UC Riverside

Aphids (Hemiptera: Aphididae) are among the most destructive agricultural pests that cause extensive economic losses to cultivated crops worldwide. Aphids feed on the phloem sap by using their specialized mouthparts known as stylets. While feeding, aphids secrete large quantities of saliva into the host cells both intercellularly and intracellularly. Aphid saliva is of two types: soluble and gelling saliva. Aphid saliva is presumed to contain a large number of proteins and some of them have been shown to be involved in altering plant defenses. Potato aphid is one of the major agricultural pests for tomato and potato. In Chapter One, by using mass spectrometry we identified plethora of proteins present in the potato aphid saliva, by feeding them on two different diets using in vitro feeding chambers. Interestingly, several of these were of aphid endosymbiont Buchnera aphidicola origin. One of these proteins GroEL that is known to induce immune responses in mammals was chosen for further characterization. This GroEL characterization is described in Chapter Two. Our analysis showed that delivering GroEL in planta by bacterial type-three secretion or expressing GroEL in planta by making stable transgenic Arabidopsis thaliana lines enhanced resistance against aphids and as well as bacteria. Additional experiments showed that GroEL is also recognized extracellularly and this extracellular recognition required the coreceptor BRASSINOSTEROID INSENSITIVE 1-ASSOCIATED KINASE 1. This study identified a novel role of endosymbionts in inducing plant defense responses.

Another indirect approach to identify secreted proteins is based on prediction of secretion signal. Using this approach, potato aphid salivary gland transcriptome was sequenced as described in Chapter Three. Putative salivary gland proteins predicted to be secreted were identified by SignalP and TargetP programs. In planta functional characterization of eight of these putative aphid secreted proteins identified roles for two, Me10 and Me23, in altering plant responses to aphid's advantage. Me10 enhanced aphid performance on tomato and Nicotiana benthamiana plants and was chosen for further analysis described in Chapter Four. A yeast two-hybrid screen with tomato cDNA library revealed that Me10 interacts with several tomato proteins including 14-3-3, which is involved in plant-pathogen immune responses.