Roots laterals

M. C. Drew, L. R. Saker, Nutrient supply and the growth of the seminal root system in barley II. Localized compensatory changes in lateral root growth and the rates of nitrate uptake when nitrate is restricted to only one part of the root system. J. E.xp. Bot. 26 79 (1976). 

In soil, the chances that any enzyme will retain its activity are very slim indeed, because inactivation can occur by denaturation, microbial degradation, and sorption (61,62), although it is possible that sorption may protect an enzyme from microbial degradation or chemical hydrolysis and retain its activity. The nature of most enzymes, particularly size and charge characteristics, is such that they would have very low mobility in soils, so that if a secreted enzyme is to have any effect, it must operate close to the point of secretion and its substrate must be able to diffuse to the enzyme. Secretory acid phosphatase was found to be produced in response to P-deficiency stress by epidermal cells of the main tap roots of white lupin and in the cell walls and intercellular spaces of lateral roots (63). Such apoplastic phosphatase is safe from soil but can be effective only when presented with soluble organophosphates, which are often present in the soil. solution (64). However, because the phosphatase activity in the rhizo-sphere originates from a number of sources (65), mostly microbial, and is much higher in the rhizosphere than in bulk soil (66), it seems curious that plants would have a need to secrete phosphatase at all. 

J. F. Johnson, C. P. Vance, and D. L. Allan, Phosphorus deficiency in Lupinus aUms altered lateral root development and enhanced expression of phosphoenol-pyruvate carboxylase. Plant Physiol. 112 31 (1996). 

Iron uptake by bacteria at sites of lateral root emergence has been further confirmed using another technique employing 7-nitrobenz-2-oxa-l,3-diazole-desferrioxamine B, which is a derivitized siderophore that becomes fluorescent after it is deferrated (78). In this case, iron uptake from the siderophore ferrox-amine B was a.ssociated primarily with microbially colonized roots, but both plant and iron uptake from this chelate occurred in the regions just behind the root tips. 

Statistical analysis of the microbial communities associated with the root tips of iron stressed and nonstressed plants revealed the formation of distinct communities in response to plant iron nutritional status (Fig. 5B and C). Communities associated with the older root parts clustered similarly, whereas sites of lateral root emergence and nongrowing root tips differentiated to a lesser degree than the communities associated with the rapidly growing primary root tips (data not shown). 

The recent report of wave-like patterns of bacteria and water soluble carbon associated with wheat roots (58) which were not strongly correlated with each other or with lateral root formation pose a challenge for models. So do the reports of highly dynamic and erratic population fluctuations of individual pseudomonad clones on sugar beet roots (59). 

Zinc concentrations in forest plants vary considerably. In oaks (Quercus spp.), for example, some species are accumulators, whereas others may be termed discriminators. For individual species, zinc concentrations tend to follow the pattern of roots > foliage > branch > trunk (Van Hook et al. 1980). Small lateral roots accumulate Zn to much greater levels than other vegetation components and are probably most sensitive to changes in zinc inputs. Half-time persistence of zinc in forest ecosystems varies from about 3 years in organic matter components to >200 years for large soil pools (Van Hook et al. 1980). 

The content of total and individual ginsenosides does not only vary between plant organs and species. In particular, the content of ginsenosides in ginseng roots also depends on growing conditions and age of the roots, and internal root size (root hairs, lateral roots, and main roots) (Christensen et al, 2006 Court et ah, 1996b Soldati and Tanaka, 1984 Wills and Stuart, 2001). 

FIGURE 1.9 Ginseng roots from 6-year-old American ginseng plants (Panax quinquefo-lium) grown in Denmark with root hairs, lateral roots, and main roots. Ginseng roots within the same species may not only differ in content of ginsenosides but also in root size. 

Figure 6.2 Cross-sections of primary rice roots, (a) Radial section close to tip showing interceUnlar spaces (I), central cylinder (CC), and rhizodermis (RH). (b) and (c) Radial sections of yonnger (39 days) and older (72 days) basal parts showing exodermis (E), schlerenchymatons cylinder (SC), parenchymatons or cortical cells (P) and aerenchyma (AE). (d) and (e) Axial sections of matnre root (72 days) showing break through of lateral roots (Butterbach-Bahl et al., 2000). Reproduced by permission of verlag...

The proportion of fine lateral roots branching off the primary root. Having high surface area to volume ratios, laterals tend to be 02-leaky. 

Eor simplicity, the effects of lateral roots are not dealt with explicitly in Armstrong and Beckett s model, but they are dealt with in Section 6.2. 

The structure of the rice root is therefore apparently dominated by the need for internal gas transport. On the face of it, this structure may conflict with the needs for efficient nutrient absorption (Kirk and Bouldin, 1991). The development of gas-impermeable layers in the root wall seems likely to impair the ability of those parts of the root to absorb nutrients, and the disintegration of the cortex might impair transport from the apoplasm to the main solute transport vessels in the stele, though these points are uncertain (Drew and Saker, 1986 Kronzucker et al, 1998a). It seems likely that the short fine lateral roots are responsible for the bulk of the nutrient absorption by the root system and compensate for any impairment of nutrient absorption by the primary roots as a result of adaptations for internal aeration. 

Further studies are required to fully elucidate the role of flavonoids in auxin regulation in vivo to determine, for example, whether changes in the synthesis or deposition of specific flavonoids within the cell act to change the rate or direction of auxin transport. There is the question of how such different organs or developmental outcomes as nodules, lateral roots,... 

Mathesius, U., Conservation and divergence of signalling pathways between roots and soil microbes — the Rhizobium-legame symbiosis compared to the development of lateral roots, mycor-rhizal interactions and nematode-induced galls. Plant Soil, 255, 105, 2003. 

Gonzalez-Rizzo S, Crespi M, Frugier F. 2006. The Medicago truncatula CRE1 cytokinin receptor regulates lateral root development and early symbiotic interaction with Sinorhizobium meliloti. Plant Cell 18 2680-2693. 

Mathesius U, Weinman JJ, Rolfe BG, Djordjevic MA. 2000. Rhizobia can induce nodules in white clover by hijacking mature cortical cells activated during lateral root development. Mol Plant Microbe Interact 13 170-182. 

The plant is a large, sturdy herbaceous perennial. The stem grows to over 1.5 m high. The leaves are orbicular-cordate, palmate-lobed, somewhat rough on the upper surface, or smooth and three- to five-ribbed. The lobes are oblong-ovate to lanceolate, dentate or pinnatisect. The root system consists of a tuber, which after a number of years measures 10 to 15 cm in diameter and has arm-thick lateral roots. 

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