DOI: 10.5061/dryad.mgqnk992g
title: Spider mite resistant maize lines, B75 and B96, maintain resistance under water-stress

description: Climate variability has major implications for agriculture due to the
 increase in the frequency and intensity of simultaneous abiotic, namely
 water-stress, and biotic stresses to crops. Plant water-stress alone harms
 crops but also can attract outbreaks of herbivores with varied host
 specialization, and plants succumb to further yield losses dealing with
 multiple stressors. Host-plant resistance provides a route to lessen yield
 losses from herbivory; however, our knowledge of the interactions between
 water-stress and pest resistance is limited, especially for mite
 herbivores of maize including the generalist two-spotted spider mite
 (Tetranychus urticae, TSM) and the specialist Banks grass mite
 (Oligonychus pratensis, BGM). We conducted parallel greenhouse and field
 experiments whereby a susceptible line (B73) and two TSM-resistant lines
 (B75 and B96) were subjected to either optimal irrigation or water-stress
 [50–60% and 5–10% volumetric water content (VWC), and 25–32% and 10–15%
 VWC, in the greenhouse and field, respectively]. As expected, we found
 that under optimal irrigation TSM and BGM populations increased readily on
 B73, while B75 and B96 were largely resistant to the TSM but not BGM.
 While plant water-stress increased the susceptibility of B73 to both mite
 species, water-stress did not disrupt initial resistance levels of B75 and
 B96 maize for either mite species. Elevated protease activity was found in
 B75 and B96 and may contribute to maize resistance. Our findings that B75
 and B96 are highly resistant to the TSM, and maintain resistance to both
 mite species with water-stress, highlights the importance of including the
 nuances of multiple stressors within the framework of host-plant
 resistance.
description: Water-stress or optimal irrigation levels were quantified by
 measuring stomatal conductance (mmolm<sup>-2
 </sup>s<sup>-1</sup>) and leaf temperature (°C) using a
 leaf porometer (Model SC-1, Meter Group, WA, USA), leaf water potential
 (bar) using a pressure chamber (Model 615, PMS Instrument Company, OR,
 USA), and stem height (cm) by using a ruler. Leaf temperature, stomatal
 conductance and stem height were measured at 3 and 7 days post-mite
 introduction, while leaf water potential was measured after sample
 collection at 7 days post-mite introduction.  At 1, 3,
 and 7 days post mite introduction, leaf samples (leaf areas inside
 Tanglefoot arenas) from eight plants of four randomly selected replicates
 (2 plants/replicate) were collected, flash-frozen using liquid nitrogen
 and stored in a freezer (-20°C) until processing. Each sample was
 processed by counting the number of eggs and all mite stages and by
 performing defense protein bioassays.  The activities
 of POD, PPO, and CHI were analyzed using a microplate reader (Biotek,
 EPOCH, VT, USA), while the activity of TI was analyzed by using radial
 diffusion techniques.
description: Mite (TSM and BGM) population sizes, assessed as the sum of eggs,
 nymphs and adults, and plant physiological measurements, and defense
 protein activity measurements from trials were analyzed using a general
 linear model (Proc Glimmix; SAS 9.4 M4 University edition). Square-root
 transformation was used for both mite population growth, plant
 physiological responses, and defensive protein activities (POD, PPO, CHI
 and TI) data to conform to the assumption of normality and
 heteroscedasticity. Data presented here are the raw numbers.