QTL were identified for root architectural traits in maize. Root architectural traits, including the number, length, orientation, and branching of the principal root classes, influence plant function by determining the spatial and temporal domains of soil exploration. To characterize phenotypic patterns and their genetic control, three recombinant inbred populations of maize were grown for 28 days in solid media in a greenhouse and evaluated for 21 root architectural traits, including length, number, diameter, and branching of seminal, primary and nodal roots, dry weight of embryonic and nodal systems, and diameter of the nodal root system. Significant phenotypic variation was observed for all traits. Strong correlations were observed among traits in the same root class, particularly for the length of the main root axis and the length of lateral roots. In a principal component analysis, relationships among traits differed slightly for the three families, though vectors grouped together for traits within a given root class, indicating opportunities for more efficient phenotyping. Allometric analysis showed that trajectories of growth for specific traits differ in the three populations. In total, 15 quantitative trait loci (QTL) were identified. QTL are reported for length in multiple root classes, diameter and number of seminal roots, and dry weight of the embryonic and nodal root systems. Phenotypic variation explained by individual QTL ranged from 0.44 % (number of seminal roots, NyH population) to 13.5 % (shoot dry weight, OhW population). Identification of QTL for root architectural traits may be useful for developing genotypes that are better suited to specific soil environments.

QTL mapping and phenotypic variation for root architectural traits in maize (Zea mays L.)

Root-root interactions are much more sophisticated than previously thought, yet the mechanisms of belowground neighbor perception remain largely obscure. Genome-wide transcriptome analyses allow detailed insight into plant reactions to environmental cues. A root interaction trial was set up to explore both morphological and whole genome transcriptional responses in roots of Arabidopsis thaliana in the presence or absence of an inferior competitor, Hieracium pilosella. Neighbor perception was indicated by Arabidopsis roots predominantly growing away from the neighbor (segregation), while solitary plants placed more roots towards the middle of the pot. Total biomass remained unaffected. Database comparisons in transcriptome analysis revealed considerable similarity between Arabidopsis root reactions to neighbors and reactions to pathogens. Detailed analyses of the functional category ‘biotic stress’ using MapMan tools found the sub-category ‘pathogenesis-related proteins’ highly significantly induced. A comparison to a study on intraspecific competition brought forward a core of genes consistently involved in reactions to neighbor roots. We conclude that beyond resource depletion roots perceive neighboring roots or their associated microorganisms by a relatively uniform mechanism that involves the strong induction of pathogenesis-related proteins. In an ecological context the findings reveal that belowground neighbor detection may occur independently of resource depletion, allowing for a time advantage for the root to prepare for potential interactions.

/pdf/belowground-neighbor-perception-in-arabidopsis-thaliana-ohzk7auz5s.pdf

Belowground neighbor perception in Arabidopsis thaliana studied by transcriptome analysis: roots of Hieracium pilosella cause biotic stress

In addition to the role of the cell wall as a physical barrier against pathogens, some of its constituents, such as pectin-derived oligogalacturonides (OGAs) are essential components to trigger signaling pathways that induce rapid defense responses. Many pathogens directly penetrate the cell wall to access water and nutrients of the plant protoplast, and a rigid cell wall can fend off pathogen attack by forming an impenetrable physical barrier. Thus, cell wall integrity sensing is one mechanism by which plants may detect pathogen attack. Moreover, when the plant-pathogen interaction occurred, OGAs released during cell wall modification can trigger plant defense (e.g., production of reactive oxygen species, production of anti-microbial metabolites and synthesis of pathogenesis-related proteins). This review documents and discusses studies suggesting that OGAs play a dual signaling role during pathogen attack by inducing defense responses and plant architecture adjustment.

https://www.tandfonline.com/doi/pdf/10.4161/psb.21779

Signaling role of oligogalacturonides derived during cell wall degradation

Plants adapt to challenging environmental factors by modulating morphogenetic processes. Although it has been speculated that activation of defense responses against pathogens leads to plant growth adjustment, little is known about developmental and architectural responses to defense stimulators. In this report we evaluated the activity of oligogalacturonides (OGs), a class of molecules directly involved in plant immunity, to modulate root system architecture in Arabidopsis thaliana. We show that OGs induce PAD3 expression and camalexin synthesis, two well-known markers of defense responses. These effects were related to primary root growth inhibition and increased lateral root and root hair formation, which are reminiscent of altered auxin responses. Cellular analysis showed that the effect of these compounds on primary root growth was due to changes in cell elongation and increased flavonoid accumulation at the root elongation region. Moreover, the observations that similar changes in primary root growth were induced by naphthylphthalamic acid supply and that auxin- or flavonoid-related mutants tir1, doc1, pgp1, pgp4, pgp19, and tt4-1 show differential responses to primary root growth inhibition by OGs suggest that auxin homeostasis plays a role in the oligogalacturonide-induced alteration of root cell patterning. Our results suggest that OGs might play a dual function in adaptation of plants to pathogen challenge by inducing defense responses and plant architecture adjustment.

Plant Immunity Induced by Oligogalacturonides Alters Root Growth in a Process Involving Flavonoid Accumulation in Arabidopsis thaliana

The aim was to test promotive effects of oligosaccharins on root growth and development at the root apical meristem and the cell cycle using the model systems, Arabidopsis thaliana and the tobacco (Nicotiana tabacum) BY-2 cell line. Arabidopsis was grown on medium supplemented with 0.1 mg L−1 oligoxyloglucan (OX), 10 mg L−1 Pectimorf® (P) or 0.5 mg L−1 indole butyric acid (IBA). Primary root length, number of lateral root primordia, root apical meristem (RAM) length and epidermal cell length were recorded. Three genotypes were used: wild type (WT) and transgenic lines expressing either Schizosaccharomyces pombe (Sp) cdc25 or over-expressing(oe) Arath;WEE1. All treatments promoted primary root elongation but repressed lateral root production. Only P had a clear positive effect on meristem length whereas all other genotype × treatment interactions showed shorter RAMs. Whilst IBA, OX and P induced an increase in cell length in Spcdc25, the same treatments caused a significant decrease in WEE1 oe . Mitotic indices were also significantly higher in roots treated with oligosaccharins suggesting a shortening of the cell cycle. This hypothesis was tested in the BY-2 cell line. Both OX and P shortened the cell cycle exclusively through a shortening of G1 whilst mitotic cell size remained constant between treatments. In conclusion, both OX and P do indeed stimulate growth and shorten the cell cycle in higher plants and at the cellular level are able to reverse large and small cell size phenotypes normally exhibited by WEE1 oe and Spcdc25 genotypes, respectively.

Oligosaccharins and Pectimorf® stimulate root elongation and shorten the cell cycle in higher plants

Phytoalexin accumulation is one of a myriad of plant defense responses; these responses can be elicited by pathogens or molecules such as oligogalacturonides (OGAs). Phytoalexin production has been considered a vital component of the resistance mechanisms that determine the outcome of many plant-microbe interactions. Besides inducing defense responses, OGAs have been shown to affect plant development, which normally is controlled by plant hormones, particularly auxin. In this work we measured phytoalexin accumulation in roots of bean (Phaseolus vulgaris L.) seedlings grown in the presence or absence of the auxin 3-naphtalenacetic acid (NAA) and treated with a decagalacturonide (OGA10). We found that OGA10 (0.01 mM) caused phytoalexin production and also inhibited main root elongation and the formation of secondary roots by ca. 33%. Expression of Cycb 2-2 was also inhibited, while pal and chs were highly expressed. The root growth inhibition was not overcome by the addition of a stimulatory concentration of auxin (NAA 0.1 µM). The data suggests that elicitation of defense responses in the root alters metabolism in such a way that results in the modification of the architecture of bean roots.

/pdf/the-architecture-of-phaseolus-vulgaris-root-is-altered-when-1yxh42n1zk.pdf

Alan Richards

Papers

The architecture of Phaseolus vulgaris root is altered when a defense response is elicited by an oligogalacturonide