Sieve element occlusion provides resistance against Aphis gossypii in TGR‐1551 melons

Feeding behavior and plant response to feeding were studied for the aphid Aphis gossypii Glover on susceptible and resistant melons (cv. Iroquois and TGR‐1551, respectively). Average phloem phase bout duration on TGR‐1551 was <7% of the duration on Iroquois. Sixty‐seven percent of aphids on TGR‐1551 never produced a phloem phase that attained ingestion (EPG waveform E2) in contrast to only 7% of aphids on Iroquois. Average bout duration of waveform E2 (scored as zero if phloem phase did not attain E2) on TGR‐1551 was <3% of the duration on Iroquois. Conversely, average bout duration of EPG waveform E1 (sieve element salivation) was 2.8 times greater on TGR‐1551 than on Iroquois. In a second experiment, liquid nitrogen was used to rapidly cryofix leaves and aphids within a few minutes after the aphids penetrated a sieve element. Phloem near the penetration site was then examined by confocal laser scanning microscopy. Ninety‐six percent of penetrated sieve elements were occluded by protein in TGR‐1551 in contrast to only 28% in Iroquois. Usually in TGR‐1551, occlusion was also observed in nearby nonpenetrated sieve elements. Next, a calcium channel blocker, trivalent lanthanum, was used to prevent phloem occlusion in TGR‐1551, and A. gossypii feeding behavior and the plant's phloem response were compared between lanthanum‐treated and control TGR‐1551. Lanthanum treatment eliminated the sieve element protein occlusion response and the aphids readily ingested phloem sap from treated plants. This study provides strong evidence that phloem occlusion is a mechanism for resistance against A. gossypii in TGR‐1551.