Transcriptome Analysis of Eucalyptus grandis Implicates Brassinosteroid Signaling in Defense Against Myrtle Rust (Austropuccinia psidii)

Eucalyptus grandis, in its native Australian range, varies in resistance to Austropuccinia psidii (syn. Puccinia psidii). The biotrophic rust fungus, A. psidii is the causal agent of myrtle rust and poses a serious threat to Australian biodiversity. The pathogen produces yellow pustules of urediniospores on young leaves and shoots, resulting in shoot tip dieback, stunted growth, and death. Dissecting the underlying mechanisms of resistance against this pathogen will contribute to improved breeding and control strategies to mitigate its devastating effects. The aim of this study was to determine the molecular dialogue between E. grandis and A. psidii, using an RNA-sequencing approach. Resistant and susceptible E. grandis seedlings grown from seed collected across its natural range were inoculated with the pandemic biotype of A. psidii. The leaf tissue was harvested at 12-h post inoculation (hpi), 1-day post inoculation (dpi), 2-dpi and 5-dpi and subjected to RNA-sequencing using Illumina 50 bp PE reads to a depth of 40 million reads per sample. Differential gene expression and gene ontology enrichment indicated that the resistant seedlings showed controlled, coordinated responses with a hypersensitive response, while the susceptible seedlings showed no systemic response against myrtle rust. Brassinosteroid signaling was apparent as an enriched term in the resistant interaction at 2-dpi, suggesting an important role of this phytohormone in defense against the pathogen. Brassinosteroid mediated signaling genes were also among the candidate genes within two major disease resistance loci (Puccinia psidii resistance), Ppr3 and Ppr5. While brassinosteroids have been tagged as positive regulators in other plant disease resistance interactions, this is the first report in the Eucalyptus – Austropuccinia psidii interaction. Furthermore, several putative resistance genes, underlying known resistance loci and implicated in the interaction have been identified and highlighted for future functional studies. This study provided further insights into the molecular interactions between E. grandis and A. psidii, contributing to our understanding of this pathosystem.

An Adenylate Kinase OsAK3 Involves Brassinosteroid Signaling and Grain Length in Rice (Oryza sativa L.)

Background Grain size is one of the major determinants of cereal crop yield. As a class of plant polyhydroxysteroids, brassinosteroids (BRs) play essential roles in the regulation of grain size and plant architecture in rice. In a previous research, we cloned qGL3/OsPPKL1 encoding a protein phosphatase with Kelch-like repeat domains, which negatively regulates BR signaling and grain length in rice. Results Here, we screened qGL3-interacting proteins (GIPs) via yeast two-hybrid assay and analyzed the phenotypes of the T-DNA insertion mutants of GIPs. Among these mutants, mutant osak3 presents shorter grain length and dwarfing phenotype. OsAK3 encodes an adenylate kinase, which regulates grain size by controlling cell expansion of rice spikelet glume. Overexpression of OsAK3 resulted in longer grain length. OsAK3 interacts with qGL3 in vivo and in vitro. Lamina inclination, coleoptile elongation and root inhibition experiments showed that the osak3 mutant was less sensitive to exogenous brassinolide (BL) treatment. The transcriptional level of OsAK3 was up-regulated under BL induction. In addition, RNA-Seq data indicate that OsAK3 is involved in a variety of biological processes that regulate BR signaling and grain development in rice. Conclusions Our study reveals a novel BR signaling component OsAK3 in the regulation of grain length, and provides novel clues for uncovering the potential functions of OsAK3 in rice growth and development.

The regeneration factors ERF114 and ERF115 act as transducers of mechanical cues to developmental pathways

Plants show an unparalleled regenerative capacity, allowing them to survive severe stress conditions, such as injury, herbivory attack and harsh weather conditions. This potential not only replenishes tissues and restores damaged organs, but can also give rise to whole plant bodies, highlighting the intertwined nature of development and regeneration. It suggests that regeneration and developmental processes respond to the same upstream signals, but how a cell knows which of the two processes to engage is currently unknown. Here, we demonstrate that next to being regulators of regeneration, ETHYENE RESPONSE FACTOR 114 (ERF114) and ERF115 govern developmental growth in the absence of wounding or injury. Increased ERF114 and ERF115 activity is correlated with enhanced xylem maturation and lateral root formation, whereas their knockout results in a decrease in lateral roots and xylem connectivity following grafting. Moreover, we provide evidence that mechanical cues contribute to ERF114 and ERF115 expression in correlation with BZR1 mediated brassinosteroid signaling under both regenerative and developmental conditions. Antagonistically, negative regulation of cell wall extensibility via cell wall-associated mechanosensory FERONIA signaling suppresses their expression under both conditions. Our data suggest a molecular framework in which mechanical perturbations too great to be compensated by adaptive cell wall remodeling results in strong ERF114 and ERF115 expression, switching their role from developmental to regenerative regulators.

Integration of Light and Brassinosteroid Signaling during Seedling Establishment

Light and brassinosteroid (BR) are external stimuli and internal cue respectively, that both play critical roles in a wide range of developmental and physiological process. Seedlings grown in the light exhibit photomorphogenesis, while BR promotes seedling etiolation. Light and BR oppositely control the development switch from shotomorphogenesis in the dark to photomorphogenesis in the light. Recent progress report that substantial components have been identified as hubs to integrate light and BR signals. Photomorphogenic repressors including COP1, PIFs, and AGB1 have been reported to elevate BR response, while photomorphogenesis-promoting factors such as HY5, BZS1, and NF-YCs have been proven to repress BR signal. In addition, BR components also modulate light signal. Here, we review the current research on signaling network associated with light and brassinosteroids, with a focus on the integration of light and BR signals enabling plants to thrive in the changeable environment.

Identification of new leaf intrinsic yield genes using cross-species network analysis in plants

Plant leaves differ in their size, form and structure, and the processes of cell division and cell expansion contribute to this diversity. Leaf transcriptional networks covering cell division and cell expansion in Arabidopsis thaliana, maize (Zea mays) and aspen (Populus tremula) were compared to identify candidate genes that are conserved in plant growth and ultimately have the potential to increase biomass (intrinsic yield, IY). Our approach revealed that genes showing strongly conserved co-expression were mainly involved in fundamental leaf developmental processes such as photosynthesis, translation, and cell proliferation. Next, known intrinsic yield genes (IYGs) together with cross-species conserved networks were used to predict novel potential Arabidopsis leaf IYGs. Using an in-depth literature screening, 34 out of 100 top predicted IYGs were confirmed to affect leaf phenotype if mutated or overexpressed and thus represent novel potential IYGs. Globally, these new IYGs were involved in processes mostly covering cell cycle, plant defense responses, gibberellin, auxin and brassinosteroid signaling. Application of loss-of-function lines and phenotypic characterization confirmed two newly predicted IYGs to be involved in leaf growth (NPF6.4 and LATE MERISTEM IDENTITY2). In conclusion, the presented network approach offers an integrative cross-species strategy to identify new yield genes and to accelerate plant breeding.

Melatonin-induced cold and drought tolerance requires brassinosteroids and hydrogen peroxide signaling in perennial ryegrass

Aims The interplay between melatonin and brassinosteroids in enhancing tolerance to cold and drought and the underlying molecular mechanisms of this relationship still remain unknown. Methods Combined physiological and transcriptomic analyses were used to clarify the crosstalk of melatonin and brassinosteroids in perennial ryegrass. Brassinosteroids biosynthesis inhibitor propiconazole, Arabidopsis brassinosteroids-receptor mutant bri1-9, bak1, and H2O2 deficient rbohC and rbohF mutants were used to analyzed the role of H2O2 in melatonin and brassinosteroids-mediated tolerance to the stress. Results Melatonin-enhanced cold and drought tolerance of perennial ryegrass depends on the duration of dose and stress that were applied. Melatonin activates the expression of NADPH oxidase-related genes to promote H2O2 production when exposed to short-term cold/drought stresses but reduced accumulation of H2O2 under long-term stress. This was associated with an increase in enzymatic and non-enzymatic antioxidant activities by regulation of their gene expression and modulation of the ascorbate-glutathione cycle. Moreover, exogenous melatonin increased the biosynthesis of melatonin and brassinosteroids or the expression of signaling-related genes. However, the effects of melatonin on the expression of BR biosynthesis and signaling-related genes were inhibited in the rbohC and rbohF mutants. Chemical scavenging of H2O2 attenuated melatonin-mediated growth in the Arabidopsis wild-type, but the bak1 and bri1.9 mutants were relatively insensitive. Consistently, Inhibition of H2O2 production impaired the effect of melatonin and brassinosteroids on seed germination and the root growth of perennial ryegrass. Conclusions This research reveals a novel regulatory mechanism of the crosstalk of H2O2 and brassinosteroid signaling in melatonin-induced cold/drought tolerance in perennial ryegrass.

BRS1 mediates plant redox regulation and cold responses

Background Brassinosteroid-insensitive 1 suppressor 1 (BRS1) is a serine carboxypeptidase that mediates brassinosteroid signaling and participates in multiple developmental processes in Arabidopsis. However, little is known about the precise role of BRS1 in this context. Results In this study, we analyzed transcriptional and proteomic profiles of Arabidopsis seedlings overexpressing BRS1 and found that this gene was involved in both cold stress responses and redox regulation. Further proteomic evidence showed that BRS1 regulated cell redox by indirectly interacting with cytosolic NADP + -dependent isocitrate dehydrogenase (cICDH). One novel alternative splice form of BRS1 was identified in over-expression mutants brs1-1D, which may confer a new role in plant development and stress responses. Conclusions This study highlights the role of BRS1 in plant redox regulation and stress responses, which extends our understanding of extracellular serine carboxypeptidases.