Phosphorus (P) acquisition, a key factor in rice productivity, is related to morphological and anatomical root traits. In this study, we examined the root traits of rice that contribute to P acquisition under low P conditions using chromosomal segment substitution lines (CSSLs) grown under non-flooded conditions. Rice plants were grown under low- and high- P conditions in a growth chamber. We tested 39 CSSLs and the parent varieties 'Sasanishiki' and 'Habataki.' Four out of the five selected genotypes from 39 CSSLs had larger root surface areas than that of Sasanishiki due to long fine root and/or coarse roots. The root surface area and shoot P uptake were significantly and positively correlated. Two of the genotypes had higher P use efficiency and shoot dry weight compared to that of Sasanishiki under low P conditions, and nodal root cross-sectional areas were larger in the two genotypes than in Sasanishiki under low P conditions. The root cortical aerenchyma in the nodal roots was well developed in all observed genotypes, which may have reduced the metabolic cost. These results suggest that the thick nodal roots with aerenchyma in contribution to the increase in root surface area are the advantage for growth under low P levels in non-flooded conditions.

A rice root system consists of main roots, and lateral roots which account for a large portion of the total length and surface area of the whole root system, indicating that lateral roots greatly contribute to its hydraulic conductance. It is therefore worthwhile measuring aquaporin activities of lateral roots in comparison with main roots to evaluate their contribution to hydraulic conductivity. However, the sampling of fine lateral roots is time-consuming, and its method can be critical because of the difficulties of handling them for sampling. In addition, root has been reported to be sensitive to the surrounding environment and affected by destructive sampling procedures in which lateral roots are removed from main roots. Therefore, in this study, we attempted to establish a method for collecting samples of lateral and main roots for aquaporin analysis while minimizing the effect of separation of roots from the shoot, and the removable of lateral roots from main roots on the transcript abundance. We conducted three experiments and measured the expression levels of four aquaporin genes including OsPIP2;1, OsPIP2;4, OsPIP2;5 and OsTIP2;1 with different sampling time length (experiment 1), with or without shoot removal (experiment 2) and light and dark periods samplings with different light conditions (experiment 3). Our results revealed that the aquaporin expression levels in separated roots did not change compared to those of the roots which were immediately preserved after sampling from the shoot, when root separation was conducted within 10 minutes without shoot removal under fluorescent light during light period of growth, as well as under indirect lighting during dark period of growth.

The root growth angle (RGA) is an important breeding target that confers high crop adaptability to deleterious environments. In barley, natural variations in RGA among accessions have been observed, but many of the genetic factors that cause of this variation remains unclear. In this study, we explored the orthologs of OsDRO1 (DEEPER ROOTING 1) and OsqSOR1 (quantitative trait locus for SOIL SURFACE ROOTING 1), which play a critical role in RGA regulation in rice, from barley genome and analyzed the polymorphisms of these genes among barley accessions. BLASTP search detected putative orthologs of OsDRO1 and OsqSOR1 in barley (HvDRO1 and HvqSOR1) with more than 60% amino acid similarity. Sequence analysis identified SNPs causing mis-splicing and nonsynonymous amino acid substitution in HvDRO1 and HvqSOR1, respectively. These SNPs were associated with RGA variation among the 47 barley accessions. Phylogenetic analysis using the 105 barley accessions revealed that the alleles of HvDRO1 and HvqSOR1 are related to the genetic background of the accessions. Furthermore, the mutant allele of HvDRO1 is mainly shared in the Hokuriku/Nagano subpopulation, suggesting that the mutant allele is involved in local adaptation of barley cultivars to the soil environment of the region. Our findings suggest that the polymorphisms of HvDRO1 and HvqSOR1 are possible determinants of RGA variation in barley, at least in Japanese accessions, and provide information on allelic variants of the genes for marker-assisted selection to genetic improvement of RGA of barley.