P7 - Rhizo Transport
Nutrient and water transporters actively shape spatiotemporal rhizosphere organization processes in maize

The role of maize radicle root hairs in seedling establishment under adverse phosphorus and water seedbed conditions
Germination constitutes a very critical growth phase during crop development. A vigorous seedling establishment in this stage will decide over the overall plant performance, stress resistance and yield formation. A vigorous seedling establishment highly depends on a sophisticated not yet understood rhizosphere organization ensuring water and nutrient uptake as well as soil anchorage. Shortly after the embryonic root penetrates the coleorhiza and is entering the soil environment, the radicle is characteristically coated in a bloomy manner by numerous root hairs. Surprisingly, their biological role is yet scarcely understood. We hypothesize that radicle root hairs represent major drivers for water and nutrient uptake, particularly under adverse nutrient and water seedbed conditions, and trigger seedling-establishment-efficiency in suboptimal agricultural sites. Thus, the main goal of this PP2089 project is to identify and characterize the role of maize root hair properties in rhizosphere organizational processes, ensuring maize seedling establishment until five days after shoot emergence under single or combined water and phosphorus (P) deficit and as a function of soil texture, in line with the overall PP2089 objectives. During germination, very defined interlinked feedback loops between nutrient and water availability, exudation processes, microbes and soil properties exist. Capturing this dynamic plasticity in its full “entirety” is experimentally feasible when focusing on radicles. To achieve our aims, growth rates, shoot biomass increase, micro- and macronutrient uptake, apoplastic barrier traits, root system architecture development and root hair characteristics will be quantified in maize wild type and mutant plants lacking proper root hairs. Data will be collected in soil column time-series experiments exploiting plants germinating under optimal or adverse water and/or P-limiting seedbed conditions in two different soils (loamy and sandy soil) and one white-peat-based substrate. Complementary, water and nutrient transporter gene expression will be determined to unravel the mutual impact of water uptake on nutrient uptake and vice versa, also at the molecular level. RNA-sequencing of root hair versus radicle transcriptomes are targeted to identify signalling pathways as well as gene expression patterns and networks driving, soil property-dependent, the responses to water and/or P availability, advancing our understanding on rhizosphere processes which are allowing plants to adapt to adverse seedbed conditions. In summary, this project will unravel the causative interplay between molecular and morphological root hair traits, soil characteristics and water and nutrient availability in the rhizosphere and therewith the role of root hairs in ensuring the hydromineral nutrition of young seedlings during germination in adverse seedbeds.
Outcome
At 5 days after maize coleoptile emergence (5 DAE), numerous radicle root hairs are present in soil-grown plants. To assess potential roles of these root hairs in adverse seedbed conditions, we compared physiological and molecular responses of wild-type maize B73 (WT) and its roothairless3 mutant (rth3) under phosphorus (P) and water-reduced conditions. Reflecting a physiological reaction to the suboptimal hydromineral status, WT plants developed a significantly shorter total root length and elongated root hairs under water-reduced conditions compared to control and P-depleted growth conditions. Root system architecture parameters of rth3 did neither react to water- nor P-reduced conditions and its total root system length was similar to the water- and/or P-depleted reduced-size root system of the WT. Only the root diameter of rth3 was treatment-independently increased compared to the WT. While the root systems of both genotypes significantly differed in all tested growth conditions already 5 DAE, surprisingly, the biomass, height, total water content and P content of WT and rth3 shoots did not differ from each other, regardless of the seedbed condition. This may suggest that molecular root traits driven by an altered gene expression adapt to various seedbed conditions allowing a vigorous seedling establishment. However, neither non-targeted RNA-seq analysis of root hairs (WT) and primary roots (WT, rth3) nor targeted qPCR analysis identified differential gene expression patterns of known P-transporters (Phts) and water transporters (PIP/TIP) independent of the water and P seedbed condition. Currently, an in-depth analyses of differential expressed gene patterns are performed to unravel transport-unrelated transcriptional responses between WT and rth3 roots and to understand roles of root hairs in WT maize in adverse seedbed conditions.In summary, all obtained results on shoot performance parameters suggest that the presence of radical root hairs do not represent an advantage for early seedling establishment neither under water- nor P-reduced conditions though comparisons of WT and root hair-less rth3 plants may be biased by secondary effects, such as an overall reduced root growth in rth3 plants. Whether the smaller and hydromineral non-responsive root system of rth3 is a result of the lacking root hairs or not is being investigated at the moment.
Link to English scientific abstract
Link to German scientific abstract