The specific rhizosheath weight is given by the ratio between the rhizosheath’s dry mass (root-adhering soil, RAS) and root tissue (RT, RAS/RT, g g
−1) after being dried at 65 °C
[51,55,82][64][69][71]. However, it can be also calculated as the ratio between the RAS and the total length of the root (RL, RAS/RL, mg cm
−1) for each plant. The specific rhizosheath weight allows us to estimate its size
[20].
5.3. Genetic Studies
Key genetic determinant studies associated with the expression of rhizosheath characteristics, focused on QTL mapping and gene identification, were conducted on several crop species, among which were cereals (e.g., wheat, pearl barley, millet, rice); others, such as lupine and tomato; and wild relatives
[29,44][29][44].
Delhaize et al.
[83][72] used multiparent whole-genome analysis (MPWGAIM) in wheat (
Triticum aestivum L.) populations grown on non-acid soils, and identified six QTLs located on chromosomes 2B, 4D, 5A, 5B, 6A and 7A. Some of them were probably linked to the basic helix–loop–helix (bHLHs) transcription factor family, influencing the root hair elongation in
Arabidopsis and rice and determining the rhizosheath extent. Others were located close to
Rht genes influencing the plant structure, such as the height or the root length. On the other hand, five QTLs located on different chromosomes (i.e., 1D, 3A, 3B, 6A2, 7B) contributed to rhizosheath size in wheat (
Triticum aestivum L.) grown in acid soil, with a notable improvement in phosphorus acquisition
[28]. George et al.
[36] found genomic regions in barley (
Hordeum vulgare L.) that were significantly associated with rhizosheath weight on the chromosome 2H. They contained a glutamate receptor and several putative candidate genes which modulate the root system development in rice and
Arabidopsis in abiotic stress conditions, such as cold and drought, or during the early and delicate plant growth stages. Drought treatment on foxtail millet (
Setaria italica) increased the expression of five root-hair-elongation-associated genes (Seita.3G196500, Seita.2G057800, Seita.9G333500, Seita. 8G104600, Seita.7G190800). This was revealed by qRT-PCR analysis demostrating the development of a larger root hairs as appendage to which soil particles bond
[42].
5.4. Microbial Investigations
The advances in genomic sequencing methods developed during the last decades has allowed us to overcome the lack of information related to non-readily culturable microbes from several environments
[85][73]. Through these revolutions, technological applications investigating the composition and the physiology of the rhizobiome can shed light on its key role in the soil ecosystem, unraveling the vital mutualistic interactions among soil, roots and microbes
[86][74]. Recent studies have suggested that rhizosheath-associated microbial communities are pivotal in its building processes, as well as in the plant-growth-promoting services provided by root–bacteria relationships. A rhizosheath was defined as an edaphic “mini-oasis” in arid habitats, where several microbial taxa presenting high functional redundancy are in competition to conquer ecological niches that support the beneficial functions of plant biofertilization, biopromotion and bioprotection
[79][75]. Root-associated microorganisms can implement several strategies aimed at improving essential nutrient availability; producing biostimulants (exopolysaccharides, phytohormones, volatile compounds, etc.) for plant growth; and at mitigating abiotic/biotic stresses affecting plants
[81][76].
Several studies based on bacterial and fungal strain inoculation techniques have demonstrated their particular role in root system development, as well as in rhizosheath evolution. Chen et al.
[67][58], in the rhizosheath of
Kengyilia hirsute, individuated the enrichment of
Massilia and
Arthrobacter species that are likely related to plant molecular mechanisms for specific taxa selection and accumulation involved in rhizosheath formation.
Trichoderma harzianum T-22 increased the rhizosheath amount in several ancient and modern wheat varieties, affecting their root systems’ architectures differently
[51][69]. The endophytic fungus
Piriformospora indica is able to modulate auxin production under moderate soil drying, enhancing the growth of rice root hairs for soil exploration by seeking water and providing a more suitable physical structure for the formation of the rhizosheath. This leads to an enrichment of
Bacillus cereus in both the rhizosphere and the rhizosheath, suggesting a strong bacteria–fungi interaction involved in the exudate compounds use
[52][68].