**2. Method and Materials**

#### *2.1. Plant Material and Growth Conditions*

*A. fistulosum* seeds and Yellow Sweet Spanish *A. cepa* seeds (Early's Farm & Garden Center, Saskatoon, SK, Canada) were grown in the Agriculture Greenhouses, University of Saskatchewan (Saskatoon, SK, Canada) in 6" pots containing Sunshine Mix No. 4 (Sungro Horticulture Canada Ltd. Seba Beach, AB, Canada) at approximately 25/22 ◦C (day/night) under natural light supplemented with high pressure sodium lights (17 h photoperiod, average 600 μmol m<sup>−</sup>2s−1). Watering (City of Saskatoon, SK, Canada, water) was applied every second day during the spring/summer months and every third day during the fall and winter months with 200 mL of 20–20–20 (NPK) fertilizer (150 g L−1) (Plant Prod, Brampton, ON, Canada) weekly during the spring/summer months and twice weekly during the fall/winter months. The experiment was arranged in a randomized complete block design across the bench.

In addition, five *A. thaliana* genotypes were analyzed: three boron transporter mutants (*nip5;1-1* (SALK\_122287C), *nip6;1-2* (SALK\_046323C) and *bor1-3* (SALK\_037312)), a pectin methylesterase inhibitor over-expression mutant (*p35S::PMEI5*) and a wild-type (Col-0) line, respectively [36–39]. The three boron mutants were obtained from the *Arabidopsis* Biological Resource Centre (ABRC) (Columbus, OH, USA). The transgenic line overexpressing PMEI5 driven by the Cauliflower mosaic virus (CaMV) 35S promoter was provided courtesy of Kerstin Müller [39]. Lines were genotyped according to Edwards et al. (1991), with some modifications [40,41]. Gene specific primers were ordered from Integrated DNA Technologies (Coralville, IA, USA) (Table S1).

*Arabidopsis* plants were grown in the Agriculture phytotron (Conviron, Winnipeg, MB, Canada) under 20 ◦C constant temperature, 50% RH, 16 h photoperiod, with an irradiance of 150 ± 10 m−2s−1, and watered every second day (City of Saskatoon, SK, Canada). Two g/L fertilizer (20–20–20 NPK) was applied weekly.
