Lateral Root Inhibition Research Articles

Low dose ribosomal DNA P-loop mutation affects development and enforces autophagy in Arabidopsis

ABSTRACT Arabidopsis contains hundreds of ribosomal DNA copies organized within the nucleolar organizing regions (NORs) in chromosomes 2 and 4. There are four major types of variants of rDNA, VAR1–4, based on the polymorphisms of 3’ external transcribed sequences. The variants are known to be differentially expressed during plant development. We created a mutant by the CRISPR-Cas9-mediated excision of ~ 25 nt from predominantly NOR4 ribosomal DNA copies, obtaining mosaic mutational events on ~ 5% of all rDNA copies. The excised region consists of P-loop and Helix-82 segments of 25S rRNA. The mutation led to allelic, dosage-dependent defects marked by lateral root inhibition, reduced size, and pointy leaves, all previously observed for defective ribosomal function. The mutation in NOR4 led to dosage compensation from the NOR2 copies by elevated expression of VAR1 in mutants and further associated single-nucleotide variants, thus, resulting in altered rRNA sub-population. Furthermore, the mutants exhibited rRNA maturation defects specifically in the minor pathway typified by 32S pre-rRNA accumulation. Density-gradient fractionation and subsequent RT-PCR of rRNA analyses revealed that mutated copies were not incorporated into the translating ribosomes. The mutants in addition displayed an elevated autophagic flux as shown by the autophagic marker GFP-ATG8e, likely related to ribophagy.

Gibberellin signaling mediates lateral root inhibition in response to K+-deprivation.

The potassium ion (K+) is vital for plant growth and development, and K+-deprivation leads to reduced crop yields. Here we describe phenotypic, transcriptomic, and mutant analyses to investigate the signaling mechanisms mediating root architectural changes in Arabidopsis (Arabidopsis thaliana) Columbia. We showed effects on root architecture are mediated through a reduction in cell division in the lateral root (LR) meristems, the rate of LR initiation is reduced but LR density is unaffected, and primary root growth is reduced only slightly. This was primarily regulated through gibberellic acid (GA) signaling, which leads to the accumulation of growth-inhibitory DELLA proteins. The short LR phenotype was rescued by exogenous application of GA but not of auxin or by the inhibition of ethylene signaling. RNA-seq analysis showed upregulation by K+-deprivation of the transcription factors JUNGBRUNNEN1 (JUB1) and the C-repeat-binding factor (CBF)/dehydration-responsive element-binding factor 1 regulon, which are known to regulate GA signaling and levels that regulate DELLAs. Transgenic overexpression of JUB1 and CBF1 enhanced responses to K+ stress. Attenuation of the reduced LR growth response occurred in mutants of the CBF1 target gene SFR6, implicating a role for JUB1, CBF1, and SFR6 in the regulation of LR growth in response to K+-deprivation via DELLAs. We propose this represents a mechanism to limit horizontal root growth in conditions where K+ is available deeper in the soil.

Systemic Stress and Recovery Patterns of Rice Roots in Response to Graphene Oxide Nanosheets.

The interactions between nanomaterials and plants have attracted increasing attention. However, the systemic stress and recovery patterns of plants in response to nanomaterials and the connections between the molecular responses and the phenotypes remain unclear. Herein, rice was exposed to graphene oxide (GO) nanosheets at 0.01-1.0 mg/L for 7 days under hydroponic exposure, followed by a 7-day post exposure (GO-free). The significant upregulation (p < 0.05) of phenylalanine metabolism, secondary metabolism, and heme peroxidase reflected the stress and recovery patterns of rice roots exposed to GO. GO triggered 27% and more than 50% decreases in hydraulic conductivity and aquaporin gene expression (PIP1-3 and PIP2-2), respectively. The uptake of GO was mediated by aquaporin inhibition. Nanomaterial biotransformation reflected the potential for rice roots to adapt to GO stress. Oxidative stress, especially the downregulation of class III peroxidase mRNAs, were suppressed by GO. Lateral root inhibition, primary root growth, and cell wall synthesis, as forms of resistance to GO stress, were related to the significant (p < 0.05) downregulation of salicylic acid and lignin biosynthesis, as well as the upregulation of jasmonic acid and laccases. The present study helps elucidate the molecular and phenotypic responses of plants to nanomaterials, which are closely linked to their environmental risk assessment.

Genetic transformation of pigeonpea (Cajanus cajan L.) and screening transgenic progenies based on lateral root inhibition

Production of transgenic pigeonpea is becoming increasingly important, but the methods currently employed in production and subsequent screening still requires improvement. Here, we describe Agrobacterium-mediated genetic transformation of pigeonpea with reporter uidA (gus) gene and selectable marker, neomycin phospho-transferase (nptII) gene. Histochemical assay demonstrate localization of gus activity in cells and transformed plants. Overall, a transformation frequency of 0.33% was achieved using the protocol. Grafting of in vitro-regenerated healthy shoots indicates higher survival percent (72.6%), when stock and scion are of the same variety. Seeds harvested from primary transgenic plants can be screened based on lateral root inhibition strategy. Approximately 87% of the screened T1 plants were found to be PCR positive. In conclusion, in vitro grafting of transgenic pigeonpea shoots leads to better plant establishment and screening based on lateral root inhibition leads to quick identification of positive segregants.

An abscisic acid-sensitive checkpoint in lateral root development of Arabidopsis.

Lateral root (LR) formation displays considerable plasticity in response to developmental and environmental signals. The mechanism whereby plants incorporate diverse regulatory signals into the developmental programme of LRs remains to be elucidated. Current concepts of lateral root regulation focus on the role of auxin. In this study, we show that another plant hormone, abscisic acid (ABA), also plays a critical role in the regulation of this post-embryonic developmental event. In the presence of exogenous ABA, LR development is inhibited. This occurs at a specific developmental stage, i.e. immediately after the emergence of the LR primordium (LRP) from the parent root and prior to the activation of the LR meristem, and is reversible. Interestingly, this inhibition requires 10-fold less ABA than the inhibition of seed germination and is only slightly reduced in characterised abi mutants, suggesting that it may involve novel ABA signalling mechanisms. We also present several lines of evidence to support the conclusion that the ABA-induced lateral root inhibition is mediated by an auxin-independent pathway. First, the inhibition could not be rescued by either exogenous auxin application or elevated auxin synthesis. Secondly, a mutation in the ALF3 gene, which is believed to encode an important component in the auxin-dependent regulatory pathway for the post-emergence LR development, does not affect the sensitivity of LRs to ABA. Thirdly, ABA and the alf3-1 mutation do not act at the same developmental point. To summarise, these results demonstrate a novel ABA-sensitive, auxin-independent checkpoint for lateral root development in Arabidopsis at the post-emergence stage. In addition, we also present data indicating that regulation of this developmental checkpoint may require novel ABA signalling mechanisms and that ABA suppresses auxin response in the LRPs.

EFFECT OF TRIFLURALIN AND METRIBUZIN ON FABA BEAN GROWTH, DEVELOPMENT, AND SYMBIOTIC NITROGEN FIXATION

Field studies were conducted to determine if trifluralin [α,α,α-trifluro-2,6,-dinitro-N,N-dipropyl-p-toluidine] and metribuzin [4-amino-6-tert-butyl-3-(methylthio)-a5-triazin-5(4H)one] affect the growth and nitrogen self sufficiency of faba bean (Vicia faba L.). Metribuzin applied at 0.42 kg/ha as a pre-plant incorporated treatment resulted in substantial crop injury in 1981. Compared to the inoculated control, overall reductions averaged over the entire growing season, in plant dry weight, nodule dry weight, nitrogenase activity, and yield due to metribuzin were 59, 85, 92, and 65%, respectively. In 1982 metribuzin at 0.4 kg/ha was not as phytotoxic. Averaged over the season, the reductions recorded in plant dry weight, nodule dry weight, nitrogenase activity, and yield were 25, 33, 49, and 9%, respectively. The differences in phytotoxicity in the 2 yr at the two sites were attributed to differences in soil organic matter, and both the degree and timing of rainfall. During both years trifluralin did not cause any significant reductions in the parameters investigated. Reductions in nodulation and nitrogen fixation as a result of metribuzin application were evident only when the crop was injured and the plants were under stress from herbicide application. Trifluralin at 1.1 kg/ha reduced the phytotoxicity caused by metribuzin when the two chemicals were tank mixed. In 1981 there was a 23% yield improvement with the tank mix compared to metribuzin applied alone. In 1982, non-significant decreases in phytotoxicity with the tank mix were evident for the parameters studied. This reduced phytotoxicity may be explained on the basis of lateral root inhibition attributable to the trifluralin in the tank mix as was evident in the growth room study. The tank mix of trifluralin and metribuzin at 1.1 and 0.4 kg/ha, respectively, caused a 56% reduction in total root length compared to the inoculated control one week after emergence. Conversely, the total root length of the plants receiving only the 0.4 kg/ha rate of metribuzin did not differ significantly from the control. This may allow for greater herbicide uptake in the plants treated with only metribuzin leading to a greater level of injury.Key words: Faba bean, growth, symbiotic nitrogen fixation, trifluralin, metribuzin

Efeitos de herbicidas na anatomia de capim-carrapicho e amendoim-bravo

Com o objetivo de melhor conhecer a ação de herbicidas, plantas de capim-carrapicho (Cenchrus echinatus L.) foram tratadas com trifluralin, a 0,84 kg/ha, incorporado nas profundidades de 0 a 3 e 5 a 11cm, e plantas de amendoim-bravo (Euphorbia hetero-phylla L.) foram tratadas com trifluralin, na mesma dose, incorporada até 8cm, pendimetalin a 1,15 kg/ha e 2,4-D amina, a 1,44 kg/ha de i.a., aplicados em pré-emergência, em vasos. O desenvolvimento das plântulas foi observado a partir de sete dias da semeadura e aos 14 dias coletou-se material para observações ao microscópio. O trifluralin causou acentuado aumento de diâmetro do ápice da raiz e do coleóptilo, inibindo o crescimento dessas partes em C. echinatus. Também ocorreu aumento pronunciado das células do córtex e da epiderme e as paredes celulares se tornaram mais delgadas, com células plurinucleadas. Em E. heterophylla houve inibição de raízes secundárias e leve aumento do diâmetro de células do córtex. O pendimetalin causou grande aumento do diâmetro do colo da planta em E. heterophylla, devido ao aumento das células do córtex e epiderme. As paredes celulares se tornaram mais delgadas. Houve pequena inibição de raízes secundárias. O 2,4-D causou a formação de células no centro da medula em algumas plantas e descolamento da epiderme em outras.

The Effect of Herbicides on Lateral Roots and Nut Quality of Pecans

Trifluralina,a,a-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine) applied at 1.12 kg/ha inhibited lateral root development on field-grown pecan(Carya illinoensis(Wang.) K. Koch) seedlings. In a greenhouse study, inhibition of lateral roots, club-shaped radicles, and restricted penetration of tap roots resulted when pecans were planted in soil treated with 1.0 ppm trifluralin. When pecans were planted above soil treated with trifluralin, normal root development occurred only in the soil which had not been treated. Lateral roots were inhibited in the trifluralin-treated soil. Pecan nuts from trifluralin plots were smaller than those collected from controls, but other factors of quality were statistically equal or superior to nuts harvested in plots which had been treated with 2-chloro-4,6-bis-(ethylamino)-s-triazine (simazine) and 3-(3,4-dichlorophenyl)-1,1-dimethylurea (diuron). The smaller nut size was attributed to lateral root inhibition by trifluralin early during nut development.