Suboptimal water and nutrient availability are primary constraints in global agriculture. Root anatomy plays key roles in soil resource acquisition. In this article we summarize evidence that root anatomical phenotypes present opportunities for crop breeding. Root anatomical phenotypes influence soil resource acquisition by regulating the metabolic cost of soil exploration, exploitation of the rhizosphere, the penetration of hard soil domains, the axial and radial transport of water, and interactions with soil biota including mycorrhizal fungi, pathogens, insects, and the rhizosphere microbiome. For each of these topics we provide examples of anatomical phenotypes which merit attention as selection targets for crop improvement. Several cross-cutting issues are addressed including the importance of phenotypic plasticity, integrated phenotypes, C sequestration, in silico modeling, and novel methods to phenotype root anatomy including image analysis tools. An array of anatomical phenes have substantial importance for the acquisition of water and nutrients. Substantial phenotypic variation exists in crop germplasm. New tools and methods are making it easier to phenotype root anatomy, determine its genetic control, and understand its utility for plant fitness. Root anatomical phenotypes are underutilized yet attractive breeding targets for the development of the efficient, resilient crops urgently needed in global agriculture.

Root anatomy and soil resource capture

Significance of root hairs for plant performance under contrasting field conditions and water deficit.

The effects of vegetation traits and their stability functions in bio-engineered slopes: A perspective review

First discovered on desert plants by Volkens 1887, rhizosheath formation, i.e. soil aggregation at the root surface, is now considered as a very promising adaptive trait to deal with abiotic stress. Indeed, the rhizosheath could help plants better cope with water stress, nitrogen and phosphorus deficiencies, and soil acidity. We have reviewed studies on the biological factors involved in rhizosheath formation, the methods used to quantify it, and its importance in plant nutrition. Thus, we have collected recent evidence that shows that the rhizosheath is an important trait arising from the morphology and physiology of plant root system, and the cooperation between plant root and its associated microbiota. In particular, the transformation of root exudates by exopolysaccharide-producing bacteria effectively contributes to soil aggregation and thus to increases the volume of the rhizosheath (i.e. root-adhering soil), thereby improving the absorption of minerals and water by plants. The growing interest for this process has led to the genetic mapping of potential plant QTLs controlling this trait in order to provide new tools for the selection of plant varieties with improved tolerance to abiotic stresses. Finally, we discussed some important issues that need to be addressed in order to develop an appropriate selection strategy focused on the rhizosheath, such as the relationship between the genes controlling rhizosheath formation and those controlling other root traits, but also the impact of rhizosheath formation on soil carbon sequestration, a potential strategy for mitigating climate change.

The rhizosheath: from desert plants adaptation to crop breeding

The interactions between plant roots and the associated microbiota impact soil aggregation, water retention and plant nutrient availability. Thus, selection of plant genotypes that promote microbial species involved in rootadhering soil aggregation and rhizosheath formation could help improve yield sustainably. Here, we tested pearl millet genotypic variation in both root-adhering soil aggregation, microbiological and biochemical characteristic. Methods: A collection of 181 pearl millet inbred lines was phenotyped for their rhizosheath size, and thirteen contrasting genotypes were selected and grown under field conditions, and their root-adhering soil (RAS) was sampled. Microbial biomass, pH, mineral N content and six enzymatic activities involved in main nutrients cycles were analyzed, and metabarcoding of 16S rDNA and ITS were performed for bacterial and fungal diversity. Results: Enzymatic activities (chitinase, acid phosphomonoesterase, FDA-hydrolysis and β-glucosidase) were higher in RAS of larger rhizosheath lines than that of smaller rhizosheath one. Bacterial β-diversity showed a separation of the most contrasting lines in the principal coordinate analysis performed with the Bray-Curtis distance. Some bacteria from the Gaiellaceae and Sphingomonadaceae families and the Bradyrhizobium genus were associated with the large rhizosheath phenotype. Concerning the fungal community, we noticed a negative correlation between the specific richness and the rhizosheath size and Trichoderma genus was positively associated to the rhizosheath size. Conclusions: This study demonstrates that in pearl millet, rhizosheath size is related to soil nutrient dynamics and microbiota diversity. However, it also shows that other factors shape this trait and their relative importance must be determined.

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Pearl millet genotype impacts microbial diversity and enzymatic activities in relation to root-adhering soil aggregation

Rhizosheath is known as a layer of adhering soil particle to the root surface. Despite several speculations, the positive function of rhizosheath in acquisition of water and nutrients from drying soil has not yet been experimentally proven. The objective of this study was to experimentally show whether an enhanced rhizosheath formation could help plants to better access water from drying soil. Eight wheat cultivars were grown in a sandy-loam soil. When plants were 35 days old let dry soil to a water content at which evident wilting symptoms appeared on the plant leaves. During this drying cycle, soil water content and transpiration rate of plants were gravimetrically measured by weighing the plant pots. At the end of this drying cycle, the roots were excavated out of the soil and the rhizosheath formation was gravimetrically quantified by weighing the soil attached to the root system. The results showed that plant cultivars with greater rhizosheath formation could sustain higher transpiration rates at dry condition (water content of 0.07 cm3 cm−3) while the plant cultivars with lower rhizosheath formation suffered from drought stress and reached their permanent wilting points at the same water content. The findings of this study gathered evidence that under severe drought condition plant cultivars with an enhanced rhizosheath formation could better survive by sustaining their transpirational and nutritional demands.

Shirzad Abbaszadeh

Papers

The rhizosheath: a potential root trait helping plants to tolerate drought stress