Interacted Effect of Arbuscular Mycorrhizal Fungi and Polyamines on Root System Architecture of Citrus Seedlings

Either arbuscular mycorrhizal fungi （AMF） or polyamines （PAs） may change root system architecture （RSA） of plants, whereas the interaction of AMF and PAs on RSA remains unclear. In the present study, we studied the interaction between AMF （Paraglomus occultum） and exogenous PAs, including putrescine （Put）, spermidine （Spd） and spermine （Spin） on mycorrhizal development of different parts of root system, plant growth, RSA and carbohydrate concentrations of 6-m-old citrus （Citrus tangerine Hort. ex Tanaka） seedlings. After 14 wk of PAs application, PA-treated mycorrhizal seedlings exhibited better mycorrhizal colonization and numbers of vesicles, arbuscules, and entry points, and the best mycorrhizal status of taproot, first-, second-, and third-order lateral roots was respectively found in mycorrhizal seedlings supplied with Put, Spd and Spm, suggesting that PAs might act as a regulated factor of mycorrhizal development through transformation of root sucrose more into glucose for sustaining mycorrhizal development. AMF usually notably increases RSA traits （taproot length, total length, average diameter, projected area, surface area, volume, and number of first-, second-, and third-order lateral roots） of only PA-treated seedlings. Among the three PA species, greater positive effects on RSA change and plant biomass increment of the seedlings generally rank as Spd〉Spm〉Put, irrespective of whether or not AMF colonization. PAs significantly changed the RSA traits in mycorrhizal but not in non-mycorrhizal seedlings. It suggests that the application of PAs （especially Spd） to AMF plants would optimize RSA of citrus seedlings, thus increasing plant growth （shoot and root dry weight）.

Identification of QTL and underlying genes for root system architecture associated with nitrate nutrition in hexaploid wheat

The root system architecture(RSA) of a crop has a profound effect on the uptake of nutrients and consequently the potential yield. However, little is known about the genetic basis of RSA and resource adaptive responses in wheat(Triticum aestivum L.). Here, a high-throughput germination paper-based plant phenotyping system was used to identify seedling traits in a wheat doubled haploid mapping population, Savannah×Rialto. Significant genotypic and nitrate-N treatment variation was found across the population for seedling traits with distinct trait grouping for root size-related traits and root distribution-related traits. Quantitative trait locus(QTL) analysis identified a total of 59 seedling trait QTLs. Across two nitrate treatments, 27 root QTLs were specific to the nitrate treatment. Transcriptomic analyses for one of the QTLs on chromosome 2 D, which was found under low nitrate conditions, revealed gene enrichment in N-related biological processes and 28 differentially expressed genes with possible involvement in a root angle response. Together, these findings provide genetic insight into root system architecture and plant adaptive responses to nitrate, as well as targets that could help improve N capture in wheat.

Constitutive basis of root system architecture:uncovering a promising trait for breeding nutrient-and drought-resilient crops

Root system architecture(RSA)plays a pivotal role in efficient uptake of essential nutrients,such as phosphorous(P),nitrogen(N),and water In soils with heterogeneous nutrient distribution,root plasticity can optimize acquisition and plant growth.Here,we present evidence that a constitutive RSA can confer benefits for sorghum grown under both sufficient and limiting growth conditions.Our studies,using P efficient SC103 and inefficient BTx635 sorghum cultivars,identified significant differences in root traits,with SC103 developing a larger root system with more and longer lateral roots,and enhanced shoot biomass,under both nutrient sufficient and deficient conditions.In addition to this constitutive attribute,under P deficiency,both cultivars exhibited an initial increase in lateral root development;however,SC103 still maintained the larger root biomass.Although N deficiency and drought stress inhibited both root and shoot growth,for both sorghum cultivars,SC103 again maintained the better performance.These findings reveal that SC103,a P efficient sorghum cultivar,also exhibited enhanced growth performance under N deficiency and drought.Our results provide evidence that this constitutive nature of RSA can provide an avenue for breeding nutrient-and drought-resilient crops.

Genetic dissection of maize seedling root system architecture traits using an ultra-high density bin-map and a recombinant inbred line population

Maize（Zea mays） root system architecture（RSA）mediates the key functions of plant anchorage and acquisition of nutrients and water. In this study,a set of 204 recombinant inbred lines（RILs） was derived from the widely adapted Chinese hybrid ZD958（Zheng58 Chang7-2）,genotyped by sequencing（GBS） and evaluated as seedlings for 24 RSA related traits divided into primary,seminal and total root classes. Signi ficant differences between the means of the parental phenotypes were detected for 18 traits,and extensive transgressive segregation in the RIL population was observed for all traits. Moderate to strong relationships among the traits were discovered. A total of 62 quantitative trait loci（QTL） were identi fied that individually explained from1.6% to 11.6%（total root dry weight/total seedling shoot dry weight） of the phenotypic variation. Eighteen,24 and 20 QTL were identi fied for primary,seminal and total root classes of traits,respectively. We found hotspots of 5,3,4 and 12 QTL in maize chromosome bins 2.06,3.02-03,9.02-04,and 9.05-06,respectively,implicating the presence of root gene clusters or pleiotropic effects. These results characterized the phenotypic variation and genetic architecture of seedling RSA in a population derived from a successful maize hybrid.

ROLE OF NITROGEN SENSING AND ITS INTEGRATIVE SIGNALING PATHWAYS IN SHAPING ROOT SYSTEM ARCHITECTURE

The Green Revolution of the 1960s boosted crop yields in part through widespread production of semidwarf plant cultivars and extensive use of mineral fertilizers.The beneficial semidwarfism of cereal Green Revolution cultivars is due to the accumulation of plant growth-repressing DELLA proteins,which increases lodging resistance but requires a high-nitrogen fertilizer to obtain high yield.Given that environmentally degrading fertilizer use underpins current worldwide crop production,future agricultural sustainability needs a sustainable Green Revolution through reducing N fertilizer use while boosting grain yield above what is currently achievable.Despite a great deal of research efforts,only a few genes have been demonstrated to improve N-use efficiency in crops.The molecular mechanisms underlying the coordination between plant growth and N metabolism is still not fully understood,thus preventing significant improvement.Recent advances of how plants sense,capture and respond to varying N supply in model plants have shed light on how to improve sustainable productivity in agriculture.This review focuses on the current understanding of root developmental and metabolic adaptations to N availability,and discuss the potential approaches to improve N-use efficiency in high-yielding cereal crops.

Statistical modeling of nitrogen-dependent modulation of root system architecture in Arabidopsis thaliana

Plant root development is strongly affected by nutrient availability. Despite the importance of structure and function of roots in nutrient acquisition,statistical modeling approaches to evaluate dynamic and temporal modulations of root system architecture in response to nutrient availability have remained as widely open and exploratory areas in root biology. In this study,we developed a statistical modeling approach to investigate modulations of root system architecture in response to nitrogen availability. Mathematical models were designed for quantitative assessment of root growth and root branching phenotypes and their dynamic relationships based on hierarchical con figuration of primary and lateral roots formulating the fishbone-shaped root system architecture in Arabidopsis thaliana. Time-series datasets reporting dynamic changes in root developmental traits on different nitrate or ammonium concentrations were generated for statistical analyses. Regression analyses unraveled key parameters associated with：（i） inhibition of primary root growth under nitrogen limitation or on ammonium;（ii） rapid progression of lateral root emergence in response to ammonium; and（iii） inhibition of lateral root elongation in the presence of excess nitrate or ammonium. This study provides a statistical framework for interpreting dynamic modulation of root system architecture,supported by metaanalysis of datasets displaying morphological responses of roots to diverse nitrogen supplies.

Evolving technologies for growing,imaging and analyzing 3D root system architecture of crop plants

A plant＇s ability to maintain or improve its yield under limiting conditions,such as nutrient de ficiency or drought,can be strongly in fluenced by root system architecture（RSA）,the three-dimensional distribution of the different root types in the soil. The ability to image,track and quantify these root system attributes in a dynamic fashion is a useful tool in assessing desirable genetic and physiological root traits. Recent advances in imaging technology and phenotyping software have resulted in substantive progress in describing and quantifying RSA. We have designed a hydroponic growth system which retains the three-dimensional RSA of the plant root system,while allowing for aeration,solution replenishment and the imposition of nutrient treatments,as well as high-quality imaging of the root system. The simplicity and flexibility of the system allows for modi fications tailored to the RSA of different crop species and improved throughput. This paper details the recent improvements and innovations in our root growth and imaging system which allows for greater image sensitivity（detection of fine roots and other root details）,higher ef ficiency,and a broad array of growing conditions for plants that more closely mimic those found under field conditions.

A protocol of field-based phenotyping procedure for no-till wheat root system architecture based on data-driven model-assist

Field-based phenotyping(FBP)of crop root systemarchitecture(RSA)provides away to quantify the root growth and distribution in fieldwith a smaller scale.Studies on a better understanding of the interrelations between field crop root physiological traits,root developmental phases and environmental changes are hindered due to deficiency of in situ root system architecture testing and quantitative methods for field crop.The present study aimed to propose a protocol for field-based wheat root system architecture with technical details of key operational procedures.Phenotyping of RSA traits from root spatial coordinate data acquisition and visualization software presented scaled illustrations of wheat RSA dynamics and root developmental phases which also revealed the root topological heterogeneities,eitherwithin a plant oramong individuals.Percentage of horizontal and vertical soil coverage by root showed that root foraging capability along soil depth was better than within the horizontal dimension.In brief,our data indicated that FBP ofwheat RSA could be achieved using the protocol of datadriven model-assisted phenotyping procedure.The proposed protocol was demonstrated useful for FBP of RSAs.It was proved effective to illustrate the topological structures of the wheat root system and to quantify RSAderived parameters,this could be a useful tool for characterizing and analyzing the structural distortion,heterogeneous distribution and the soil space exploration characteristics of wheat root.

Development of Root Phenotyping Platforms for Identification of Root Architecture Mutations in EMS-Induced and Low-Path-Sequenced Sorghum Mutant Population

Sorghum’s natural adaptation to a wide range of abiotic stresses provides diverse genetic reserves for potential improvement in crop stress tolerance. Growing interest in sorghum research has led to the expansion of genetic resources though establishment of the sorghum association panel (SAP), generation of mutagenized populations, and recombinant inbred line (RIL) populations, etc. Despite rapid improvement in biotechnological tools, lack of efficient phenotyping platforms remains one of the major obstacles in utilizing these genetic resources. Scarcity of efforts in root system phenotyping hinders identification and integration of the superior root traits advantageous to stress tolerance. Here, we explored multiple approaches in root phenotyping of an ethyl methanesulfonate (EMS)-mutagenized sorghum population. Paper-based growth pouches (PGP) and hydroponics were employed to analyze root system architecture (RSA) variations induced by mutations and to test root development flexibility in response to phosphorus deficiency in early growing stages. PGP method had improved capabilities compared to hydroponics providing inexpensive, space-saving, and high-throughput phenotyping of sorghum roots. Preliminary observation revealed distinct phenotypic variations which were qualitatively and quantitatively systemized for association analysis. Phenotypes/ideotypes with root architecture variations potentially correlated with Pi acquisition were selected to evaluate their contribution to P-efficiency (PE). Sand mixed with P-loaded activated alumina substrate (SAS) provided closely to natural but still controlled single-variable conditions with regulated Pi availability. Due to higher labor and cost input we propose SAS to be used for evaluating selected sorghum candidates for PE. The ability of rapidly screening root phenotypes holds great potential for discovering genes responsible for relevant root traits and utilizing mutations to improve nutrient efficiency and crop productivity.

Phenotyping of Weedy Rice to Assess Root Characteristics Associated with Allelopathy

Weedy rice is a species of Oryza, and is a wild relative of cultivated rice. The weed possesses unique hardiness that allows them to thrive in dynamic and stressful environments. These characteristics suggest that weedy rice is a stored source of novel genes for competitive traits. One such trait is allelopathy, where a species releases secondary metabolites that suppress the growth and development of neighboring species. Weed competition is a limiting factor in rice production systems;therefore, it is critical to identify specific allelopathic weedy rice accessions to determine the genetic pathways and mechanisms associated with allelopathy to be used in breeding programs. Due to the complex nature of allelochemical production and the lack of knowledge of allelopathy mechanisms in weedy rice, phenotypic traits, particularly root traits, can be used to overcome this limitation and serve as target characteristics for breeding weed suppressive rice varieties. Five weedy rice accessions were chosen from preliminary screenings of larger sample sizes with the ability to suppress barnyardgrass weed seedling growth. Another five weedy rice with low barnyardgrass suppression was selected for the current root phenotypic study. Five cultivated rice lines were used as a comparison. All plants were propagated in a transparent germination pouch for four weeks. Roots were scanned and analyzed for root length and area covered. No differences were found in the seedling root area among weedy rice and rice accessions;however, allelopathic weedy rice plants exhibited a 14% increase in root length than non-allelopathic weedy rice plants. The allelopathic weedy rice accession B2 possessed the most extended root system (22.4 cm root length). The highly allelopathic weedy rice accessions (including B2) screened and phenotyped in this study are ideal candidates for identifying the genetic controls of early root length, a possible trait contributing to underground allelopathic production and competitive advantage.

The mitigation effects of exogenous dopamine on low nitrogen stress in Malus hupehensis

Dopamine plays numerous physiological roles in plants.We explored its role in the regulation of growth,nutrient absorption,and response to nitrogen(N)deficiency in Malus hupehensis Rehd.Under low N condition,plant growth slowed,and the net photosynthetic rates,chlorophyll contents,and maximal quantum yield of PSII(Fv/Fm)decreased significantly.However,the application of 100μmol L−1 exogenous dopamine significantly reduced the inhibition of low N stress on plant growth.In addition to modifying root system architecture under low N supply,exogenous dopamine also changed the uptake,transport,and distribution of N,P,and K.Furthermore,exogenous dopamine enhances the tolerance to low nitrogen stress by increasing the activity of enzymes(nitrate reductase,nitrite reductase,glutamic acid synthase and glutamine synthetase)involved in N metabolism.We also found that exogenous dopamine promoted the expression of ethylene signaling genes(ERF1,ERF2,EIL1,ERS2,ETR1,and EIN4)under low N stress.Therefore,we hypothesized that ethylene might be involved in dopamine response to low N stress in M.hupehensis.Our results suggest that exogenous dopamine can mitigate low N stress by regulating the absorption of mineral nutrients,possibly through the regulation of the ethylene signaling pathway.

Rice roots avoid asymmetric heavy metal and salinity stress via an RBOH-ROS-auxin signaling cascade

Root developmental plasticity is crucial for plants to adapt to a changing soil environment,where nutrients and abiotic stress factors are distributed heterogeneously.How plant roots sense and avoid heterogeneous abiotic stress in soil remains unclear.Here,we show that,in response to asymmetric stress of heavy metals(cadmium,copper,or lead)and salt,rice roots rapidly proliferate lateral roots(LRs)in the stress-free area,thereby remodeling root architecture to avoid localized stress.Imaging and quantitative analyses of reactive oxygen species(ROS)showed that asymmetric stress induces a ROS burst in the tips of the exposed roots and simultaneously triggers rapid systemic ROS signaling to the unexposed roots.Addition of a ROS scavenger to either the stressed or stress-free area abolished systemic ROS signaling and LR proliferation induced by asymmetric stress.Asymmetric stress also enhanced cytosolic calcium(Ca^(2+))signaling;blocking Ca^(2+)signaling inhibited systemic ROS propagation and LR branching in the stress-free area.We identified two plasma-membrane-localized respiratory burst oxidase homologs,OsRBOHA and OsRBOHI,as key players in systemic ROS signaling under asymmetric stress.Expression of OsRBOHA and OsRBOHI in roots was upregulated by Cd stress,and knockout of either gene reduced systemic ROS signaling and LR proliferation under asymmetric stress.Furthermore,we demonstrated that auxin signaling and cell wall remodeling act downstream of the systemic ROS signaling to promote LR development.Collectively,our study reveals an RBOH-ROS-auxin signaling cascade that enables rice roots to avoid localized stress of heavy metals and salt and provides new insight into root system plasticity in heterogenous soil.

Morphological and kinetic parameters of the absorption of nitrogen forms for selection of Eucalyptus clones

Eucalyptus clones are selected according to productivity,wood quality,rooting capacity,and resistance to drought,frost and diseases.However,kinetic and morphological parameters that determine the absorption efficiency of nutrients such as nitrate(NO_(3)^(-)) and ammonium(NH_(4)^(+))are often not considered in breeding programs.The objective of this study was to evaluate the morphological,physiological and kinetic parameters of nitrogen uptake by clones of Eucalyptus saligna(32,864) and Eucalyptus grandis(GPC23).Morphological parameters in shoot and root systems,biomass and N concentrations in different organs,photosynthetic pigment concentrations,parameters of chlorophyll a fluorescence and photosynthetic rates were evaluated.Kinetic parameters,maximum absorption velocity(V_(max)),Michaelis-Menten constant(K_(m)),minimum concentration(C_(min)) and influx(I) were calculated for NO_(3)^(-)and NH_(4)^(+) in the two clones.E.granais clone was more efficient in the uptake of NO_(3)^(-)and NH_(4)^(+),and showed lower K_(m) and C_(min)values,allowing for the absorption of nitrogen at low concentrations due to the high affinity of the absorption sites of clone roots to NO_(3)^(-)and NH_(4)^(+).Higher root lengths,area and volume helped the E.grandis clone in absorption efficiency and consequently,resulted in higher root and shoot biomass.The E.saligna clone had higher K_(m) and Cmin for NO_(3)^(-)and NH_(4)^(+),indicating adaptation to environments with higher N availability.The results of NO_(3)^(-)and NH_(4)^(+) kinetic parameters indicate that they can be used in Eucalyptus clone selection and breeding programs as they can predict the ability of clones to absorb NO_(3)^(-)and NH_(4)^(+) at different concentrations.

MADS-Box Transcription Factor AGL21 Regulates Lateral Root Development and Responds to Multiple External and Physiological Signals

Plant root system morphology is dramatically influenced by various environmental cues. The adaptation of root system architecture to environmental constraints, which mostly depends on the formation and growth of lateral roots, is an important agronomic trait. Lateral root development is regulated by the external signals coordinating closely with intrinsic signaling pathways. MADS-box transcription factors are known key regulators of the transition to flowering and flower development. However, their functions in root development are still poorly understood. Here we report that AGL21, an AGL17-clade MADS-box gene, plays a crucial role in lateral root development. AGL21 was highly expressed in root, particularly in the root central cylinder and lateral root primordia. AGL21 overexpression plants produced more and longer lateral roots while ag121 mutants showed impaired lateral root development, especially under nitrogen-deficient conditions. AGL21 was induced by many plant hormones and environmental stresses, suggesting a function of this gene in root system plasticity in response to various signals. Furthermore, AGL21 was found positively regulating auxin accumulation in lateral root primordia and lateral roots by enhancing local auxin biosynthesis, thus stimulating lateral root initiation and growth. We propose that AGL21 may be involved in various environmental and physiological signals-mediated lateral root development and growth.

Root developmental responses to phosphorus nutrition

Phosphorus is an essential macronutrient for plant growth and development. Root system architecture(RSA) affects a plant's ability to obtain phosphate, the major form of phosphorus that plants uptake. In this review, I first consider the relationship between RSA and plant phosphorusacquisition efficiency, describe how external phosphorus conditions both induce and impose changes in the RSA of major crops and of the model plant Arabidopsis, and discuss whether shoot phosphorus status affects RSA and whether there is a universal root developmental response across all plant species. I then summarize the current understanding of the molecular mechanisms governing root developmental responses to phosphorus deficiency. I also explore the possible reasons for the inconsistent results reported by different research groups and comment on the relevance of some studies performed under laboratory conditions to what occurs in natural environments.

L-Cysteine inhibits root elongation through auxin/PLETHORA and SCR/SHR pathway in Arabidopsis thaliana

L-Cysteine plays a prominent role in sulfur metabo- lism of plants. However, its role in root development is largely unknown. Here, we report that L-cysteine reduces primary root growth in a dosage-dependent manner. Elevating cellular L-cysteine level by exposing Arabidopsis thaliana seedlings to high L-cysteine, buthionine sulphoximine, or O-acetylserine leads to altered auxin maximum in root tips, the expression of quiescent center cell marker as well as the decrease of the auxin carriers PIN1, PIN2, PIN3, and PIN7 of primary roots. We also show that high L-cysteine significantly reduces the protein level of two sets of stem cell specific transcription factors PLETHORA1/2 and SCR/SHR. However, L-cysteine does not downregulate the transcript level of PiNs, PLTs, or SCR/SHR, suggesting that an uncharacterized post-transcriptional mechanism may regulate the accumulation of PIN, PLT, and SCR/SHR proteins and auxin transport in the root tips. These results suggest that endogenous L-cysteine level acts to maintain root stem cell niche by regulating basal- and auxin-induced expression of PLT1/2 and 5CR/SHR. L-Cysteine may serve as a link between sulfate assimilation and auxin in regulating root growth.

Long-distance blue light signalling regulates phosphate deficiency-induced primary root growth inhibition

Although roots are mainly embedded in the soil, recent studies revealed that light regulates mineral nutrient uptake by roots. However, it remains unclear whether the change in root system architecture in response to different rhizosphere nutrient statuses involves light signaling. Here, we report that blue light regulates primary root growth inhibition under phosphate-deficient conditions through the cryptochromes and their downstream signaling factors. We showed that the inhibition of root elongation by low phosphate requires blue light signal perception at the shoot and transduction to the root. In this process, SPA1 and COP1 play a negative role while HY5 plays a positive role. Further experiments revealed that HY5 is able to migrate from the shoot to root and that the shoot-derived HY5 autoactivates root HY5 and regulates primary root growth by directly activating the expression of LPR1, a suppressor of root growth under phosphate starvation. Taken together, our study reveals a regulatory mechanism by which blue light signaling regulates phosphate deficiency-induced primary root growth inhibition, providing new insights into the crosstalk between light and nutrient signaling.

Sulfur nutrient availability regulates root elongation by affecting root indole-3-acetic acid levels and the stem cell niche

Sulfur is an essential macronutrient for plants with numerous biological functions. However, the influence of sulfur nutrient availability on the regulation of root development remains largely unknown. Here, we report the response of Arabidopsis thaliana L. root development and growth to different levels of sulfate, demonstrating that low sulfate levels promote the primary root elongation. By using various reporter lines, we examined in vivo IAA level and distribution, cell division,and root meristem in response to different sulfate levels.Meanwhile the dynamic changes of in vivo cysteine, glutathione,and IAA levels were measured. Root cysteine, glutathione, and IAA levels are positively correlated with external sulfate levels in the physiological range, which eventually affect root system architecture. Low sulfate levels also downregulate the genes involved in auxin biosynthesis and transport, and elevate the accumulation of PLT1 and PLT2. This study suggests that sulfate level affects the primary root elongation by regulating the endogenous auxin level and root stem cell niche maintenance.

The Salt Overly Sensitive （SOS） Pathway： Established and Emerging Roles

Soil salinity is a growing problem around the world with special relevance in farmlands. The ability to sense and respond to environmental stimuli is among the most fundamental processes that enable plants to survive. At the cellular level, the Salt Overly Sensitive （SOS） signaling pathway that comprises SOS3, SOS2, and SOS1 has been proposed to mediate cellular signaling under salt stress, to maintain ion homeostasis. Less well known is how cellularly heterog- enous organs couple the salt signals to homeostasis maintenance of different types of cells and to appropriate growth of the entire organ and plant. Recent evidence strongly indicates that different regulatory mechanisms are adopted by roots and shoots in response to salt stress. Several reports have stated that, in roots, the SOS proteins may have novel roles in addition to their functions in sodium homeostasis. SOS3 plays a critical role in plastic development of lateral roots through modulation of auxin gradients and maxima in roots under mild salt conditions. The SOS proteins also play a role in the dynamics of cytoskeleton under stress. These results imply a high complexity of the regulatory networks involved in plant response to salinity. This review focuses on the emerging complexity of the SOS signaling and SOS protein functions, and highlights recent understanding on how the SOS proteins contribute to different responses to salt stress besides ion homeostasis.

Plant phosphate nutrition:sensing the stress

Phosphorus(P)is obtained by plants as phosphate(Pi)from the soil and low Pi levels affects plant growth and development.Adaptation to low Pi condition entails sensing internal and external Pi levels and translating those signals to molecular and morphophysiological changes in the plant.In this review,we present findings related to local and systemin Pi sensing with focus the molecular mechanisms behind root system architectural changes and the impact of hormones and epigenetic mechanisms affecting those changes.We also present some of the recent advances in the Pi sensing and signaling mechanisms focusing on inositol pyrophosphate InsP8 and its interaction with SPX domain proteins to regulate the activity of the central regulator of the Pi starvation response,PHR.