Abstract
BackgroundThe parasitic plant Orobanche cumana is one of the most important threats to sunflower crops in Europe. Resistant sunflower varieties have been developed, but new O. cumana races have evolved and have overcome introgressed resistance genes, leading to the recurrent need for new resistance methods. Screening for resistance requires the phenotyping of thousands of sunflower plants to various O. cumana races. Most phenotyping experiments have been performed in fields at the later stage of the interaction, requiring time and space. A rapid phenotyping screening method under controlled conditions would need less space and would allow screening for resistance of many sunflower genotypes. Our study proposes a phenotyping tool for the sunflower/O. cumana interaction under controlled conditions through image analysis for broomrape tubercle analysis at early stages of the interaction.ResultsWe optimized the phenotyping of sunflower/O. cumana interactions by using rhizotrons (transparent Plexiglas boxes) in a growth chamber to control culture conditions and Orobanche inoculum. We used a Raspberry Pi computer with a picamera for acquiring images of inoculated sunflower roots 3 weeks post inoculation. We set up a macro using ImageJ free software for the automatic counting of the number of tubercles. This phenotyping tool was named RhizOSun. We evaluated five sunflower genotypes inoculated with two O. cumana races and showed that automatic counting of the number of tubercles using RhizOSun was highly correlated with manual time-consuming counting and could be efficiently used for screening sunflower genotypes at the tubercle stage.ConclusionThis method is rapid, accurate and low-cost. It allows rapid imaging of numerous rhizotrons over time, and it enables image tracking of all the data with time kinetics. This paves the way toward automatization of phenotyping in rhizotrons that could be used for other root phenotyping, such as symbiotic nodules on legumes.
Highlights
Sunflower (Helianthus annuus) is one of the most important crops for oil production worldwide, and sunflowercultivated areas have increased in recent decades [1], notably due to the use of hybrid varieties with improved yield, oil content and oil quality
Tubercle observation of sunflower plants inoculated with O. cumana grown in rhizotrons O. cumana-inoculated sunflower plantlets were grown in rhizotrons consisting of transparent Plexiglas boxes containing a layer of rockwool and glass fiber paper watered with nutrient solution (Additional file 1: 1a–c, e)
In contrast to phenotyping in the field, the use of rhizotrons for growing O. cumana-inoculated sunflower allows the observation of early stages of the interaction between the parasitic plant and its host from the germination induction of O. cumana seeds to the tubercle stage
Summary
Sunflower (Helianthus annuus) is one of the most important crops for oil production worldwide, and sunflowercultivated areas have increased in recent decades [1], notably due to the use of hybrid varieties with improved yield, oil content and oil quality. O. cumana is an obligate parasitic organism of sunflower (H. annuus). The broomrape takes up water and nutrients from the host and forms storage organs, called tubercles, each of which subsequently develops a single flowering shoot that emerges above ground, forming thousands of tiny seeds within a few months. The parasitic plant Orobanche cumana is one of the most important threats to sunflower crops in Europe. Resistant sunflower varieties have been developed, but new O. cumana races have evolved and have overcome introgressed resistance genes, leading to the recurrent need for new resistance methods. Screening for resistance requires the phenotyping of thousands of sunflower plants to various O. cumana races. Our study proposes a phenotyping tool for the sunflower/O. cumana interaction under controlled conditions through image analysis for broomrape tubercle analysis at early stages of the interaction
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