The Department of Biochemistry's weekly BCH 252 seminar series is presented this week by:

Jacob MacWilliams, Biochemistry and Molecular Biology Graduate Student, UC Riverside 

Seminar Title: "Elucidating how cowpea aphids affect plant host immunity and physiology"

Abstract: One of the biggest threats to cowpea, an important crop to drought-ridden areas of the world, is the cowpea aphid, Aphis craccivora. The cowpea aphid feeds by inserting its stylets into the host tissues and navigating to the phloem element where it acquires plant sap. During this navigation to the phloem, aphids deposit saliva, into cells and the apoplast, containing effector proteins that alter plant immune responses and physiology for the benefit of the aphid. Here we report the salivary profile of the cowpea aphid native to California. Over 76% of the proteins identified in the A. craccivora saliva, were not previously reported from this aphid species. One of the enzymes found in this saliva, not previously identified in the saliva of any aphid species, is diacetyl/L-xylulose reductase (DAX). DAX is a member of the short-chain dehydrogenases/reductases. Using an orthologous expression system, A. craccivora DAX (AcDAX) was expressed and purified. AcDAX found to both oxidize xylitol to xylulose, via the reduction of NADP⁺ to NADPH, and to reduce methylglyoxal, via the oxidation of NADPH to NADP⁺. The enzyme was found to have a K_m of 0.46 mM and k_cat of 1.45 s^-1 in converting xylitol to xylulose, comparable to its human homolog (DCXR) (K_m = 0.11 mM, k_cat = 1.7 s^-1), and to have a K_m of 1.57 mM and k_cat of 0.24 s^-1 in the reduction of methylglyoxal. The conversion of xylitol to xylulose, an intermediate in the non-oxidative phase of the pentose phosphate pathway, by this aphid salivary enzyme suggests that the aphid utilizes an alternate energy source. While the reduction of methylglyoxal by DAX is a way for the aphid to reduce plant cytotoxicity and alter plant immune signaling. Preliminary results indicate that transient overexpression of DAX in planta improves the aphid’s fecundity, supporting a role for DAX in enhancing aphid performance on its host.