Abdul Rahman, Muhamad Faiz
(2018)
Understanding crop domestication : responses of lettuce roots under differential phosphorus conditions.
PhD thesis, University of Nottingham Malaysia Campus.
Abstract
Root system architecture (RSA) is a dynamic system of root network capable of adapting to changes in soil environment such as decline in soil moisture and nutrient deficiency. RSA responses to different nutrient concentration levels provide a unique system to study interactions between plants and their soil environment and the effect of different nutrient levels on root development. Wild (Lactuca serriola) and domesticated (Lactuca sativa) lettuce root systems have contrasting RSA and therefore populations developed from these two contrasting parents provide unique resources to explore root traits between cultivated and domesticated crop species.
Wild lettuce has a deeper root system with capability to exploit deeper soil horizons for nutrients and water while the domesticated lettuce possesses a shallow root system capable of acquiring resources mostly from the topsoil. Although there are clear RSA differences between wild and domesticated lettuce grown under normal soil condition, an understanding of the effect of different phosphorus (P) levels on RSA is lacking. P is one of the most important macronutrients for most crops after nitrogen, especially used as one of the building blocks of nucleic acid, phospholipids and many metabolites. Furthermore, P is often immobilised in the soil, therefore understanding the optimal uptake of P through RSA is important.
The present study aims to provide a better understanding of the effect of crop domestication on root traits by evaluating lettuce RSA, specifically the contrasting features of wild and domesticated lettuce, in response to a wide array of P levels. Specifically, the root systems of lettuce parental lines, wild lettuce (Lactuca serriola acc. UC96US23) and domesticated lettuce (Lactuca sativa cv. Salinas) were evaluated using agar-based and paper-based root phenotyping methods. This was followed by the QTL analysis of the lettuce parental lines and an recombinant inbred lines (RIL) mapping population derived from the cross of the two lettuce parental lines. The present study also explored the use of x-ray microcomputed tomography (µCT) to visualise the undisturbed lettuce RSA in 3D.
The agar-based root phenotyping method utilised vertical agar-filled petri dishes at five different P levels (0, 6, 312, 625 and 1250 µM P) and images of the roots were obtained through a flatbed scanner and analysed in silico. Seven RSA traits showed significant difference (P≤0.049) between lettuce parental genotypes × P levels interaction. The subsequent multiple comparison tests implied that the wild lettuce showed significant enhanced primary root (PR) growth (P<0.001) while domesticated lettuce significant showed enhanced lateral roots (LR) formation (P<0.001), especially at very low and high P levels.
The paper-based root phenotyping method utilised vertical paper pouches and images were obtained through simple DSLR camera setup, and then analysed in silico. The results showed significant mean differences (P≤0.006) between the parental genotypes in most of the measured traits. The trait means of domesticated lettuce were consistently higher than the wild. Additionally, the correlation tests revealed strongest significant correlation (r ≥ 0.82, P<0.001) of similar trait classes (i.e. lateral-lateral, primary-primary and global-global root traits), suggesting similar growth mechanisms between highly related traits.
The confirmation of significant genotypic differences in previous experiments led to the QTL mapping of the traits using an F8 RIL mapping population. From multiple QTL mapping (MQM) analysis, six QTLs and a putative QTL were obtained, mostly clustered in a hotspot in linkage group (LG) 1. The traits were mainly of the primary and global root traits. The primary root length (PRL) in this hotspot was driven by wild lettuce, which may imply association of domestication QTL in lettuce rooting depth as opposed to interval mapping (IM) or multiple QTL mapping (MQM) analysis. Using a non-parametric Kruskal-Wallis (KW) QTL analysis, 48 QTLs were identified, in which some clustered at hotspots (i.e. LG1, LG4, LG5 and LG8) dominated by lateral root traits. These clusters of trait loci may imply similar mechanisms control similar growth-related traits.
The overall differences seen between wild and domesticated lettuce RSA have provided an understanding of the effects of domestication on RSA traits. The present study showed some deviation in P adaptation between the parental lines, suggesting the novel domestication QTL identified particularly in LG1 which relates to the PRL. The outcome of this study could potentially be applied in identifying RSA traits that should be maintained or selected in other species, particularly the underutilised crops, during improvement process. Development of improved varieties with superior root traits such as deep rooting system, may pave the way for more sustainable agricultural practice thereby reducing dependency of crops on inputs such as chemical fertilisers and excess water.
The results obtained from the present study were obtained from 2D images, which may not entirely represent the 3D architecture of the roots in the soil. The utilisation of x-ray microcomputed tomography (µCT) in visualising the lettuce RSA in a preliminary study have shown interesting ‘umbrella-shaped’ root architecture, which cannot be clearly identified in experiments using 2D images. The study could be potentially expanded to explore more traits, especially using 3D-related root traits, to better understand the lettuce RSA, particularly responses towards different P levels.
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