counteracted by an abrupt switch of aphid behaviour. Each of the seven aphid species tested interrupted its feeding behaviour and started secreting watery saliva. The protein composition of watery saliva appeared strikingly different between aphid species with less than 50% overlap. Secretion of watery saliva seems to be a universal means to suppress sieve-plate occlusion, although the protein composition of watery saliva seems to diverge between species.
Highlights
Sieve tubes – arrays of longitudinally arranged sieve elements (SEs) – are the transport conduits for an assortment of metabolites such as carbohydrates, amino acids, phytohormones and vitamins
We investigated whether watery saliva acts as a universal tool in aphid–plant interactions through suppression of sieve-tube occlusion
Time course of events in sieve tubes after leaf-tip burning carboxyfluorescein diacetate (CFDA) was applied to the loading site on B. napus leaves and transported downstream in the CF form through the sieve tubes
Summary
Sieve tubes – arrays of longitudinally arranged sieve elements (SEs) – are the transport conduits for an assortment of metabolites such as carbohydrates, amino acids, phytohormones and vitamins (reviewed by van Bel, 2003). Following production in the mesophyll and loading into sieve tubes, plant nutrients are translocated through the transport phloem towards the sinks, e.g. growing leaves, roots, stem or storage organs (van Bel, 1996). Mass flow through sieve tubes is driven by a turgorpressure gradient between sources (high pressure) and sinks (low pressure) (Münch, 1930; Gould et al, 2005). Aphids are confronted with sieve-tube occlusion mechanisms (Sjölund, 1997; Knoblauch and van Bel, 1998; Ehlers et al, 2000). Occlusion of sieve pores prevents the loss of sieve-tube sap (Evert, 1982; Schulz, 1998) and the invasion of pathogens through the wound site (van Bel, 2003)
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