As aged kiwifruit trees often reduce the fruit productivity and quality, those aged kiwifruits are replaced with juvenile vigorous plants. However, the productivity and quality of the replaced kiwifruits remain relatively low. In the present research, autotoxicity and allelopathy of kiwifruit roots were evaluated. Aqueous methanol extracts of kiwifruit roots inhibited the growth of cress, lettuce, alfalfa, Lolium multiflorum, Phleum pretense and Echinochloa crus-galli. The extracts of kiwifruit roots also showed an inhibitory effect on the growth of kiwifruits themselves. These results suggest that kiwifruit roots may contain allelopathic and autotoxic substances. The accumulation of autotoxic substances in orchard soil may occur by continuous exudation of autotoxic substances from kiwifruit roots over the long-term cultivations, and by liberation of the substances from the root residues. Accumulated autotoxic substances may suppress the fruit productivity and quality of the replaced kiwifruits. Therefore, autotoxicity may be involved in the replant problem of kiwifruits.

Root growth is under constant dynamic regulation for optimal response to developmental and environmental stimuli. At the posttranslational level, protein abundance is controlled by proteasomal degradation of targeted proteins. The substrate-specificity of this process is exerted by F-box proteins taking part in the SCFs (SKP1-CULLIN-F-box protein ligase) E3 ubiquitin protein ligases. In this work an Arabidopsis thaliana AtF-box, which regulates leucine homeostasis, was analyzed in the context of root development. Publicly available data sets and reporter lines revealed AtF-box expression in the primary and lateral roots. Aberrant stem cell divisions were detected in the distal stem cells (DSC) of the AtF-box knockdown lines (AtF-box^amiRNA), suggesting that AtF-box is required for the optimal cell division. Microscopic observations revealed the premature exit from cell proliferation and slower cell division activity. Conversely, in AtF-box overexpression (AtF-box^OE) lines the cell division phase was prolonged. The root growth rate was respectively reduced and enhanced in the AtF-box^amiRNA and AtF-box^OE lines compared to the control. From the results of these studies, we concluded that the AtF-box gene is important for the fine-tuning of root growth.

Surface humic soils, where fine roots are mainly distributed, can be accidentally buried due to coverage by deposits such as volcanic ash. This buried humic soil may influence the vertical distribution of fine roots because soil organic matter strongly affects soil functions. However, fine root distributions in buried humic soils are little understood. In order to elucidate the effects of buried humic soils on fine root distribution, we investigated fine root biomass and soil characteristics in a soil profile down to 3.3 m with two buried humic soils formed by tephra in Tomakomai, Hokkaido, Japan. In this profile, fine root biomass decreased with soil depth, but increased in buried humic soils that had higher soil total carbon (C) content and higher fine soil ratio than buried nonhumic soils. These results lead us to surmise a preferential development of active fine roots in buried humic soils rich in organic C rather than nonhumic soils.

A buttress is a prominent lateral-vertical structures of surface roots of a canopy or emergent trees frequently observed in tropical forests. Buttresses are probably formed to enhance mechanical stability of tree trunks and/or promote nutrient acquisition. However, the morphological diversity and control of buttress development remain unclear. Therefore, we aimed to clarify variations in buttress development related to prevailing wind and declination of tree trunks in warm temperate forests of Japan. We chose two pairs of forest sites with similar precipitation and geographical locations, but with contrasting wind regimes. Buttresses of two Castanopsis species, C. sieboldii and C. cuspidata, were assessed, and the size and direction of the most developed buttress (MDB) were measured for each individual. The average MDB height at the stem of trees at the strong wind site, Sumoto, was less than half of that at the control site, Himeji. However, the result was the opposite at the other strong wind site, Muroto, and control site, Kochi. The average MDB length did not differ between the strong wind and control sites. MDBs were formed corresponding to the direction of the most frequent wind at strong wind sites but not at control sites. The direction of growth of MDBs was almost the same as that at slopes at all the sites. Our results suggest that wind loading likely influences the development of buttresses; however, its effect could also be site-dependent. These results suggest the potential roles of buttress formation on mechanical stability of trees, which have not been appreciated in temperate forests.

Root form and function is key to the productivity of natural and agricultural ecosystems. The growth and development of roots is highly adaptable to the environment. Studies show that species differ in their root system adaptation and function in response to high CO₂, which may lead to changes in global species composition in the future. However, knowledge of genetic variation and molecular responses in roots to elevated CO₂ is still lacking. This review examines the effects of elevated CO₂ on root system and offers perspectives for future research.

The radial oxygen loss (ROL) barrier formed on the outer cell layers of roots of rice (Oryza sativa) contributes to efficient oxygen transfer through the aerenchyma from the aerial parts to the root apex. It is hypothesized that suberin accumulation in the exodermis contributes to an apoplastic barrier and plays a vital role in ROL barrier formation. A previous study reported that some genes encoding cytochrome P450 family 86 (CYP86) and glycerol-3-phosphate acyltransferase (GPAT) might be involved in suberin biosynthesis during ROL barrier development of rice roots. However, how these genes are expressed and their contribution to the sequential development of suberin accumulation in rice roots remains unclear. In this study, four CYP86 and five GPAT genes of rice were identified as candidate genes involved in suberin biosynthesis in roots using sequence homology alignment with Arabidopsis suberin and cutin biosynthesis genes. Gene expression analyses revealed that expression of the candidate genes was induced at the region where suberin biosynthesis occurred under stagnant deoxygenated conditions. These genes showed two types of spatiotemporal expression patterns, at the regions of 5–25 mm and 25–35 mm from the root apex. Tissue-specific expression analyses using laser microdissection and histochemical GUS staining revealed that candidate gene expression was similar in the cell layer of the root exodermis. These results suggest that the selected CYP86 and GPAT genes are involved in suberin biosynthesis in the exodermis, and that suberin biosynthesis in the root may be controlled by the spatiotemporal expression of two groups of genes.

Water stress such as drought and waterlogging is considered to be a major limiting factor in crop production. Roots play important roles in crop adaptation to water stress. This study aimed to characterize the vertical root distribution patterns and analyze the root-shoot relationships of different cereal species with different water requirements in response to different soil moisture conditions. Sorghum, maize, and rice were grown under 5% w/w soil moisture content (SMC5), 20% w/w soil moisture content (SMC20) and in waterlogged soil (WL) for 35 days using root box pin-board method. For sorghum and maize, the optimal soil water condition was SMC20 which produced the greatest shoot and root growth, while rice had greatest shoot and root growth under WL. Sorghum significantly increased root to shoot ratio in both water stress conditions, suggesting that sorghum prioritizes carbon partitioning of assimilates towards the roots. Although whole root dry weight and total root length were reduced by water stress, vertical distribution of root traits varied with soil water conditions and promoted root response was observed in specific soil layer. A highly positive relationship between root and shoot traits was observed in rice, suggesting that root and shoot trait responses are coupled with changing soil water conditions. Further studies are needed to confirm root architectural changes focusing on different root component traits as well as other root traits related to root architectural structure.

The fine root systems of ericaceous plants possess both absorptive fibrous roots and skeletal pioneer roots. This functional differentiation (heterorhizy) can be coupled with a specific mycorrhization pattern, in which fibrous roots form more ericoid mycorrhizas than pioneer roots. However, the behaviors of root-associated fungi on the different individual roots remain largely unknown. We investigated the colonization patterns and root modifications of Leohumicola verrucosa, Oidiodendron maius, and Leptobacillium leptobactrum strains on Vaccinium oldhamii hypocotyl cuttings in vitro. Three putative and true mycorrhizal fungal strains (one L. verrucosa and two O. maius) exhibited typical heterorhizic colonization patterns and tended to increase pioneer root branching. However, a root endophytic fungus (L. leptobactrum) uniformly exhibited a lower colonization across the different roots and did not increase the number of pioneer root branches. The colonization patterns and root modification ability of mycorrhizal fungi may be linked, thereby ensuring sufficient colonization sites.