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Root System Structure and Function: A Themed Issue in Honor of Professor Yoav Waisel

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A special issue of Plants (ISSN 2223-7747).

Deadline for manuscript submissions: closed (31 October 2019) | Viewed by 89479

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Special Issue Editor

Prof. Dr. Amram Eshel

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Guest Editor

Plant Science, Tel-Aviv University, Israel
Interests: plant ecophysiology, especially as related to root-system structure and function; plant ecology and agro-ecosystems and in application of computer simulation

Special Issue Information

Dear Colleagues,

In Memoriam Yoav Waisel (1931–2010)

This Special Issue is dedicated to the memory of Prof. Dr. Yoav Waisel (1931–2010), a world leader in root research. He was well known for the series of text books “Plant Roots: The Hidden Half” a phrase he coined, and which became a popular expression among root scientists. He was active in the International Society for Root Research from its early days, and a leader in a COST Action on Woody Root Processes. One of his main achievements was the establishment of The Sarah Racine Root Research Laboratory at Tel-Aviv University, which is the world’s largest aeroponics facility.

This Special Issue emphasizes the definitive connection between the structure and function of plant root systems. Root systems encounter soil media that vary to a great extent, both spatially and temporally. Spatial variability manifests in the non-uniform soil structure of the soil profile and the distribution of stones, cracks, wormholes, and organic residues. Even more dynamic is the variance in soil composition, especially its water content that, in turn, affects solute concentrations and their division between soluble and solid phases. Such non-uniformities occur not only in the natural soil, but also in plant pots filled originally with uniform media. The activity of the roots is responsible for creating further changes in soil structure and composition. Other variable components of the rooting media are distribution and the activities of soil microorganisms, including bacteria and fungi. These are, in turn, affected by root presence and activities such as mucilage secretion and ion and water uptake.

The adaptation of a root system to such variable conditions is expressed through the distribution of its components in the rooting medium. The resulting root system structure is the outcome of two main processes: The elongation and branching of its various components. Other processes, such as secondary thickening and sloughing-off of fine roots, also take part in shaping the structure of the root system. The rates and allocation of these processes result from the interaction between the plant genomic characteristics and the environmental cues. Important features of root structure, which have a major role in determining their function, are the microscopic characteristics of the root surface and its physiological activities.

Manuscripts addressing the genomic basis of key developmental processes that underlie all aspects of formation of the root system structure that affect its function are invited to this Special Issue. Articles related to crop and forestry plants, as well as model plant species, are welcome. This Special Issue will also include technical papers concerning measurement and quantitative description of the complex structure of plant root systems.

Prof. Dr. Amram Eshel
Guest Editor

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Plants is an international peer-reviewed open access semimonthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2700 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

root branching
root elongation
gravitropism
hydrotropism
thigmotropism
hormonal control
gene function
water uptake
mineral nutrition
crop species
plant model species
early tracheophytes
measurement techniques
modeling

Published Papers (11 papers)

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Research

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16 pages, 2882 KiB

Open AccessArticle

Variation in Root and Shoot Growth in Response to Reduced Nitrogen

by Seth Tolley and Mohsen Mohammadi

Plants 2020, 9(2), 144; https://doi.org/10.3390/plants9020144 - 23 Jan 2020

Cited by 17 | Viewed by 4351

Abstract

Recently, root traits have been suggested to play an important role in developing greater nitrogen uptake and grain yield. However, relatively few breeding programs utilize these root traits. Over a series of experiments at different growth stages with destructive plant biomass measurements, we analyzed above-ground and below-ground traits in seven geographically diverse lines of wheat. Root and shoot biomass allocation in 14-day-old seedlings were analyzed using paper roll-supported hydroponic culture in two Hoagland solutions containing 0.5 (low) and 4 (high) mM of nitrogen (N). For biomass analysis of plants at maturity, plants were grown in 7.5 L pots filled with soil mix under two nitrogen treatments. Traits were measured as plants reached maturity. High correlations were observed among duration of vegetative growth, tiller number, shoot dry matter, and root dry matter. Functionality of large roots in nitrogen uptake was dependent on the availability of N. Under high N, lines with larger roots had a greater yield response to the increase in N input. Under low N, yields were independent of root size and dry matter, meaning that there was not a negative tradeoff to the allocation of more resources to roots, though small rooted lines were more competitive with regards to grain yield and grain N concentration in the low-N treatment. In the high-N treatment, the large-rooted lines were correlated to an increase in grain N concentration (r = 0.54) and grain yield (r = 0.43). In low N, the correlation between root dry matter to yield (r = 0.20) and grain N concentration (r = −0.38) decreased. A 15-fold change was observed between lines for root dry matter; however, only a ~5-fold change was observed in shoot dry matter. Additionally, root dry matter measured at the seedling stage did not correlate to the corresponding trait at maturity. As such, in a third assay, below-ground and above-ground traits were measured at key growth stages including the four-leaf stage, stem elongation, heading, post-anthesis, and maturity. We found that root growth appears to be stagnant from stem elongation to maturity. Full article

(This article belongs to the Special Issue Root System Structure and Function: A Themed Issue in Honor of Professor Yoav Waisel)

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13 pages, 2512 KiB

Open AccessFeature PaperArticle

Root Development of Bell Pepper (Capsicum annuum L.) as Affected by Water Salinity and Sink Strength

by Ran Erel, Thuc T. Le, Amram Eshel, Shabtai Cohen, Rivka Offenbach, Tobias Strijker and Ilana Shtein

Plants 2020, 9(1), 35; https://doi.org/10.3390/plants9010035 - 25 Dec 2019

Cited by 5 | Viewed by 7110

Abstract

Fruits are the dominant sinks for assimilates. At optimal conditions, assimilates supply can meet the demand of fruits and those of the vegetative organs; however, extreme circumstances such as strong sink strength or an environmental stress may disturb this fine balance. While most studies focus on aboveground parameters, information regarding root growth dynamics under variable sink strength are scarce. The objective of this study was to evaluate the effect of sink strength (represented by fruit load) and salinity on bell-pepper root development. Three levels of fruit load were combined with two salinity levels in plants grown in an aeroponic system. Root growth was determined both by root capacitance and destructive measurements. Salinity and sink strength significantly affected root, shoot and fruit growth dynamics. Root growth was less affected by fruit load. Salinity stress was negatively associated with shoot growth, but after an acclimation period, salinity enhanced root development. Additionally, this study shows for the first time that root capacitance is a valid approach for non-destructive measurement of root development in aeroponic systems. The good correlation measured by us (r² 0.86) opens new opportunities for continuous root growth monitoring in aeroponic systems in the future. Full article

(This article belongs to the Special Issue Root System Structure and Function: A Themed Issue in Honor of Professor Yoav Waisel)

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18 pages, 2271 KiB

Open AccessArticle

Characterization of Cover Crop Rooting Types from Integration of Rhizobox Imaging and Root Atlas Information

by Gernot Bodner, Willibald Loiskandl, Wilfried Hartl, Eva Erhart and Monika Sobotik

Plants 2019, 8(11), 514; https://doi.org/10.3390/plants8110514 - 17 Nov 2019

Cited by 12 | Viewed by 4686

Abstract

Plant root systems are essential for sustainable agriculture, conveying resource-efficient genotypes and species with benefits to soil ecosystem functions. Targeted selection of species/genotypes depends on available root system information. Currently there is no standardized approach for comprehensive root system characterization, suggesting the need for data integration across methods and sources. Here, we combine field measured root descriptors from the classical Root Atlas series with traits from controlled-environment root imaging for 10 cover crop species to (i) detect descriptors scaling between distant experimental methods, (ii) provide traits for species classification, and (iii) discuss implications for cover crop ecosystem functions. Results revealed relation of single axes measures from root imaging (convex hull, primary-lateral length ratio) to Root Atlas field descriptors (depth, branching order). Using composite root variables (principal components) for branching, morphology, and assimilate investment traits, cover crops were classified into species with (i) topsoil-allocated large diameter rooting type, (ii) low-branched primary/shoot-born axes-dominated rooting type, and (iii) highly branched dense rooting type, with classification trait-dependent distinction according to depth distribution. Data integration facilitated identification of root classification variables to derive root-related cover crop distinction, indicating their agro-ecological functions. Full article

(This article belongs to the Special Issue Root System Structure and Function: A Themed Issue in Honor of Professor Yoav Waisel)

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17 pages, 3618 KiB

Open AccessArticle

Two Wheat Cultivars with Contrasting Post-Embryonic Root Biomass Differ in Shoot Re-Growth after Defoliation: Implications for Breeding Grazing Resilient Forages

by Ana Paez-Garcia, Fuqi Liao and Elison B. Blancaflor

Plants 2019, 8(11), 470; https://doi.org/10.3390/plants8110470 - 02 Nov 2019

Cited by 4 | Viewed by 6244

Abstract

The ability of forages to quickly resume aboveground growth after grazing is a trait that enables farmers to better manage their livestock for maximum profitability. Leaf removal impairs root growth. As a consequence of a deficient root system, shoot re-growth is inhibited leading to poor pasture performance. Despite the importance of roots for forage productivity, they have not been considered as breeding targets for improving grazing resilience due in large part to the lack of knowledge on the relationship between roots and aboveground biomass re-growth. Winter wheat (Triticum aestivum) is extensively used as forage source in temperate climates worldwide. Here, we investigated the impact of leaf clipping on specific root traits, and how these influence shoot re-growth in two winter wheat cultivars (i.e., Duster and Cheyenne) with contrasting root and shoot biomass. We found that root growth angle and post-embryonic root growth in both cultivars are strongly influenced by defoliation. We discovered that Duster, which had less post-embryonic roots before defoliation, reestablished its root system faster after leaf cutting compared with Cheyenne, which had a more extensive pre-defoliation post-embryonic root system. Rapid resumption of root growth in Duster after leaf clipping was associated with faster aboveground biomass re-growth even after shoot overcutting. Taken together, our results suggest that lower investments in the production of post-embryonic roots presents an important ideotype to consider when breeding for shoot re-growth vigor in dual purpose wheat. Full article

(This article belongs to the Special Issue Root System Structure and Function: A Themed Issue in Honor of Professor Yoav Waisel)

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15 pages, 2158 KiB

Open AccessEditor’s ChoiceArticle

Early Season Drought Largely Reduces Grain Yield in Wheat Cultivars with Smaller Root Systems

by Victoria Figueroa-Bustos, Jairo A. Palta, Yinglong Chen and Kadambot H.M. Siddique

Plants 2019, 8(9), 305; https://doi.org/10.3390/plants8090305 - 27 Aug 2019

Cited by 25 | Viewed by 3667

Abstract

In the Australian grainbelt, early winter rainfall has declined during the last 30 years, and farmers sow their crops dry, increasing the risk of early season drought. This study aimed to examine whether differences in the root systems were associated with tolerance to early season drought. Three wheat cultivars with different root systems were grown in 1 m columns in a glasshouse. Immediately after sowing in dry soil, 440 mL water (equivalent to 25 mm rainfall) was supplied to each column, and no water was added to induce the early-season drought for the next 30 days. Shoot and root traits were measured at the end of the early season drought, anthesis and at maturity, respectively. The restricted water supply reduced Ψ_leaf, stomatal conductance, leaf photosynthetic rate, shoot and root biomass. Early season drought delayed phenology in all cultivars, but there was recovery of root and shoot biomass at anthesis in all three cultivars. Leaf area and shoot biomass at anthesis in Bahatans-87 (large root system) recovered better than Tincurrin (small root system). At maturity, early season drought reduced grain yield more in Tincurrin than Bahatans-87. The slow phenology of Bahatans-87 allowed greater recovery after the drought in leaf area and shoot biomass, which may explain the smaller reduction in grain yield after early season drought. Full article

(This article belongs to the Special Issue Root System Structure and Function: A Themed Issue in Honor of Professor Yoav Waisel)

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16 pages, 2221 KiB

Open AccessArticle

Clothing the Emperor: Dynamic Root–Shoot Allocation Trajectories in Relation to Whole-Plant Growth Rate and in Response to Temperature

by David Robinson and John Henry Peterkin

Plants 2019, 8(7), 212; https://doi.org/10.3390/plants8070212 - 10 Jul 2019

Cited by 8 | Viewed by 4017

Abstract

We quantified how root–shoot biomass allocation and whole-plant growth rate co-varied ontogenetically in contrasting species in response to cooling. Seven grass and four forb species were grown for 56 days in hydroponics. Growth was measured repeatedly before and after day/night temperatures were reduced at 28 days from 20 °C/15 °C to 10 °C/5 °C; controls remained unchanged. Sigmoid trajectories of root and shoot growth were reconstructed from the experimental data to derive continuous whole-plant relative growth rates (RGRs) and root mass fractions (RMFs). Root mass fractions in cooled plants generally increased, but this originated from unexpected and previously uncharacterised differences in response among species. Root mass fraction and RGR co-trajectories were idiosyncratic in controls and cooled plants. The RGR–RMF co-trajectories responded to cooling in grasses, but not forbs. The RMF responses of stress-tolerant grasses were predictably weak but projected to eventually out-respond faster-growing species. Sigmoid growth constrains biomass allocation. Only when neither root nor shoot biomass is near-maximal can biomass allocation respond to environmental drivers. Near maximum size, plants cannot adjust RMF, which then reflects net above- and belowground productivities. Ontogenetic biomass allocations are not equivalent to those based on interspecific surveys, especially in mature vegetation. This reinforces the importance of measuring temporal growth dynamics, and not relying on “snapshot” comparisons to infer the functional significance of root–shoot allocation. Full article

(This article belongs to the Special Issue Root System Structure and Function: A Themed Issue in Honor of Professor Yoav Waisel)

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16 pages, 2489 KiB

Open AccessArticle

Intraspecific Fine-Root Trait-Environment Relationships across Interior Douglas-Fir Forests of Western Canada

by Camille E. Defrenne, M. Luke McCormack, W. Jean Roach, Shalom D. Addo-Danso and Suzanne W. Simard

Plants 2019, 8(7), 199; https://doi.org/10.3390/plants8070199 - 30 Jun 2019

Cited by 23 | Viewed by 5245

Abstract

Variation in resource acquisition strategies enables plants to adapt to different environments and may partly determine their responses to climate change. However, little is known about how belowground plant traits vary across climate and soil gradients. Focusing on interior Douglas-fir (Pseudotsuga menziesii var. glauca) in western Canada, we tested whether fine-root traits relate to the environment at the intraspecific level. We quantified the variation in commonly measured functional root traits (morphological, chemical, and architectural traits) among the first three fine-root orders (i.e., absorptive fine roots) and across biogeographic gradients in climate and soil factors. Moderate but consistent trait-environment linkages occurred across populations of Douglas-fir, despite high levels of within-site variation. Shifts in morphological traits across regions were decoupled from those in chemical traits. Fine roots in colder/drier climates were characterized by a lower tissue density, higher specific area, larger diameter, and lower carbon-to-nitrogen ratio than those in warmer/wetter climates. Our results showed that Douglas-fir fine roots do not rely on adjustments in architectural traits to adapt rooting strategies in different environments. Intraspecific fine-root adjustments at the regional scale do not fit along a single axis of root economic strategy and are concordant with an increase in root acquisitive potential in colder/drier environments. Full article

(This article belongs to the Special Issue Root System Structure and Function: A Themed Issue in Honor of Professor Yoav Waisel)

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Review

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18 pages, 1622 KiB

Open AccessReview

Reducing Cadmium Accumulation in Plants: Structure–Function Relations and Tissue-Specific Operation of Transporters in the Spotlight

by Xin Huang, Songpo Duan, Qi Wu, Min Yu and Sergey Shabala

Plants 2020, 9(2), 223; https://doi.org/10.3390/plants9020223 - 09 Feb 2020

Cited by 93 | Viewed by 5667

Abstract

Cadmium (Cd) is present in many soils and, when entering the food chain, represents a major health threat to humans. Reducing Cd accumulation in plants is complicated by the fact that most known Cd transporters also operate in the transport of essential nutrients such as Zn, Fe, Mn, or Cu. This work summarizes the current knowledge of mechanisms mediating Cd uptake, radial transport, and translocation within the plant. It is concluded that real progress in the field may be only achieved if the transport of Cd and the above beneficial micronutrients is uncoupled, and we discuss the possible ways of achieving this goal. Accordingly, we suggest that the major focus of research in the field should be on the structure–function relations of various transporter isoforms and the functional assessment of their tissue-specific operation. Of specific importance are two tissues. The first one is a xylem parenchyma in plant roots; a major “controller” of Cd loading into the xylem and its transport to the shoot. The second one is a phloem tissue that operates in the last step of a metal transport. Another promising and currently underexplored avenue is to understand the role of non-selective cation channels in Cd uptake and reveal mechanisms of their regulation. Full article

(This article belongs to the Special Issue Root System Structure and Function: A Themed Issue in Honor of Professor Yoav Waisel)

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16 pages, 888 KiB

Open AccessEditor’s ChoiceReview

Potassium in Root Growth and Development

by Marek Sustr, Ales Soukup and Edita Tylova

Plants 2019, 8(10), 435; https://doi.org/10.3390/plants8100435 - 22 Oct 2019

Cited by 116 | Viewed by 11545

Abstract

Potassium is an essential macronutrient that has been partly overshadowed in root science by nitrogen and phosphorus. The current boom in potassium-related studies coincides with an emerging awareness of its importance in plant growth, metabolic functions, stress tolerance, and efficient agriculture. In this review, we summarized recent progress in understanding the role of K⁺ in root growth, development of root system architecture, cellular functions, and specific plant responses to K⁺ shortage. K⁺ transport is crucial for its physiological role. A wide range of K⁺ transport proteins has developed during evolution and acquired specific functions in plants. There is evidence linking K⁺ transport with cell expansion, membrane trafficking, auxin homeostasis, cell signaling, and phloem transport. This places K⁺ among important general regulatory factors of root growth. K⁺ is a rather mobile element in soil, so the absence of systemic and localized root growth response has been accepted. However, recent research confirms both systemic and localized growth response in Arabidopsis thaliana and highlights K⁺ uptake as a crucial mechanism for plant stress response. K⁺-related regulatory mechanisms, K⁺ transporters, K⁺ acquisition efficiency, and phenotyping for selection of K⁺ efficient plants/cultivars are highlighted in this review. Full article

(This article belongs to the Special Issue Root System Structure and Function: A Themed Issue in Honor of Professor Yoav Waisel)

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24 pages, 3660 KiB

Open AccessEditor’s ChoiceReview

What Makes Adventitious Roots?

by Mathieu Gonin, Véronique Bergougnoux, Thu D. Nguyen, Pascal Gantet and Antony Champion

Plants 2019, 8(7), 240; https://doi.org/10.3390/plants8070240 - 22 Jul 2019

Cited by 73 | Viewed by 28259

Abstract

The spermatophyte root system is composed of a primary root that develops from an embryonically formed root meristem, and of different post-embryonic root types: lateral and adventitious roots. Adventitious roots, arising from the stem of the plants, are the main component of the mature root system of many plants. Their development can also be induced in response to adverse environmental conditions or stresses. Here, in this review, we report on the morphological and functional diversity of adventitious roots and their origin. The hormonal and molecular regulation of the constitutive and inducible adventitious root initiation and development is discussed. Recent data confirmed the crucial role of the auxin/cytokinin balance in adventitious rooting. Nevertheless, other hormones must be considered. At the genetic level, adventitious root formation integrates the transduction of external signals, as well as a core auxin-regulated developmental pathway that is shared with lateral root formation. The knowledge acquired from adventitious root development opens new perspectives to improve micropropagation by cutting in recalcitrant species, root system architecture of crops such as cereals, and to understand how plants adapted during evolution to the terrestrial environment by producing different post-embryonic root types. Full article

(This article belongs to the Special Issue Root System Structure and Function: A Themed Issue in Honor of Professor Yoav Waisel)

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19 pages, 1046 KiB

Open AccessReview

Root Plasticity in the Pursuit of Water

by Hillel Fromm

Plants 2019, 8(7), 236; https://doi.org/10.3390/plants8070236 - 22 Jul 2019

Cited by 44 | Viewed by 7107

Abstract

One of the greatest challenges of terrestrial vegetation is to acquire water through soil-grown roots. Owing to the scarcity of high-quality water in the soil and the environment’s spatial heterogeneity and temporal variability, ranging from extreme flooding to drought, roots have evolutionarily acquired tremendous plasticity regarding their geometric arrangement of individual roots and their three-dimensional organization within the soil. Water deficiency has also become an increasing threat to agriculture and dryland ecosystems due to climate change. As a result, roots have become important targets for genetic selection and modification in an effort to improve crop resilience under water-limiting conditions. This review addresses root plasticity from different angles: Their structures and geometry in response to the environment, potential genetic control of root traits suitable for water-limiting conditions, and contemporary and future studies of the principles underlying root plasticity post-Darwin’s ‘root-brain’ hypothesis. Our increasing knowledge of different disciplines of plant sciences and agriculture should contribute to a sustainable management of natural and agricultural ecosystems for the future of mankind. Full article

(This article belongs to the Special Issue Root System Structure and Function: A Themed Issue in Honor of Professor Yoav Waisel)

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