Most of the preliminary studies on Xerobranching were performed in crop plants using low-throughput soil assays and X-ray CT imaging. Therefore, I first designed ‘Agar-Based Air-gap Assay’ (AAA) to study XB responses in model plant, Arabidopsis. This allowed me to take best advantage of the wealth of genetic resources available in Arabidopsis and proved instrumental in dissecting the molecular mechanisms underlying XB. With the help of the agar-based bioassay, I endeavoured to address the proposed key questions:
1. Do roots exhibit Xerobranching in response to ABA-dependent changes in SUMOylation?
To determine whether Xerobranching is dependent on ABA, I first examined the XB responses of ABA biosynthetic/signalling mutants. Strikingly, unlike wild-type (WT) the Arabidopsis ABA biosynthetic mutant, aba2-1 produced lateral roots in air-gaps. The mutant phenotype was rescued by expressing the wildtype ABA2 gene using the phloem-expressed SUC2 promoter. This result revealed the importance of ABA originating from the phloem, which is also the source of water a root needs to maintain growth in drying soil (Mehra et al., 2022 Science). I further established that that ABA-dependent XB response pathway acts independently of SUMOylation-dependent ABA response pathway.
2. Does ABA trigger SUMOylation of auxin response factors during Xerobranching?
The hormone auxin regulates transcription of many auxin-responsive genes dependent on ‘Aux/IAA-ARF’ signalling modules. The host laboratory discovered another water-related root adaptive trait termed ‘hydropatterning’ which favours lateral root positioning towards moist agar/soil patches. This phenomenon is regulated by differential SUMOylation of ARF7 on dry vs moist sides of a root. SUMOylation blocks ARF7 activity on the dry side of a root, thereby suppressing expression of its downstream target (e.g. LBD16), resulting in no LR formation. To validate the functional importance of SUMOylation during Xerobranching, I examined XB response of Arabidopsis mutants of several SUMO/DeSUMOylation components, some of which also display hydropatterning defects. However, like WT none of these mutants formed lateral roots in air gap suggesting XB response is independent of SUMO/DeSUMOylation processes.
3. Uncovering Xerobranching’s impact on soil exploration
The Xerobranching response ensures roots only branch when in contact with moist soil. I found that besides monocot cereal crops, the response is also conserved in millets such as Setaria and dicot species such as tomato. Using X-ray CT, I observed that tomato mutants impaired in ABA biosynthesis continue to branch in air gaps in soil. This confirmed the role of ABA in regulating XB in crop plants as well. To discover phenotypic variation in this adaptive trait, a diversity population of >300 Setaria accessions was screened for their branching response. The generated phenotypic datasets will be next utilized to perform GWAS analysis to discover the underlying candidate genes/alleles involved in regulation of Xerobranching as well as other root traits in Setaria.
The project outcomes were disseminated through conferences, press releases and via media platforms:
2023 Concurrent session talk at IPGSA, South Korea
2022 Plenary session talk at ICAR, Belfast
Press release:
https://www.nottingham.ac.uk/news/research-reveals-plant-roots-change-shape-and-branch-out-for-water#(odnośnik otworzy się w nowym oknie)Media highlights: The amazing system plants use to shape their roots and why it could help protect crops from climate change. The Conversation
