Pioneer Roots Research Articles

Fine-root system development and susceptibility to pathogen colonization

Root development may exert control on plant-pathogen interactions with soil-borne pathogens by shaping the spatial and temporal availability of susceptible tissues and in turn the impact of pathogen colonization on root function. To evaluate the relationship between root development and resistance to apple replant disease (ARD) pathogens, pathogen abundance was compared across root branching orders in a bioassay with two rootstock genotypes, M.26 (highly susceptible) and CG.210 (less susceptible). Root growth, anatomical development and secondary metabolite production were evaluated as tissue resistance mechanisms. ARD pathogens primarily colonized first and second order roots, which corresponded with cortical tissue senescence and loss in second and third order roots. Defense compounds were differentially allocated across root branching orders, while defense induction or stress response was only detected in first order and pioneer roots. Our results suggest disease development is based largely on fine-root tip attrition. In accordance, the less susceptible rootstock supported lower ARD pathogen abundance and altered defense compound production in first order and pioneer roots and maintained higher rates of root growth in both the ARD soil and pasteurized control compared to the more susceptible. Thus, this rootstock's ability to maintain shoot growth in replant soil may be attributable to relative replant pathogen resistance in distal root branches as well as tolerance of infection based on rates of root growth.

Avoiding transport bottlenecks in an expanding root system: Xylem vessel development in fibrous and pioneer roots under field conditions

Root systems develop to effectively absorb water and nutrients and to rapidly transport these materials to the transpiring shoot. In woody plants, roots can be born with different functions: fibrous roots are primarily used for water and nutrient absorption, whereas pioneer roots have a greater role in transport. Because pioneer roots extend rapidly in the soil and typically quickly produce fibrous roots, they need to develop transport capacity rapidly so as to avoid becoming a bottleneck to the absorbed water of the developing fibrous roots and, as we hypothesized, immediately activate a specific type of autophagy at a precise time of their development. Using microscopy techniques, we monitored xylem development in Populus trichocarpa roots in the first 7 d after emergence under field conditions. Newly formed pioneer roots contained more primary xylem poles and had larger diameter tracheary elements than fibrous roots. While xylogenesis started later in pioneer roots than in fibrous, it was completed at the same time, resulting in functional vessels on the third to fourth day following root emergence. Programmed cell death was responsible for creating the water conducting capacity of xylem. Although the early xylogenesis processes were similar in fibrous and pioneer roots, secondary vascular development proceeded much more rapidly in pioneer roots. Compared to fibrous roots, rapid development of transport capacity in pioneer roots is not primarily caused by accelerated xylogenesis but by larger and more numerous tracheary elements and by rapid initiation of secondary growth.

Growth and physiology of olive pioneer and fibrous roots exposed to soil moisture deficits

In woody plants, pioneer roots are the main roots used to expand the root system horizontally and vertically whereas fibrous 'feeder' roots are chiefly used in the absorption of water and nutrients. Because of their different roles, we expected newly emerged pioneer and fibrous roots to respond differently to restrictions in soil moisture. We hypothesized that fibrous roots would exhibit greater growth plasticity and greater physiological impairment from soil moisture deficits, especially under heterogeneous conditions. We compared the responses of fibrous and pioneer roots of olive seedlings (Olea europaea) to localized and uniform soil moisture deficits in transparent containers in the greenhouse. In comparison with uniformly wet conditions, uniformly dry conditions caused reduced shoot photosynthesis and reduced shoot growth, but no significant effect on root morphology, root respiration (measured in aerated buffer solution using excised roots) or electrolyte leakage as a function of root age. Under heterogeneous soil moisture conditions, root growth tended to preferentially occur in the moist sector, especially in the pioneer roots. In comparison with pioneer roots in the moist sector, pioneer roots in the dry sector had higher tissue density and higher suberin content, but no shift in root respiration, non-structural carbohydrates or electrolyte leakage. In contrast, fibrous roots in the dry sector exhibited evidence of impaired physiology in older (>38 days) roots compared with similar age fibrous roots in the moist sector. While we anticipated that, compared with pioneer roots, fibrous roots would be more sensitive to soil moisture deficits as expressed by higher electrolyte leakage, we did not expect the strong growth plasticity of pioneer roots under heterogeneous soil moisture conditions. Differentiating the responses of these two very different root types can improve our understanding of how different portions of the root system of woody plants cope with soil moisture deficits.

Contrasting the morphology, anatomy and fungal colonization of new pioneer and fibrous roots

Not all roots born as first-order branches are the same and this has important consequences for overall function. We hypothesized that, compared with fibrous roots, pioneer roots are built to live longer at the expense of absorptive capacity. We tested this hypothesis by investigating pioneer and fibrous roots in their first 14 d of life in the arbuscular mycorrhizal tree species: Acer negundo, Acer saccharum, Juglans nigra, Liriodendron tulipifera and Populus tremuloides. Root observations were made with root-access boxes that allowed roots to be sampled at known ages in field-grown trees. Compared to fibrous roots, pioneer roots had larger diameter, lower specific root length, greater average length and a lack of mycorrhizal or nonmycorrhizal fungal colonization. Pioneer roots < 14 d old had more layers of hypodermis with a lower percentage of putative passage cells and more protoxylem groups than similar age fibrous roots. Our results suggest that pioneer roots are constructed for defense against biotic and abiotic challenges, exploration of soil distal to the stem, high fibrous root branching and secondary development with high axial hydraulic conductivity at the expense of mycorrhizal colonization and high absorptive capacity for water and nutrients.

Anatomical characteristics of roots of citrus rootstocks that vary in specific root length.

Among citrus rootstocks, higher specific root length (SRL, root length/d. wt) has been linked to several specific morphological and physiological traits, including smaller average root diameter, higher root hydraulic conductivity and higher rates of root proliferation. In this study, thickness of the outer tangential exodermal (hypodermal) wall and its suberin layer, number of passage cells, presence of epidermis, and stelar anatomy were examined and related to variation in root diameter of field roots of known maximum age. We also compared root morphology and anatomy of young roots in the field with those of potted rootstock seedlings in the glasshouse. Fibrous roots were measured separately from pioneer (framework) roots. Among the fibrous roots, only the first-order (root links having a root tip) and second-order (root links bearing first-order roots) laterals were examined. Among first-order field roots, larger root diameter was caused by larger rather than more numerous cells in the cortex. Root diameter of first-order roots was positively correlated with both number of passage cells in the exodermis and thickness of the secondary walls of the exodermis in both field and potted plants. Exodermal walls were about 80% thicker in field- than pot-grown roots. In the field, in more than 50% of the first-order roots examined less than 30% of the root surface was still covered by epidermis, with few differences among rootstocks. In contrast, in roots of 19-wk-old glasshouse plants generally 70-100% of the epidermis was still intact. There was no evidence of secondary xylem development in second-order fibrous roots in the field; in seedling, pot-grown rootsystems, 75-97% of second-order roots had formed secondary xylem despite their small diameter (<0.8 mm). It is argued that there can be suites of physiological, morphological and anatomical traits in roots that co-vary with specific root length. Investigations of how root morphology and anatomy are linked to root function, moreover, need to recognize trait variability and the potentially important differences between field- and pot- grown (seedling) roots.

Anatomical Observations on the Accumulation and Utilization of Storage Starch and Epidermis-related Development in Roots of Vigorous and Non-vigorous Satsuma Mandarin Trees from Early Winter to Early Summer.

Accumulation and utilization of storage starch and epidermis-related development in roots of vigorous and non-vigorous satsuma mandarin (Citrus unshiu Marc. cv. Miyagawa Wase) were compared anatomically from early winter to early summer. Only a few starch grains were deposited in the cortex of the pioneer roots of both kinds of trees in December. In vigorous trees, the number of starch grains per cortical cell and the ratio of areas of starch grain to the area of cortex (area-ratio) increased considerably during January and February, attaining a maximum in March. Area-ratio gradually decreased from April to May ; by June, it approached zero as the starch grains almost disappeared. In contrast, the area-ratio in non-vigorous trees was unchanged from January to February and increased slightly in March. The area-ratio reached the highest level in April, followed by rapid reduction in May, as the starch grains disappeared in June. Neither the vigorous nor the non-vigorous trees contained starch grains in July. Fibrous roots on vigorous trees were filled with many starch grains up to April, then the grains disappeared completely in June ; whereas in non-vigorous trees starch grain depletion started in June but the grains persisted until July. The pioneer roots on vigorous trees had but one layer of phellogen cells beneath the epidermis in January. The periderm was distinct with the formation of phellem, phellogen, and phelloderm in March. Periderm formation in non-vigorous trees was initiated in March, and it continued slowly into April. Root hair and epidermal development was faster in roots of vigorous tree.

Salt Tolerance, Ion Relations and the Effect of Root Medium on the Response of Citrus to Salinity

This paper describes the ion relations of two citrus genotypes, Etrog citron (Citrus medica L.), a salt-sensitive genotype, and Rangpur lime (Citrus reticulata var. austera hybrid?), a salt-tolerant genotype, under conditions of high NaCl concentrations. Root morphology was modified by establishing seedlings for 6 months in solution or sand culture. When established in sand culture, both genotypes displayed a high efficiency, relative to plants established in solution culture, to exclude Na from leaves, but differed in their capacities to exclude Cl; this capacity was much lower in Etrog citron than Rangpur lime. The net Cl root to shoot transport rates over 7 weeks for Etrog citron and Rangpur lime, treated with 50 mol m-3 NaCl, were 0.25 and 0.07 μmol g-1 root FW h-1, respectively. The net transport rates of Cl in Etrog citron were 10 times those in Rangpur lime between weeks 4 and 7. In Rangpur lime Cl reached pseudo steady-state levels in root and leaf tissue by week 4, thereafter, the net Cl root to shoot transport rate of Rangpur lime decreased while the rates in Etrog citron increased. The accumulation of Na and Cl in leaves of Rangpur lime, in contrast to Etrog citron, was not affected by transpiration. The high salt tolerance of Rangpur lime appears to be associated with low rates of net uptake and transport of Cl, feedback control of net root to shoot transport by Cl levels in the shoot, a low interaction between water flow and Cl and Na fluxes, and a reduction in the leafiroot ratio in response to salinity. When plants were established in solution culture the pioneer roots became elongated and the normal development of the laterals into a fibrous root system was suppressed. Plants of both genotypes, previously established in solution culture and then treated with 50 mol m-3 NaCl, accumulated levels of Na and Cl in leaves up to 200-400 mol m-3. Under these conditions the rates of net Cl root to shoot transport over the 7 weeks were 0.53 and 0.56 μmol g-1 root FW h-1 for Etrog citron and Rangpur lime, respectively. These high rates of Cl uptake and transport were attributed to changes in root permeability and increases in passive ion fluxes. Keywords: citrus, root morphology, root medium, transpiration, Cl, Na, salt accumulation, salinity.

MORPHOLOGICAL STUDIES OF THE ROOTS OF RED PINE, PINUS RESINOSA. II. FUNGAL COLONIZATION OF ROOTS AND THE DEVELOPMENT OF MYCORRHIZAE

Observations are reported on the distribution and appearance of long‐ and short‐root apices of nursery seedlings and plantation trees. Only emerging lateral roots over 50% of the diameter of the mother roots become long‐roots, and only from 10 to 21% attain this initial size. Short‐roots have rounded apices with small, sharply delimited, hemispherical or lens‐shaped meristematic zones. Long‐roots have pointed apices with paraboloid‐shaped meristematic zones showing light‐absorbing zones near the apex as well as near the base of the paraboloid. In small‐diameter long‐roots, lateral short‐roots tend to be uniformly distributed throughout the increment, but in large‐diameter roots they are closer together at the two ends of the increment. Lateral roots may be missing completely in the largest‐diameter pioneer roots. Both long‐ and short‐roots are inhabited by various species of fungi, mycorrhizal and non‐mycorrhizal. The patterns of fungal distribution in long‐roots vary from the proximal to the distal ends of the seasonal growth increments, depending on root growth and morphology. Mycorrhizal‐forming fungal species appear to predominate during the growing season but non‐mycorrhizal species may become prominent during the dormant season. The reactivation of apical and lateral meristems following dormancy occurs somewhat in advance of growth resumption by the fungi. However, delay in growth of the fungi is usually not long enough to permit emerging laterals to escape infection from the developing Hartig net in the mother root. Very large first‐order laterals of 3–0 seedlings and large pioneer roots in the plantations may ultimately acquire a sufficiently rapid growth rate to escape the fungus. When this occurs lateral roots may be initiated from a fungal‐free zone in the mother root and may become infected later. A very sparse Hartig net suffices for a strong invasion of emerging laterals, because hyphal growth is stimulated in the vicinity of a developing lateral.