Uncovering phenotypic traits and the molecular basis for PUE are important early steps in breeding for phosphate use efficiency. No commercial breeding programs exist for watercress worldwide, but germplasm collections are emerging. Development of new watercress varieties is also no doubt limited by the lack of genomic information for this crop. Payne screened a germplasm collection under indoor and outdoor conditions and identified significant variation in several traits including stem length, stem diameter and antioxidant concentrations. Voutsina used RNA-seq to analyse the first watercress transcriptome and identified differences in antioxidant capacity and glucosinolate biosynthesis across a germplasm collection of watercress, with 71% of the watercress transcripts annotated based on orthology to Arabidopsis. Jeon et al. independently used RNA-seq approaches to assemble the watercress transcriptome de novo. They identified 33 candidate genes related to glucosinolate biosynthetic pathways using Arabidopsis glucosinolate genes to search for homologous sequences in the watercress transcriptome. Additionally, a watercress mapping population comprising 259 F2 individuals was established by Voutsina using parents with contrasting nutrient and growth phenotypes. Genotype-by-sequencing of this mapping population enabled the construction of first genetic linkage map for watercress and identified 17 QTL for morphological traits of interest, antioxidant capacity and cytotoxicity against human cancer cells. However, no root traits were assessed in this work. Screening this mapping population and wider germplasm for root traits may reveal individuals with extreme PUE phenotypes,indoor farming equipment which could allow the development of markers and QTL associated with this complex trait.Together these findings will assist in developing commercial cultivars with a reduced need for phosphate, and a reduced negative environmental impact.

To assess whether homologs of the candidate PUE genes identified in this study exist in watercress, available watercress transcriptome data was mined for 13 key PUE genes selected from Table 2. This included genes whose expression was induced more than tenfold in at least 2 independent studies , plus PHR1, the global regulator of P starvation responses. Annotated transcripts were obtained from transcriptomic studies of watercress by Voutsina et al., 2016; Jeon et al., 2017; Müller et al., 2021 and matches for these candidate genes were assessed by searching for genes using AGI [47, 133, 221, 222]. Across all studies, strong matches were found for PHT1;4, SPX1, PHR1, MDG3, PEPC1, PLDζ1/2, PSR2 and SQD2, with all corresponding e-values ranging from 0 to 3.00E-32. Additionally, Müller et al. identified homologous transcripts for MDG2. Voutsina et al. had transcripts corresponding to PHT1;3, and MDG2, and Jeon et al. had hits for PHT1;2 and PHT1;3. No matching transcripts were found for PHT1;1 in any of the three studies. Where FDR values were < 0.05, changes to expression patterns were noted. Interestingly, Müller et al. also observed varying levels of upregulation of PHT1;4 and PHR1 following submergence, which may suggest a link between phosphate starvation and submergence responses.A number of pyrethroids, such as permethrin, cypermethrin, deltamethrin, and esfenvalerate, have been reported to be present in house dust with detection frequency ranges from various studies of 45–100%, 5–64%, 5–17% and 5–29%, respectively . Much of this data was collected before or in the same year as the federally mandated phase-out of residential uses of the organophosphate pesticides chlorpyrifos and diazinon in 2001, which subsequently caused household pyrethroid use to increase . This can be seen in the above mentioned studies, with the highest %Ds of pyrethroids occurring in studies whose samples were collected during or after 2001. Although pyrethroids have low toxicity, particularly compared to other insecticides, studies have shown that high levels of exposure to pyrethroids may cause significant toxicity and health effects, including acute neurotoxic effects , immunotoxic effects and negative effects on mammalian reproduction .

Pyrethroids are also possible human carcinogens . Families living in close proximity to farms may have higher than average pyrethroid exposure due to household pesticide use, drift from agricultural application and take-home exposure pathways from occupational use by another family member . High levels of pesticides in carpet dust are a particular concern for young children who, due to their continual exploration of their environments, spend a large amount of time on the floor and have increased hand to mouth activity, resulting in increased exposure to pollutants through dermal and non-dietary ingestion routes . These two factors combined make children living in agricultural communities especially susceptible to pesticide exposure . Data on pyrethroid concentrations in the house dust of rural farm worker homes is limited. This study was conducted in order to address participant concerns about pesticide exposure in the community-based Mexican Immigration to California: Agricultural Safety and Acculturation study. Our objectives were to characterize the levels of pyrethroid pesticides in the house dust of farm worker families and characterize their residential pesticide application practices in order to evaluate possible associations between the dust levels and pesticide use practices. We report the pesticide use data and levels of pyrethroid pesticides in indoor dust collected in 2009 as measured by questionnaires and dust concentrations of the pyrethroids cis– and trans-permethrin, cypermethrin, deltamethrin, esfenvalerate and resmethrin among 55 households of farm worker families living in Mendota, CA. Single dust samples were collected from 105 homes of families participating in the MICASA study. Of the 105 available samples, 70 had sufficient quantities of dust after sieving for instrumental analysis of pyrethroids. Of those, there were 55 samples selected, with relatively higher selection probabilities assigned to those households with elevated levels of the common pyrethroid urinary metabolite 3-phenoxybenzoic acid in urine samples collected from the children in order to increase the probability of having detectable levels of pyrethroids in the dust. Data on the 55 dust samples that were analyzed are presented here. Data on urine concentrations will be reported in a future publication.

The MICASA study is a prospective cohort sample of 467 hired farm worker family households from Mendota, CA, designed to evaluate occupational and environmental exposures of significance for a farm worker population. Households were sampled from randomly selected census blocks and, following door-to-door enumeration, those households containing at least one hired farm worker were contacted for recruitment. Eligible participants in the MICASA study were men and women, residing in Mendota, CA, ages 18–55 years, self-identified as Mexican or Central American, and with at least one household member who worked in agriculture 45 days or more in the previous year, with both members of the household completing the interview . MICASA recruitment was conducted between January 2006 and May 2007. Recruitment for the home pyrethroid exposure study began in February of 2009, and sample collection took place between June and December of 2009. The analysis highlighted in this paper was designed to look at levels of pyrethroid pesticides in the homes of the MICASA study population. Because children typically have higher levels of exposure to pesticides , we restricted eligibility to those MICASA families with at least one child aged 7 or under at the time of recruitment in order to better understand pyrethroid sources in this potentially highly exposed population. Among the MICASA households completing baseline interviews, 175 were eligible for participation in the home pyrethroid exposure study. Eligible households were listed in random order for contact. One hundred twenty seven households were contacted for recruitment before reaching our goal of 105 households who agreed to participate and were enrolled in the study. The remaining 22 households either could not be contacted or declined to participate. If a family had multiple eligible children, one child was randomly selected and enrolled. At the time of sample collection, children ranged from 2 to 8 years of age. Written informed consent was obtained from each participant. Each study component was described verbally and in writing to the participant prior to obtaining written informed consent. Spanish was the primary language of the participants,farm shelving thus the study description and written informed consent were provided in Spanish. All study procedures were approved by the University of California, Davis, Institutional Review Board. Dust samples were collected and questionnaires were conducted between June and December of 2009. Dust samples were collected in the main living area of the home, which was defined as the most frequently used room in the house that was not a bedroom or kitchen. Dust samples were collected using a Eureka Mighty-Mite vacuum cleaner and standard crevice tool attachment modified to collect dust into a 19 × 90 mm cellulose extraction thimble that was secured to the crevice tool using a rubber O-ring. More detailed information on collection methods using the Eureka Mighty Mite have been described elsewhere . The square footage of the main living area was measured and recorded as well as the temperature and humidity. Dust was collected over the equivalent of the entire measured floor area.

Once sampling was complete, the thimble was removed from the Mighty Mite, wrapped in cleaned foil, weighed, placed in a polyethylene zip-top bag and labeled with the household ID number. Dust samples were then refrigerated at the MICASA field office for generally less than one day and delivered on ice to UC Davis, where they were stored in a −20 °C freezer until sample extraction and analysis. All Mighty-Mite equipment was cleaned using a 1% solution of detergent and hot water and allowed to air-dry between home visits in order to prevent cross-contamination. At the time of sample collection, a questionnaire was administered to the mothers. We obtained the frequency of pesticide use in both the hot and cold season of the previous year, including sprays, foggers, sticky traps, bait traps, gels, and any application by professional exterminators. Participants were asked if anyone living in the home had seen rodents, rodent feces, live or dead roaches, roach feces or ants inside the home at any time in the last year, with answer options including: large amounts, moderate amounts, none or don’t know. On the day of dust collection a staff member conducted a pesticide inventory in which detailed information on all pesticide products in the home was recorded, this included the name of each product, the size of the product container, the EPA registration number and all active ingredients. Summary statistics for the pyrethroid data were calculated. For concentrations below the limit of detection , an imputed value was assigned equal to the LOD divided by the square root of 2 . A Spearman rank-order correlation procedure was used to determine the intra-household correlations between particular pyrethroid concentrations, with significance set at p < 0.05. A Spearman rank-order correlation procedure and 95% confidence intervals were used to evaluate associations between interview questionnaire variables and the presence of pyrethroid pesticides in the house dust, with significance set at p < 0.05. As part of the main MICASA study questions on pesticide use were asked of the full cohort of 436 households in both an interview conducted from January 2006 to May 2007 and an interview conducted from February 2009 to June 2010. These questions were asked of both the male and female heads of household. Responses to these questions allowed us to look at the consistency of reporting pesticide use among family members as well as the consistency of reporting pesticide use over time. In both interviews the male and female heads of household were asked separately if either they or anyone in the household uses indoor and/or outdoor pesticide sprays. The consistency of responses to these pesticide use questions between the men and women from the same household was assessed using Cohen’s kappa, a measure of chance-corrected agreement . Temporal comparisons from the same participant between the two interviews conducted approximately 3 years apart were also made using Cohen’s kappa. All statistical analyses were performed using SAS version 9.2 . We assessed the levels of pyrethroid pesticides in 55 homes in a farm worker population by laboratory measurements of permethrin, cypermethrin, resmethrin, esfenvalerate and deltamethrin in house dust samples and by questionnaire data. This population had a relatively low educational level, with less than half of the participants reporting a 6th grade education or higher, in contrast to the 85% of U.S. adults who have a high school diploma . Detectable levels of the common pyrethroids permethrin, cypermethrin, deltamethrin, esfenvalerate and resmethrin were found in the dust samples collected in this study.