agriculture Article Tomato Seed Coat Permeability: Optimal Seed Treatment Chemical Properties for Targeting the Embryo with Implications for Internal Seed-Borne Pathogen Control Hilary Mayton 1,† , Masoume Amirkhani 1,† , Daibin Yang 2, Stephen Donovan 3 and Alan G. Taylor 1,* 1 Cornell AgriTech, School of Integrative Plant Science, Horticulture Section, Cornell University, New York, NY 14456, USA; [email protected] (H.M.); [email protected] (M.A.) 2 Institute of Plant Protection, Chinese Academy of Agricultural Sciences, Beijing 100193, China; [email protected] 3 The Center for Forensic Science Research & Education, Willow Grove, PA 19090, USA; [email protected] * Correspondence: [email protected]; Tel.: +1-315-787-2243 † These authors contributed equally in this study. Abstract: Seed treatments are frequently applied for the management of early-season pests, including seed-borne pathogens. However, to be effective against internal pathogens, the active ingredient must be able to penetrate the seed coat. Tomato seeds were the focus of this study, and the objectives were to (1) evaluate three coumarin fluorescent tracers in terms of uptake and (2) quantify seed coat permeability in relation to lipophilicity to better elucidate chemical movement in seed tissue. Uptake in seeds treated with coumarin 1, 120, and 151 was assessed by fluorescence microscopy. For quantitative studies, a series of 11 n-alkyl piperonyl amides with log Kow in the range of 0.02–5.66 Citation: Mayton, H.; Amirkhani, M.; were applied, and two portions, namely, the embryo, and the endosperm + seed coat, were analyzed Yang, D.; Donovan, S.; Taylor, A.G. by high-performance liquid chromatography (HPLC). Coumarin 120 with the lowest log Kow of 1.3 Tomato Seed Coat Permeability: displayed greater seed uptake than coumarin 1 with a log Kow of 2.9. In contrast, the optimal log Kow Optimal Seed Treatment Chemical for embryo uptake ranged from 2.9 to 3.3 derived from the amide series. Therefore, heterogeneous Properties for Targeting the Embryo coumarin tracers were not suitable to determine optimal log Kow for uptake. Three tomato varieties with Implications for Internal were investigated with the amide series, and the maximum percent recovered in the embryonic tissue Seed-Borne Pathogen Control. ranged from only 1.2% to 5%. These data suggest that the application of active ingredients as seed Agriculture 2021, 11, 199. https:// treatments could result in suboptimal concentrations in the embryo being efficacious. doi.org/10.3390/agriculture11030199 Keywords: tissue lipophilicity; systemic uptake; coumarin; piperonyl amides Academic Editor: Rentao Song Received: 13 February 2021 Accepted: 23 February 2021 Published: 28 February 2021 1. Introduction Seed-borne pathogens are responsible for the initiation of numerous plant diseases and Publisher’s Note: MDPI stays neutral are one of the primary mechanisms for the global spread of plant pathogens [1–4]. Internal with regard to jurisdictional claims in infection of seeds and colonization of the embryo and endosperm are most often associated published maps and institutional affil- with infection of the mother plant via the xylem, stigma, or non-vascular tissue [4–6]. Seed- iations. borne pathogens have been observed in the seed embryo, storage tissue (endosperm and perisperm), and seed coat or testa [4,7]. Disinfection techniques can be used to remove and clean contaminants from the seed surface; however, plant pathogenic organisms located within the seed endosperm and embryo are much more difficult to control. Tomatoes Copyright: © 2021 by the authors. are an important high-value vegetable crop and are susceptible to multiple pathogens. Licensee MDPI, Basel, Switzerland. Tomato seeds can harbor fungal, bacterial, and viral pathogens [8–10]. Several systemic This article is an open access article conventional pesticide seed treatments are available for fungal pathogens of tomato, but distributed under the terms and options are more limited for organic production and control of bacterial pathogens [3,11,12]. conditions of the Creative Commons Seed treatments are applied worldwide for crop protection against pests and plant Attribution (CC BY) license (https:// pathogens [13,14]; however, the systemic uptake and distribution of active ingredients creativecommons.org/licenses/by/ of pesticide seed treatments in seed tissue have not been as well defined as root and leaf 4.0/). Agriculture 2021, 11, 199. https://doi.org/10.3390/agriculture11030199 https://www.mdpi.com/journal/agriculture Agriculture 2021, 11, 199 2 of 11 transport [15,16]. The long-term efficacy of seed treatments and control of seed-borne pathogens are dependent on seed coat permeability, as the active ingredients must be able to penetrate the seed coat and diffuse to the embryo. There have been several studies focused on the physiochemical barriers that prevent or allow a chemical to permeate the seed coats of several plant species [17–19]. Taylor and Salanenka (2012) developed a system to classify seed coat permeability based on the passage of ionic and non-ionic compounds through the seed coat of ten plant species from seven plant families [18]. Tomato seeds have selective permeability defined as only non-ionic compounds diffusing through the seed coat, while ionic compounds are blocked [17,18]. Seed uptake research on potential chemical pesticides applied as seed treatments is problematic due to the potential human risk of exposure to agrochemicals and/or radioactively labeled compounds. Fluorescent tracers provide an alternative approach, and coumarins are one group that includes several fluorescent, non-ionic tracers differing in chemical properties and that allows for a more comprehensive analysis of seed coat permeability characteristics [16]. Therefore, coumarin compounds were used both for qualitative uptake [16,17,19] and, using a single coumarin compound, for quantitative uptake research [20]. One objective of this research is to use three coumarin compounds with different chemical properties for tomato seed uptake to assess optimum log Kow. A key chemical property that affects the uptake of an organic compound in a seed is the log Kow, also known as the log P [20–23]. A compound’s lipophilicity is measured as the log Kow and is the ratio of its chemical concentration in octanol (o) to its concentration in the aqueous (w) phase expressed on a log10 scale [24]. A series of fluorescent piperonyl amides were synthesized, and a novel combinatorial pharmacokinetic technique was developed to provide a range of compounds with log Kow from 0.2 to 5.8. This series of fluorescent piperonyl amides was used to explore seed coat permeability and systemic uptake in soybean and corn seeds [23]. This same approach was adopted for tomato seed in this study. Understanding the chemical/physical properties associated with the uptake of active ingredients in tomato seed tissue will aid in the development of new products for the control of internal seed-borne pathogens. The key objectives of this study were to evaluate the movement of selected coumarin compounds in uptake by fluorescence imaging and assess the role of log Kow in seed tissue permeability using a homologous series of 11 fluorescent piperonyl amides quantified by high-performance liquid chromatography (HPLC). 2. Materials and Methods 2.1. Fluorescence Microscopy of Coumarin 1, 120, and 151 in Tomato Seeds The first study was on the uptake of selected coumarin tracers in tomato seeds imaged by fluorescence microscopy. Tomato seeds of the variety “Hypeel 696” were provided by Seminis, Oxnard, CA, and coumarin 1, 120, and 151 were purchased from TCI America, Portland, OR. The chemical and other properties of these three coumarin compounds are shown in Table1. Tomato seeds were treated with 3 µmoles of each coumarin per gram of seed, which was 0.833, 0.631, and 0.825 mg coumarin 1, 120, or 151, respectively, per gram of seed. Each coumarin compound was mixed with 3.8 mg L650 seed treatment binder (Incotec, Salinas, Canada), 250 µL deionized water, and mixed in a 50 mL centrifuge tube using a vortex mixer (Scientific Industries, Inc., Model 2-Genie No. G560, New York, NY, USA). Ten non-treated and treated tomato seeds of each tracer were sown in 20% moisture content silica sand (#1 Q-ROK, 0.15–0.84 mm, New England Silica, Inc., South Windsor, CT, USA) and maintained in a germinator at 20 ◦C for 40 h in the dark. Imbibed seeds were then removed and washed with deionized water, and then the seeds were dissected with scalpel blades and imaged under an Olympus microscope (SZX12, Tokyo, Japan), imaging camera (Infinity 3- 3URC, Lumenera Corp., Ottawa, ON, Canada), and Infinity Analyze (Revision 6.5.2, Teledyne Lumenera, Ottawa, ON, Canada). Seed tissue was illuminated Agriculture 2021, 11, x FOR PEER REVIEW 3 of 12 Agriculture 2021, 11, 199 3 of 11 (SZX12, Tokyo, Japan), imaging camera (Infinity 3- 3URC, Lumenera Corp., Ottawa, ON, Canada), and Infinity Analyze (Revision 6.5.2, Teledyne Lumenera, Ottawa, ON, Canada). withSeed longtissue UV was light, illuminated UV lamp with (Model long 9-circular UV light, illuminator, UV lamp (Model Stocker 9-circular & Yale, Salem,illuminator, NH, USA).Stocker Non-treated & Yale, Salem, seeds NH, were USA). used Non-tr as theeated control. seeds were used as the control. TableTable 1. 1.Physical/chemical Physical/chemical propertiesproperties ofof coumarincoumarin 120,120, 151,151, andand 1.1. * Water Molar Abs Coumarin CAS MW,* Water Excitation/EmissionMolar Abs Quantum Coumarin CAS MW, * Log Kow Excitation/Emissionsolubility, Coefficient,Quantum * Log Kow Solubility, Coefficient, Compound Number Compoundg/mol Number g/mol Max, nm Max, nm −1Yield Yield Log S Log S cm−1 cm 120 26093-31-2 175.2 1.25 1.25 342/409 3.50 × 108 0.63 120 26093-31-2 175.2 1.25 1.25 342/409 3.50 × 108 0.63 151 53518-15-3 229.2 1.62 −3.56 364/460 4.58 × 108 0.53 151 53518-15-3 229.2 1.62 −3.56 364/460 4.58 × 108 0.53 8 1 91-44-1 231.31 91-44-1 2.90 231.3 −3.692.90 −3.69 369/431 369/4314.63 × 108 4.63 × 100.73 0.73 * Log Kow and water solubility data obtained from Chemicalize, ChemAxon’s cheminformatic tool.