DOI: 10.25338/b8tw2n
title: Effect of Drought Stress on the Genetic Architecture of Photosynthate Allocation and Remobilization in Pods of Common Bean (Phaseolus vulgaris L.), a Key Species for Food Security

description: Background: Common bean is the most important staple grain legume for
 direct human consumption and nutrition. It complements major sources of
 carbohydrates, including cereals, root crop, or plantain, as a source of
 dietary proteins. It is also a significant source of vitamins and minerals
 like iron and zinc. To fully play its nutritional role, however, its
 robustness against stresses needs to be strengthened. Foremost among these
 is drought, which commonly affects its productivity and seed quality.
 Previous studies have shown that photosynthate remobilization and
 partitioning is one of the main mechanisms of drought tolerance and
 overall productivity in common bean. Results: In this study, we sought to
 determine the inheritance of pod harvest index (PHI), a measure of the
 partitioning of pod biomass to seed biomass, relative to that of grain
 yield. We evaluated a recombinant inbred population of the cross of ICA
 Bunsi and SXB405, both from the Mesoamerican gene pool, to determine the
 effects of intermittent and terminal drought stresses on the genetic
 architecture of photosynthate allocation and remobilization in pods of
 common bean. The population was grown for two seasons, under well-watered
 conditions and terminal and intermittent drought stress in one year, and
 well-watered conditions and terminal drought stress in the second year.
 There was a significant effect of the water regime and year on all the
 traits, at both the phenotypic and QTL levels. We found nine QTLs for pod
 harvest index, including a major (17% of variation explained), stable QTL
 on linkage group Pv07. We also found eight QTLs for yield, three of which
 clustered with PHI QTLs, underscoring the importance of photosynthate
 remobilization in productivity. We also found evidence for substantial
 epistasis, explaining a considerable part of the variation for yield and
 PHI. Conclusion: Our results highlight the genetic relationship between
 PHI and yield and confirm the role of PHI in selection of both additive
 and epistatic effects controlling drought tolerance. These results are a
 key component to strengthen the robustness of common bean against drought
 stresses.
description: Population
 development To investigate the genetic basis
 of PHI [defined as seed dry weight per pod at harvest or whole-seed weight
 (WSW) over average dry pod weight, including seeds, at harvest or
 whole-pod weight (WPW)] and other yield-related traits, a bi-parental
 population was evaluated consisting of 226 F<sub>9</sub>
 Recombinant Inbred Lines (RILs) from the cross of ICA Bunsi and SXB405,
 developed at the Centro Internacional de Agricultura Tropical (CIAT),
 Colombia. This population was selected for a QTL analysis because a
 previous study using a subset of this population (78 lines,
 F<sub>4:6</sub>) found large phenotypic variation and
 transgressive segregation for PHI, as well as relatively high heritability
 and correlation with yield [27]. This study provides a more extensive
 analysis on the entire population; it also combines phenotypic and
 genotypic data, leading to an as yet unperformed QTL analysis in this
 population. ICA Bunsi is a
 navy type bean developed in 1968 by the Instituto Colombiano Agropecuario
 (ICA) from a cross between Magdalena 8 and Japón 3 [66]. It has been used
 extensively in Canadian breeding programs [67], due to its high
 productivity and resistance to white mold (<i>Sclerotinia
 sclerotiorum</i>) [68,69]. ICA Bunsi carries the
 <i>I</i> gene, which confers Bean Common Mosaic Virus
 resistance [70].  SXB405 is a cream-colored type experimental line
 developed at CIAT from a four-way cross (A 686/A 774//NXB 80/SEA 15) and
 selected for its high productivity under drought and common bacterial
 blight (<i>Xanthomonas axonopodis </i>pv.
 <i>phaseoli</i>) resistance [27,71]. Both genotypes belong to
 the Mesoamerican gene pool, are photoperiod-neutral, and have a type II
 (indeterminate, bush) growth habit [27]. Trial design Field experiments were carried
 out at the Plant Sciences Farm at the University of California, Davis
 (38.53 °N, 121.78 °W) for a total of five environments in two years and
 three water regime treatments. The soil type of the site belongs to the
 Yolo series, a member of fine-silty loam, mixed, nonacid, thermic Mollic
 Xerofluvents, considered well-drained, with slow to medium
 runoff and moderate permeability
 (https://soilseries.sc.egov.usda.gov). The seeding was carried out on the
 5<sup>th</sup> of June in 2013 and 8<sup>th</sup>
 of June in 2014. The plants were harvested on the
 10<sup>th</sup> of September in 2013 and
 12<sup>th</sup> of September in 2014. All water was provided
 by irrigation as there were no rain events during the experiments. In
 2013, there were three irrigation treatments: terminal drought (TD),
 intermittent drought (ID), and full irrigation (well-watered). The full
 irrigation treatment received four irrigations while the intermittent
 drought received the first and third irrigation, and the terminal drought
 received the first and second irrigation. Only terminal drought and full
 irrigation treatments were carried out in 2014, receiving 4 irrigations
 for the full irrigation treatment, and the first two irrigations for
 terminal drought. The second irrigation of the terminal drought was
 applied during early flowering. The timing of the irrigation was decided
 according to weather and evapotranspiration (Fig. 4). The agricultural
 management was according to standard practices [72]. The parental lines
 and 226 of the RIL lines were planted in a split plot design with 3 blocks
 and one repetition per block. The experimental unit was a plot of 60
 plants grown in two 3 m-long rows and 76 cm between rows (density of
 131,578 plants per hectare). Phenotyping From each plot, 15 to 20 pods
 were collected at random across the plot during harvest and dried for 5
 days at 50<sup>O</sup>C. Seed and pod number and pod mass were
 recorded. The seeds and pod walls were weighed separately. Pod harvest
 index was calculated as the proportion of seed biomass to the overall pod
 biomass (sum of pod wall + seed biomass) [17]. Days to flowering (DTF) was
 assessed when at least 50% of the plants in the plot had an open flower.
 Agronomic yield was weighed after four days of drying at
 40<sup>0</sup>C to standardize grain water content. Yield was
 not measured on the intermittent drought treatment in 2013 as the plots
 were mixed after cutting due to unexpectedly high winds.
description: There are two worksheets in this dataset. Worksheet A provides
 segregation data for phenotypic traits in the ICA Bunsi x SXB405
 recombinant inbred population. Values are averages  across replications.
 C: control; ID: Intermittent drought; TD: Terminal drought. DTF: Days to
 flowering; PWW: Pod whole-weight; WSW: whole-seed weight; WPW: Whole-pod
 weight; SW100: 100-seed weight; SPP: Seeds per pod; PHI: Pod harvest
 index; YLD: yield.. Values are averages over replicates.
 Worksheet B provides SNP segregation in the recombinant inbred
 population ICA Bunsi x SXH405. A: ICA-Bunsi allele; B: SXB 405
 allele. Missing data are shown with a dash.