Declining Phosphorus Availability Increases Sucrose Synthase Activity in ‘Beauregard’ Sweetpotato Proximal Adventitious Root Tissue Undergoing Storage Root Initiation

Sucrose synthase ( SuSy ) is recognized as a marker for sink strength in sweetpotato adventitious roots (ARs) undergoing storage root (SR) initiation. However, little is known about specific variables that modulate AR SuSy activity. Two studies were performed using a vertical split-root culture system to test the hypothesis from model systems that declining rhizosphere phosphorus (Pi) availability leads to increased carbohydrate accumulation in the root system. In the first study, an experiment was conducted to characterize tissue-specific SuSy expression in sweetpotato cv. Beauregard plants subjected to declining Pi treatments during early AR development and SR formation stages. A subsequent experiment was performed to measure SuSy activity in the AR proximal tissue (APT) under various Pi availability scenarios. The Pi treatments applied in either compartment was +P/+P, +P/–P, –P/–P, and 0P/0P (+P = Pi sufficient control; 0P = Pi omission; –P = declining Pi). The –P treatment was implemented by reducing Pi by 25%, 50%, 75%, and 100% at 7, 9, 11, and 13 days after planting (DAP), respectively. In the first experiment, APT SuSy expression increased from 12 to 15 DAP relative to the root tip and immature leaves, confirming tissue specificity of activity at the onset of SR initiation. In the second experiment, APT SuSy activity was higher with –P/–P relative to the control (+P/+P) at 10 to 15 DAP, aligning with prior anatomic evidence regarding the location and timing of SR initiation. Neither the 0P/0P treatment nor the +P/–P treatment elicited a response, suggesting that systemic Pi depletion rate and timing, rather than deficiency alone, are critical for increased sink strength. A second study, which characterized plant height and root system architecture responses, confirmed this presumptive systemic signaling. Taken together, these data support the hypothesis that SuSy is a marker for sink strength and that a declining Pi level acts as a developmental cue that signals sweetpotato SR initiation.