Browsing by Subject "Root and rhizosphere"

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Dissecting the genetic basis of root- and rhizosphere-related phosphorususe efficiency in European elite maize (Zea mays L.) lines and landraces
(2021) Li, Xuelian; Ludewig, Uwe
In agriculture, farmers massively apply P fertilizer to maintain high yield. Due to the long-term high fertilization rates and long-term organic residue accumulation, the total P pool per hectare has increased between 1900 and 2020. Since modern varieties have often been selected in high-nutrient input conditions for high yields, concerns are being raised that the beneficial traits for P uptake under a limited P supply will gradually decline in elite varieties. Regarding to maize (Zea mays L.), thousands of varieties have been bred since it was domesticated as a food product. It is an open question whether traits and genes related to P deficiency in European maize have changed since the Green Revolution, the start of hybrid breeding and high-intensity fertilization. This is the core research question of this dissertation. Here I present the analysis of roots in response to P deficiency using a diverse panel of European maize genotypes via several experiments. In Chapter I, we focus on whether maize seedlings of the flint and dent heterotic pools vary in the P acquisition and utilization since the onset of hybrid breeding using 34 genotypes in mini-rhizotrons. These genotypes included 16 flint lines that were released over more than five decades ago, 7 doubled haploid lines from the flint landraces (DH_LR), 8 dent lines, and 3 hybrids. Seedling P use efficiency (PUE) and related traits were measured and compared at two P levels in a calcareous soil. In Chapter II, we compared the root exudated organic acids and mycorrhizal fungi colonization degree among 24 genotypes which have been evaluated in Chapter I. These genotypes included 16 flint lines, 6 DH_LR and 2 old dent lines. Seedling colonization with arbuscular mycorrhizal fungi (AMF) and organic acid anion release were measured. P-uptake-related root traits were compared under P-sufficient and P-deficient conditions. In Chapter III, using nearly isogenic maize lines, the B73 wild type and the rth3 root hairless mutant, we quantified the effect of root hairs and AMF infection in a calcareous soil under P deficiency. Wild-type root hairs extended the rhizosphere for acid phosphatase activity by 0.5 mm compared with the rth3 hairless mutant. Total root length of the wild type was longer than that of rth3 under P deficiency. Higher AMF colonization and mycorrhiza-induced phosphate transporter gene expression were identified in the mutant under P deficiency, but plant growth and P acquisition were similar between mutant and the wild type. The mycorrhizal dependency of maize was 33 % higher than the root hair dependency. Root hairs and AMF inoculation are two alternative ways to increase Pi acquisition under P deficiency, but these two strategies compete with each other. In Chapter IV again two nearly isogenic maize lines, the B73 wild type and the rth2 root hairless mutant, were used to address the importance of root hairs during drought and under P deficiency. The results indicate that drought and P deficiency synergistically impair maize growth; while P concentrations were little affected by the loss of root hairs, the P content was massively reduced at combined stress, showing that P deficiency is much more severe under drought. In Chapter V, we first compared the root traits response to low P and high P of six preselected genotypes in European flint in Chapter I. We then generated RNA libraries from the roots of these lines under both low P and high P. Using an expressed genes matrix, we conducted a Weighted Genomic Coexpression Network Analysis (WGCNA), and detected general low P-induced modules and modules that were higher in founder flints. The P deficiency-responsive metabolic processes common to all six genotypes included: (1) acceleration of carbon supply for organic acid synthesis through glycolysis and TCA cycle; (2) alteration of lipid metabolism; (3) changes of activity of transmembrane transporters; (4) carotenoid metabolism. Additionally, the founder flint line EP1, F2 and doubled haploid landrace SM1 have their specific strategies and mechanism to cope with low P. Our findings well support other studies with transcriptome, proteome and metabolome experiments in maize and other species, and point to molecular events involved in the efficient alleviation of P stress in efficient maize accessions. Altogether, this study presents informative analyses in how maize genotypes with distinct breeding history adapt to P deficiency in regard of root, rhizosphere traits and root transcription. It showed correlation between phenotypic traits and gene transcription, which is much more complex than previously reported. It also opened a novel insight into molecular regulation on Pi utilization, resulting in promotion of vegetative biomass in P deficiency. These findings will also provide precious knowledge for plant breeders and agronomists who work on P research in maize and other cereal crops.