Plants constantly sense changes in their environment. When nutrient elements are scarce, plants often allocate a greater proportion of their biomass to the root system. In the present study, we characterized the early root development of rice reduced culm number (rcn) mutants under standard growth conditions. Short lateral root length was consistent in five rcn mutants, and the remaining root traits varied between the rcn mutants. Subsequently, we characterized the effects of rcn1 mutation in response to nutrient shortage. Root development, involving the elongation of seminal, crown, and lateral roots and branching of the lateral roots, was promoted in wild type in response to nutrient shortage. However, in rcn1 mutants, crown root elongation was constant and lateral root branching was reduced in response to nutrient shortage. The present results indicate that RCN1/OsABCG5 is associated with root development in response to nutrient shortage.

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Zea nicaraguensis (teosinte), a wild relative of maize (Zea mays ssp. mays), constitutively forms aerenchyma, which contributes to plant waterlogging tolerance, in the root cortex in drained soil, whereas maize (inbred line Mi29) does not. One with highest logarithm of odds (LOD) among quantitative trait loci (QTLs) that control constitutive aerenchyma formation in Z. nicaraguensis is Qaer1.05-6 on chromosome 1. Here, we attempted to identify genes in Qaer1.05-6 by comparing cDNA libraries from Mi29, Z. nicaraguensis and a line BC₄F₁ #62 carrying Qaer1.05-6. We first confirmed that constitutive aerenchyma formation was apparently observed in the order Z. nicaraguensis > BC₄F₁ #62 > Mi29. Contigs were assembled from cDNAs pooled from the three lines. We identified 1,868 contigs in the region on chromosome 1 that contained Qaer1.05-6. These contigs were screened for contigs that were predominantly composed of cDNAs from BC₄F₁ #62 and Z. nicaraguensis (no more than 10% of cDNAs from Mi29). Twenty-one such contigs were found and the genes they encoded were identified. In a real-time reverse transcription-polymerase chain reaction (RT- PCR) analysis, expression of six of these genes in BC₄F₁ #62 was at least double that in Mi29, making them candidates for genes associated with constitutive aerenchyma formation.

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To explore seasonality of root functions, we analyzed the concentrations of 8 minerals in leaves of Populus maximowiczii (Japanese native poplar) by inductively coupled plasma atomic emission spectroscopy. These concentrations were used as indices of root mineral translocation activity. In leaves close to the shoot apex, dramatic increases in Ca concentration, and similar but slight increase in Mg and Mn, were observed after the onset of dormancy. Because of the constant concentration of Na, which is not essential for plant growth, the increase of Ca concentration was mainly derived from not by the increase of transpiration rate but by the enhancement of root activity of xylem loading. Leaf Ca concentration in August 2010 was approximately fivefold higher than before dormant bud formation. To investigate the shifts in Ca-translocation activity during dormancy induction, we grew saplings hydroponically under light- and temperature-controlled conditions and subsequently analyzed the distribution of ⁴⁵Ca absorbed by roots using a Bio-Image Analyzer. In this pulse chase experiment, the enhancement of ⁴⁵Ca translocation to the shoot was not observed in early dormancy. This suggested the increase of leaf Ca in early dormancy was caused by the Ca loading into root xylem vessels using the root Ca absorbed before the onset of dormancy. These changes in mineral translocation activities indicate that alterations in Ca distribution are most probably triggered by bud dormancy. Furthermore, several root functions were regulated by the dormancy induction process.

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Cell death in the lateral root tip of rye seedlings under salt stress conditions was analyzed quantitatively by the modified terminal deoxynucleotidyl transferase (TdT)-mediated dUTP nick end labeling (TUNEL) method and the frequency of cell death was compared in the seminal root tip. There were no significant differences in total root length and the number of root tips among control, 10 mM and 100 mM NaCl treatments, although the root elongation and initiation was inhibited in the 250 mM NaCl treatment. The frequency of cell death was increased in 100 mM and 250 mM NaCl treatments compared with the control, significantly. There was no significant difference in the frequency of cell death between seminal root and lateral root in all stress treatments. Moreover, technical advantage of the modified TUNEL method was discussed by comparing with the classical TUNEL method.

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