With the advent of the molecular era of plant biology, the location and activity of the quiescent centre (QC) within the root meristem were reappraised with respect to the transport and distribution of hormones, especially auxin. Later, when methods for probing gene activity became established, the genes and their regulators that were identifiably specific to the QC were also actively studied, at first in relation to the establishment of the root and its QC in the proembryo and later in relation to the interaction of the QC with neighbouring meristem cells. Auxin distribution in and around the QC was found to be associated with co-located oxidative enzymes which established a redox system within the root apex. This system is pivotal in both maintenance of quiescence and the activation of cell proliferation in the QC via the generation of reactive oxygen species (ROS) and their interaction with mitochondria. These and other features of QC biology are summarised.
Within the tip of roots meristems of angiosperms and gymnosperms there is a small group of cells known as the quiescent centre (QC). The concept of the QC was developed 60 years ago by FAL Clowes, working in the Botany School, Oxford University, UK. To celebrate the Jubilee of the QC, a brief outline of the work that led to its demonstration by autoradiography was presented by Dubrovsky and Barlow (2015). The present article traces Clowes’s subsequent experimental studies of the QC, especially with regard to how X-irradiation became an important tool for elucidating the properties and significance of the QC for root development. Also reviewed are some of the consequences that subsequently arose from this work with radiation, in particular the concerns over the use of radioisotopes in attempts to describe the kinetics of cell proliferation in the root meristem.
Understanding morphology of lateral roots in rice is important in modeling different agricultural management system. The objective of the study was to explore the morphology of lateral roots of lowland transplanted-aman rice cultivars to mechanistically model length, surface area and volume of an individual root. Seedlings of twelve selected rice cultivars at 30 days of age were transplanted and a series of measurements was carried out on 14, 20, 33 and 60 days after transplantation (DAT). Lateral roots and root hairs were studied under a light microscope. Individual main axes produced up to second-order laterals. Mean main axis diameter and length of twelve cultivars measured 0.94 mm and 20.4 cm respectively at 60 days after transplantation. Diameter reduced at the first-order and second-order laterals up to 4.56 and 21.4 times respectively compared to main axis on 60 DAT. Root hair diameter measured 4.0 μm. An individual root on 60 DAT estimated 911 m in the length, 1714 cm2 in the surface area and in the 467 mm3 volume. Root hairs had the highest contribution towards total length and surface area of an individual root whereas main axis and first order laterals mostly contributed root volume.
Phosphorus (P) is patchily distributed in soil because of its slow diffusion, especially in soil with a high phosphate absorption coefficient (PAC). Root responses to localized supply of phosphate were studied in Sesbania cannabina grown in volcanic andosol, which has a high PAC. Seedlings were grown in soil that was supplied with 0, 10, 100, 500, or 1000 mg P kg-1. After 30 days, analyses of plant P and root morphological were conducted. Further rhizobox experiments were also conducted. Seedlings were grown with layered P sources or localized P patches. Densely branched lateral roots (DBLRs) developed only in the 10 and 100 mg P kg-1 treatments. Although an increase in shoot dry weight (DW) was observed in the 500 and 1000 mg P kg-1 treatments, DBLRs were not observed. The number of DBLRs was positively correlated with shoot DW, root DW, and number of nodules, and negatively correlated with phosphorus use efficiency of shoots and roots. The rhizobox experiment showed that most DBLRs were observed in the layer with added P and in the position where P fertilizer was present. DBLRs developed so as to monopolize the P fertilizer by completely enveloping the area around it. The results suggest that DBLR formation is one of S. cannabina’s P acquisition strategies.
Two Melaleuca species, M. cajuputi and M. bracteata, were compared to identify the factors determining their distinct aluminum (Al) resistance levels. The presence of Al in a liquid culture medium (maximum tested concentration, 2 mM) did not affect the growth of M. cajuputi, but severely inhibited the growth of M. bracteata. The Al content in the roots was 50% higher in Al-sensitive M. bracteata than in Al-resistant M. cajuputi. Al penetration and tissue damage were obvious in the roots of M. bracteata, but only mild in the roots of M. cajuputi. Relatively high levels of fumarate were released by the roots of M. cajuputi, but not by those of M. bracteata. Supplementation of Al-containing liquid media with fumarate resulted in a reduction of Al toxicity on M. bracteata. These results suggest that Al-resistant M. cajuputi releases fumarate from its roots, thereby detoxifying Al.
To analyze the function of SYNC1, an Arabidopsis asparaginyl-tRNA synthetase gene, the FOX-hunting system using super-growing roots (SR) from the legume species Lotus corniculatus was employed. One transformed line, FSL#121, was compared to parental SR and to an SR line harboring the empty vector (Control), all of which were grown in vivo using vermiculite pots. The level of several free amino acids was higher in FSL#121 than SR. Concomitantly, FSL#121 had a distinct phenotype of greater shoot length, stem diameter and shoot fresh weight compared with SR. Also, the root length, root diameter and fresh root weight were greater in FSL#121 than SR. Furthermore, the greater number of nodules in FSL#121 increased the nitrogen fixation activity per whole plant. Therefore, SYNC1 overexpression caused distinct changes in plant growth, increased the number of root nodules, and may be involved in increasing the amount of free amino acids, especially asparagine.