Conserved phosphoserine/threonine-binding proteins in phloem-feeding insect saliva suppress plant defenses by attenuating the transactivation activity of ABI5

Phosphoserine/threonine-binding proteins from the 14-3-3 class are involved in a variety of regulatory processes in all eukaryotic cells, including yeast, protozoa and mammals. Recently, 14-3-3 proteins have been identified in the saliva of some phloem-feeding insects, but their function remains poorly understood. 14-3-3 protein, secreted by the phloem-feeding whitefly Bemisia tabaci (Bt14-3-3), is delivered into tobacco plants during feeding. Within the plant, Bt14-3-3 interacts with the tobacco abscisic acid-insensitive 5-like (ABI5) basic leucine zipper transcription factor, repressing its ability to bind cis-acting activating elements present in the promoter region of the plant defensin J1-2 (PDF1.2) gene. Downregulation of PDF1.2 expression suppresses plant defenses, leading to enhanced whitefly performance. A homologous 14-3-3 protein secreted by the phloem-feeding aphid Myzus persicae was found to similarly enhance aphid performance on tobacco plants by interfering with the ability of ABI5 to transactivate the PDF1.2 gene. These results raise the possibility that during the evolutionary ‘arms-race’ between plants and phloem-feeding insects, 14-3-3 proteins present in the insects' saliva might have evolved novel functions that contribute to the ability of these insects to counterbalance their host plant defenses, transforming them into a suitable habitat for feeding and reproduction.