An aphid's Odyssey--the cortical quest for the vascular bundle

Area of Science:

• Plant-insect interactions
• Aphid feeding behavior
• Plant physiology

Background:

• Aphids possess sophisticated mechanisms to locate nutrient-rich sieve tubes within plant vascular bundles.
• Previous research indicated aphids sense pH and sucrose concentration for sieve tube recognition.
• The precise navigational cues used by aphids to reach vascular bundles remained largely unknown.

Purpose of the Study:

• To investigate the orientation mechanisms aphids employ to find vascular bundles in plant stems.
• To determine if electrical or chemical gradients in the stem cortex guide aphid stylet penetration.
• To elucidate the role of sucrose and pH in aphid phloem-feeding site selection.

Main Methods:

• Measurement of membrane potentials in Vicia faba stem cortex.
• Analysis of aphid feeding preferences using artificial diets with varying oxygen levels.
• Tracking of aphid salivary sheath deposition patterns.
• Optical examination and Electrical Penetration Graph (EPG) recordings of aphid probing behavior.
• Experimental manipulation of phloem transport in Ricinus communis to alter sugar composition.

Main Results:

• No detectable electrical gradient was found in the stem cortex to guide aphid stylets.
• Aphids showed no preference for diets with differing dissolved oxygen levels.
• Aphids exhibit a radial search pattern with regular lateral punctures, probing cortex cells.
• Acidic pH and low sucrose in cortex cell vacuoles prompted stylet retraction.
• Aphids consumed less phloem sap from plants engineered to transport hexoses instead of sucrose.

Conclusions:

• Aphids do not rely on cortical electrical or oxygen gradients for vascular bundle orientation.
• A rejection-acceptance behavior, driven by sensing unfavorable cell sap (low pH, low sucrose), guides aphids.
• Aphids retract stylets from unfavorable cells and probe adjacent ones until reaching sieve tubes.
• Sucrose acts as a critical cue, with aphids preferring sucrose over hexoses.
• This rejection-acceptance mechanism provides a universal plant-directed strategy for aphid phloem location.