Rock Fragment Content in Soils Shift Root Foraging Behavior in Xerophytic Species

Aims Root traits associated with resource foraging, including fine-root branching intensity, root hair and mycorrhiza, may change in soils with various physical structure indicated by rock fragment content (RFC), while how these traits covariate at the level of individual root branching order is largely unknown.Methods We subjected two xerophytic species, Artemisia vestita (subshrub) and Bauhinia brachycarpa (shrub), to increasing RFC gradients (0%, 25%, 50% and 75%, v v-1) in an arid environment and measured fine-root traits related to resource foraging.Results Root hair density and mycorrhizal colonization of both species decreased with increasing root order, but increased in 3rd- and 4th-order roots at high RFCs (50% or 75%). The two species tend to produce more root hairs than mycorrhizas under the high RFCs. For both species, root hair density and mycorrhizal colonization intensity were negatively correlated with root length and root diameter. Rockiness reduced root branching intensity in both species comparing with rock-free soil. At the same level of RFC, A. vestita had thicker roots and lower branching intensity than B. brachycarpa, and tended to produce more root hairs.Conclusion Our results suggest the high RFC soil conditions stimulated greater foraging functions in higher root orders. We found evidence for a greater investment in root hairs and mycorrhizal symbioses as opposed to building an extensive root system in rocky soils. The subshrub and shrub species took different approaches to foraging in the rocky soil through distinctive trait syndromes of fine-root components.

The Steroid Saponin Protodioscin Modulates Arabidopsis thaliana Root Morphology Altering Auxin Homeostasis, Transport and Distribution

To date, synthetic herbicides are the main tools used for weed control, with consequent damage to both the environment and human health. In this respect, searching for new natural molecules and understanding their mode of action could represent an alternative strategy or support to traditional management methods for sustainable agriculture. Protodioscin is a natural molecule belonging to the class of steroid saponins, mainly produced by monocotyledons. In the present paper, protodioscin’s phytotoxic potential was assessed to identify its target and the potential mode of action in the model plant Arabidopsis thaliana. The results highlighted that the root system was the main target of protodioscin, which caused a high inhibitory effect on the primary root length (ED50 50 μM) with morphological alteration, accompanied by a significant increase in the lateral root number and root hair density. Through a pharmacological and microscopic approach, it was underlined that this saponin modified both auxin distribution and transport, causing an auxin accumulation in the region of root maturation and an alteration of proteins responsible for the auxin efflux (PIN2). In conclusion, the saponin protodioscin can modulate the root system of A. thaliana by interfering with the auxin transport (PAT).

Abscisic Acid Mediates Drought-Enhanced Rhizosheath Formation in Tomato

The rhizosheath, commonly defined as soil adhering to the root surface, may confer drought tolerance in various crop species by enhancing access to water and nutrients under drying stress conditions. Since the role of phytohormones in establishing this trait remains largely unexplored, we investigated the role of ABA in rhizosheath formation of wild-type (WT) and ABA-deficient (notabilis, not) tomatoes. Both genotypes had similar rhizosheath weight, root length, and root ABA concentration in well-watered soil. Drying stress treatment decreased root length similarly in both genotypes, but substantially increased root ABA concentration and rhizosheath weight of WT plants, indicating an important role for ABA in rhizosheath formation. Neither genotype nor drying stress treatment affected root hair length, but drying stress treatment decreased root hair density of not. Under drying stress conditions, root hair length was positively correlated with rhizosheath weight in both genotypes, while root hair density was positively correlated with rhizosheath weight in well-watered not plants. Root transcriptome analysis revealed that drought stress increased the expression of ABA-responsive transcription factors, such as AP2-like ER TF, alongside other drought-regulatory genes associated with ABA (ABA 8′-hydroxylase and protein phosphatase 2C). Thus, root ABA status modulated the expression of specific gene expression pathways. Taken together, drought-induced rhizosheath enhancement was ABA-dependent, but independent of root hair length.

First report of Pythium myriotylum causing root rot in Cannabis sativa (L.) in California

In June of 2020 we observed greenhouse grown Cannabis sativa in Sonoma County, CA and Monterey County, CA showing stress symptoms: stunting, leaf chlorosis, and leaf senescence, when moved to flower production conditions. We uprooted symptomatic and healthy plants and observed disease symptoms only in symptomatic plants: reduced root mass, reduced root hair density, and necrosis. Roots and growth substrate samples were taken from infected and healthy plants for further analysis. Approximately one gram of soil was added to 20.0 mL deionized water and 5.0 mL of the resulting slurry was spread on water agar plates. Plates were rinsed free of soil after 24 hours of incubation, then incubated for an additional two days. Diffuse mycelial growth was observed on all soil plates from symptomatic plant pots and not healthy plant pots. Four subcultures were transferred to V8 media and grown for three days. Roots with brown lesions and healthy roots were surface sterilized by soaking in 0.1% sodium hypochlorite for five minutes, rinsed in sterile deionized water, and two-centimeter segments plated on V8 agar. After 24 hours mycelial growth was observed growing from the cut ends of the lesion roots and not the healthy-looking roots. Four subcultures were transferred to V8 and grown for three days. Mycelium from water sample isolates and root isolates were collected and DNA extractions performed using Quick DNA Fungi/Bacterial Kit (Zymo Research Irvine, CA, USA), then PCR amplified using ribosomal internal transcribed spacer (ITS) primers ITS100/ITS4 as described by Riit et al. and cytochrome oxidase I (COX) primers OomCox-Levup/OomCox-Levlo as described by Robideau et al. (Riit et al., 2016; Robideau et al., 2011). Amplicons from all eight isolates for each region were Sanger sequenced and the found to be identical, and consensus sequences deposited in Genebank with accession numbers MW436422 and MW448569 for ITS and COX sequences, respectively. Observation of cultures under light microscope revealed morphological characteristics congruent with P. myriotylum (Watanabe, 2002). Two V8 cultures isolated from roots were cut into approximately 2-3mm2 pieces and transferred to a one-liter flask of water. A negative control using clean V8 was also prepared. The flasks were placed on a rotary shaker and incubated at 150 rpm at ambient temperature for 48 hours. The resulting suspensions for zoospore and control treatments were observed under a light microscope and motile zoospores present in the water suspension from P. myriotylum cultures only. The zoospore suspension was then divided into six equal portions and applied to the soil of six C. sativa rooted cuttings in one-gallon pots. The control slurry was added to two C. sativa rooted cuttings. All plants were grown in a controlled environment for 28 days with 16-hour photoperiod. All plants were then removed from their pots and roots observed for symptoms. Plants that were treated with zoospore suspensions had tan to brown lesions on significant numbers of roots, and reduced root hair density compared to the plants treated with the control V8 agar suspension. Roots samples from all eight plants were then surfaced sterilized in bleach as previously described and five root sections from each plant plated on V8 media. After 48 hours mycelial growth was observed from root sections from P. myriotylum zoospore treated plants and not control plants. DNA extraction and PCR amplification for ITS and COX amplicons was performed from one representative culture for each plant, Sanger sequenced, and aligned with the previous sequences. All ITS and COX sequences were identical to the original sequences from greenhouse samples. P. myriotylum may cause root rot in C. sativa in greenhouse cultivation.

Vascular transcription factors guide plant epidermal responses to limiting phosphate conditions

Optimal plant growth is hampered by deficiency of the essential macronutrient phosphate in most soils. Plant roots can, however, increase their root hair density to efficiently forage the soil for this immobile nutrient. By generating and exploiting a high-resolution single-cell gene expression atlas of Arabidopsis roots, we show an enrichment of TARGET OF MONOPTEROS 5/LONESOME HIGHWAY (TMO5/LHW) target gene responses in root hair cells. The TMO5/LHW heterodimer triggers biosynthesis of mobile cytokinin in vascular cells and increases root hair density during low-phosphate conditions by modifying both the length and cell fate of epidermal cells. Moreover, root hair responses in phosphate-deprived conditions are TMO5- and cytokinin-dependent. Cytokinin signaling links root hair responses in the epidermis to perception of phosphate depletion in vascular cells.

Diverse phenotypic responses and phosphate content in foxtail millet genotypes under greenhouse and field conditions

Phosphorous (P) is an important macronutrient for the growth of all agricultural crops. This study reports phenotype analysis for P responses in field (two different seasons, monsoon and summer) and greenhouse, using 54 genotypes of foxtail millet (Setaria italica) under P-fertilized (P+) and unfertilized (P-) conditions. Variation was seen for plant height, leaf number and length, tillering ability and seed yield traits. Genotypes ISe 1234 and ISe 1541 were P+ responders, and the genotypes ISe 1181, ISe 1655, ISe 783 and ISe 1892 tend more towards low P tolerance for total seed yield. Genotypes that performed well under P-conditions were almost as productive as genotypes that performed well under P+ conditions suggesting some genotypes are well adapted to nutrient-poor soils. In the greenhouse, significant variation was seen for root hair density and root hair number and for fresh and dry weights of shoot and root under P-stress. However, there was not much difference in the shoot and root total P and inorganic phosphate (Pi) levels of five selected high and low responding genotypes. In the root and leaf tissues, total P and Pi contents of five high responding genotypes were higher than the five low responding genotypes.HighlightEnormous phenotypic and phosphate content variation of foxtail millet under low-phosphate supply in greenhouse and natural field conditions identifies genotypic plasticity for future breeding for improved P use efficiency.

KAI2 regulates root and root hair development by modulating auxin distribution

ABSTRACTStrigolactones (SLs) are endogenous signalling molecules that play important roles in controlling plant development. SL perception is closely related to that of karrikins, smoke-derived compounds presumed to mimic endogenous signalling molecules (KLs). SLs have been suggested to regulate root development. However, perception of both molecules requires the F-box protein MAX2 and the use of max2 mutants has hampered defining the exact role of SLs in roots. Here we dissect the role of SL and KL signalling in Arabidopsis root development using mutants defective in the α/β hydrolase receptors D14 and KAI2, which specifically perceive SLs and KLs, respectively. Both pathways together regulate lateral root density (LRD), but contrary to previous reports, KL signalling alone controls root hair density, root hair length and additionally root skewing, straightness and diameter. Members of the SMXL protein family are downstream targets of SL (SMXL6, 7, 8) and KL (SMAX1, SMXL2) signalling. We identified distinct and overlapping roles of these proteins in the regulation of root development. Both SMAX1/SMXL2 and SMXL6/SMXL7/SMXL8 regulate LRD, confirming that SL and KL signalling act together to regulate this trait, while the KL-signalling specific SMAX1 and SMXL2 regulate all other investigated root traits. Finally, we show that KL signalling regulates root hair development by modulating auxin distribution within the root.

Increased root hair density by loss of WRKY6 in Arabidopsis thaliana

Root hairs are unicellular elongations of certain rhizodermal cells that improve the uptake of sparingly soluble and immobile soil nutrients. Among different Arabidopsis thaliana genotypes, root hair density, length and the local acclimation to low inorganic phosphate (Pi) differs considerably, when analyzed on split agar plates. Here, genome-wide association fine mapping identified significant single nucleotide polymorphisms associated with the increased root hair density in the absence of local phosphate on chromosome 1. A loss-of-functionmutant of the candidate transcription factor gene WRKY6, which is involved in the acclimation of plants to low phosphorus, had increased root hair density. This is partially explained by a reduced cortical cell diameter in wrky6-3, reducing the rhizodermal cell numbers adjacent to the cortical cells. As a consequence, rhizodermal cells in positions that are in contact with two cortical cells are found more often, leading to higher hair density. Distinct cortical cell diameters and epidermal cell lengths distinguish other Arabidopsis accessions with distinct root hair density and −Pi response from diploid Col-0, while tetraploid Col-0 had generally larger root cell sizes, which explain longer hairs. A distinct radial root morphology within Arabidopsis accessions and wrky6-3explains some, but not all, differences in the root hair acclimation to –Pi.