The plant hormones ethylene and auxin mediate the formation of root hairs on lettuce seedlings that are transferred from pH 6.0 to pH 4.0 medium. To investigate the regulatory mechanism of ethylene, we isolated ethylene receptor genes from lettuce. Three putative transmembrane domains were found in Ls-ERS1 and Ls-ETR1 and four in Ls-ETR2 and Ls-ETR3. Five bacterial histidine kinase motifs were highly conserved in Ls-ERS1 and Ls-ETR1, but not Ls-ETR2 or Ls-ETR3. Phylogenetic analysis supported these similarities among family members. Genomic Southern hybridization revealed that each gene existed as a single copy in the genome. mRNAs of these genes were detected in seedling roots after pre-culture at pH 6.0. After transfer to pH 4.0 medium, Ls-ERS1 and Ls-ETR2 expression and ethylene produc-tion increased and were maintained at higher levels than those found at pH 6.0. The addition of 1-aminocyclopropane-1-carboxylic acid (ACC) to the pH 6.0 medium noticeably induced both ethylene production and Ls-ERS1 and Ls-ETR2 expression. A marked increase in the mRNA level of Ls-ERS1, with a slight increase in Ls-ETR2 mRNA level, was noted with the addition of indole-3-acetic acid (IAA); however, ethylene production was also induced. Simultaneous treatment with an ethylene biosynthesis inhibitor and IAA markedly inhibited ethylene production and ethylene receptor gene expression. These results suggest that ethylene receptor gene expression is differentially regulated among the family members during low pH-induced root hair formation in lettuce seedlings, and that the increased expression of Ls-ERS1 and Ls-ETR2 during this process is induced by ethylene rather than by auxin.
Many facets reflecting the autopoietic process of Life and Living can be found in plant roots at many levels relevant to their organisation, from cells to ecosystems. At each level, there are sub-processes dedicated to both the auto-reproduction and the self-maintenance of that level, these processes being contained within a boundary appropriate for that level. Auto-reproduction and self-maintenance unite with a third sub-process, cognition, and provide the basis of a coherent multi-levelled programme of root-research.
Aerenchyma promotes gas exchange between shoots and roots that supports plant to survive under waterlogged conditions. To understand the process of aerenchyma formation under waterlogged conditions, we developed a method for creating hypoxic pot-culture conditions using different water depths, and used this system to examine the effects of hypoxia on seedling growth and the anatomy of the seminal roots of spring wheat (Triticum aestivum cv. Bobwhite line SH 98 26). After 72 h of waterlogging, the redox potentials of a well-drained control and treatments with a water depth 15 cm below (T-15) and 3 cm above (T+3) the soil surface were +426, +357, and +292 mV, respectively. The root growth of the seedlings was reduced in T+3 plants while the shoot growth did not change significantly during 72 h waterlogging. Root anatomy study showed that wheat formed no aerenchyma under our control condition, but formed aerenchyma in the root cortex in response to hypoxia in T-15 and T+3 conditions. The aerenchyma was initially formed at 2 to 5 cm behind the root tip after 72 h in T-15 and 48 h in T+3. The aerenchyma in T+3 plants then extended by an additional 5 cm towards root base during the next 24 h. Evans blue staining indicated that wheat aerenchyma was lysigenous which resulted from degradation of cortical cells. Thus, the combination of the plant material and the pot-culture method can be used for a basic tool with which to analyse the molecular and physiological mechanisms of lysigenous aerenchyma formation in wheat.
The distribution of roots in soil determines their acquisition of spatially varying resources. It may be altered by changing the response of roots to gravity. The aim of the study was to assess gravitropic set-point angles (GSAs) of maize (Zea mays L.) roots, their response to temperature and the feasibility to measure them in growth pouches. The GSAs of the primary, seminal and crown roots of a set of nine temperate inbred lines were measured. The lines were grown under controlled conditions in growth columns either at 15/13°C or 24/20°C (day/night) until the two-leaf stage (V2). The GSA was measured as the deviation of the initial 3 cm of root axis from the vertical zero. Low temperature resulted in a decrease in the GSAs of the crown roots by 10°, i.e. the roots oriented more vertically. The effect of the GSAs on the distribution of the roots was verified in wider columns using two extreme inbred lines. The proportion of roots in the upper 5 cm of the columns was 78% for the line S335 with the strongest tendency to horizontal root growth and only 39% for CM105 with almost vertical orientation of the roots. The differences in GSAs between these two genotypes were even more pronounced in growth pouches, thus proving the feasibility of this system for rapid screening. The results indicate that there is a huge genetic variability available to alter the growth direction of the seedling roots of maize. However, there was little effect of the temperature.
Row crops commonly grown under irrigation in the Vertisols of north-western New South Wales, Australia, include summer crops such as corn (Zea mays L.) and cotton (Gossypium hirsutum L.). Soil organic carbon (SOC) and residue (SOR) dynamics in these farming systems have been analysed primarily in terms of inputs of above-ground material and root mass towards the end of a growing season. Addition of root material to SOC and SOR stocks either in the form of roots dying and decaying during and after the crop's growing season may, however, be significant. Carbon inputs by roots of irrigated corn to an irrigated Vertisol were evaluated in an experiment near Narrabri, Australia, where corn grown as a monoculture was compared with corn sown in rotation with cotton. Root growth in the surface 0.10 m was measured with the core-break method, and that in the 0.10 to 1.0 m depth with a minirhizotron and I-CAP image capture system. These measurements were used to derive root length per unit area (LA), root C added to soil through intra-seasonal root death (Clost), C in roots remaining at end of season (Croot) and root C potentially available for addition to soil (Ctotal). Ctotal averaged 5.0 Mg ha-1 with cotton-corn and 9.3 Mg ha-1 with corn monoculture, with average Clost accounting for 11%. Intra-seasonal root death from corn made only a small contribution to soil carbon stocks. LA of corn was higher with corn monoculture than with cotton-corn.
A simple method for the evaluation of respiration activity of root cells of intact plants grown hydroponically and/or in agar medium was developed. The novelty of the present method is based on visual detection of dehydrogenase activity of plant roots by use of tetrazolium violet dye without destructive steps, allowing follow up of living and photosynthetically active growing plants and the impact of inhibitors such as sodium azide and cycloheximide. The results of this approach demonstrated that root tip cells comprise the highest dehydrogenase activity compared to other root parts. The non-expensive assay is easy to perform and allows to experiment a large variety of chemical compounds with potential inhibitory characteristics for plants.
To elucidate the involvement of gibberellin (GA) in the growth regulation of Arabidopsis roots, effects of shoot-applied GA and GA-biosynthesis inhibitors on the root were examined. Applying GA to the shoot of Arabidopsis slightly enhanced the primary root elongation. Treating shoots with uniconazole, a GA biosynthesis inhibitor, also resulted in enhancement of primary root elongation, while shoots treated with uniconazole were stunted and bolting was delayed. Analysis of the expression of GA3ox and GA20ox confirmed the up-regulation of these genes in roots following the inhibitor application to shoots. The results suggest that the inhibition of GA-biosynthesis enhances the production of bioactive GAs in roots and promotes root elongation.