Transcriptome analysis of early stages of sorghum grain mold disease reveals defense regulators and metabolic pathways associated with resistance

Background Sorghum grain mold is the most important disease of the crop. The disease results from simultaneous infection of the grain by multiple fungal species. Host responses to these fungi and the underlying molecular and cellular processes are poorly understood. To understand the genetic, molecular and biochemical components of grain mold resistance, transcriptome profiles of the developing grain of resistant and susceptible sorghum genotypes were studied. Results The developing kernels of grain mold resistant RTx2911 and susceptible RTx430 sorghum genotypes were inoculated with a mixture of fungal pathogens mimicking the species complexity of the disease under natural infestation. Global transcriptome changes corresponding to multiple molecular and cellular processes, and biological functions including defense, secondary metabolism, and flavonoid biosynthesis were observed with differential regulation in the two genotypes. Genes encoding pattern recognition receptors (PRRs), regulators of growth and defense homeostasis, antimicrobial peptides, pathogenesis-related proteins, zein seed storage proteins, and phytoalexins showed increased expression correlating with resistance. Notably, SbLYK5 gene encoding an orthologue of chitin PRR, defensin genes SbDFN7.1 and SbDFN7.2 exhibited higher expression in the resistant genotype. The SbDFN7.1 and SbDFN7.2 genes are tightly linked and transcribed in opposite orientation with a likely common bidirectional promoter. Interestingly, increased expression of JAZ and other transcriptional repressors were observed that suggested the tight regulation of plant defense and growth. The data suggest a pathogen inducible defense system in the developing grain of sorghum that involves the chitin PRR, MAPKs, key transcription factors, downstream components regulating immune gene expression and accumulation of defense molecules. We propose a model through which the biosynthesis of 3-deoxyanthocynidin phytoalexins, defensins, PR proteins, other antimicrobial peptides, and defense suppressing proteins are regulated by a pathogen inducible defense system in the developing grain. Conclusions The transcriptome data from a rarely studied tissue shed light into genetic, molecular, and biochemical components of disease resistance and suggested that the developing grain shares conserved immune response mechanisms but also components uniquely enriched in the grain. Resistance was associated with increased expression of genes encoding regulatory factors, novel grain specific antimicrobial peptides including defensins and storage proteins that are potential targets for crop improvement.

Uncovering the chemistry behind inducible morphological defences in the crustacean Daphnia magna via micro-Raman spectroscopy

The widespread distribution of Crustacea across every aquatic ecological niche on Earth is enabled due to their exoskeleton's versatile properties. Especially mineralization of the exoskeleton provides protection against diverse environmental threats. Thereby, the exoskeleton of some entomostracans is extremely phenotypically plastic, especially in response to predators. For instance, the freshwater zooplankton Daphnia forms conspicuous inducible morphological defenses, such as helmets, and can increase the stability of its exoskeleton, which renders them less vulnerable to predation. In this study, we reveal for the first time the chemical composition of the exoskeleton of Daphniamagna, using Raman spectroscopy, to be composed of α-chitin and proteins with embedded amorphous calcium carbonate (ACC). Furthermore, we reveal the exoskeleton's chemical changes associated with inducible defense mechanisms in the form of more substantial mineralization, which is probably correlated with enhanced carapace stability. We, therefore, highlight the importance of calcium-biominerals for inducible morphological defenses in Daphnia.

Reversible colony formation and the associated costs in Scenedesmus obliquus

Grazer-induced colony formation as a defense strategy in microalgae such as Scenedesmus species has been widely reported, but the associated costs and reversibility of the colonies are rarely studied. We experimentally showed that Scenedesmus obliquus formed chained colonies in the presence of a predator, including predators separated from the algae by a membrane, but quickly reverted to single cells after the removal of the predator—a defining characteristic of an inducible defense. We detected stress indicators—astaxanthin esters—in the algal populations in the presence of grazers but not when grazers were absent. We found significant costs associated with S. obliquus colony formation in terms of lower population growth rate, lower photosystem II efficiency and lower cellular chlorophyll a content. These results together show that colony formation as an inducible defense in S. obliquus against grazers comes at a substantial cost such that the defense must be switched off and the colonies revert to single cells when the predation risk disappears.