The spatial organization of the genome plays an important role in the regulation of gene expression. However, the core structural features of animal genomes, such as topologically associated domains (TADs) and chromatin loops, are not prominent in the extremely compact Arabidopsis genome. In this study, we examine the chromatin architecture, as well as their DNA methylation, histone modifications, accessible chromatin, and gene expression, of maize, tomato, sorghum, foxtail millet, and rice with genome sizes ranging from 0.4 to 2.4 Gb. We found that these plant genomes can be divided into mammalian-like A/B compartments. At higher resolution, the chromosomes of these plants can be further partitioned to local A/B compartments that reflect their euchromatin, heterochromatin, and polycomb status. Chromatins in all these plants are organized into domains that are not conserved across species. They show similarity to the Drosophila compartment domains, and are clustered into active, polycomb, repressive, and intermediate types based on their transcriptional activities and epigenetic signatures, with domain border overlaps with the local A/B compartment junctions. In the large maize and tomato genomes, we observed extensive chromatin loops. However, unlike the mammalian chromatin loops that are enriched at the TAD border, plant chromatin loops are often formed between gene islands outside the repressive domains and are closely associated with active compartments. Our study indicates that plants have complex and unique 3D chromatin architectures, which require further study to elucidate their biological functions.

Leaves of C4 crops usually have higher radiation, water and nitrogen use efficiencies compared to the C3 species. Engineering C4 traits into C3 crops has been proposed as one of the most promising ways to repeal the biomass yield ceiling. To better understand the function of C4 photosynthesis, and to identify candidate genes that are associated with the C4 pathways, a comparative transcription network analysis was conducted on leaf developmental gradients of three C4 species including maize, green foxtail and sorghum and one C3 species, rice. By combining the methods of gene co-expression and differentially co-expression networks, we identified a total of 128 C4 specific genes. Besides the classic C4 shuttle genes, a new set of genes associated with light reaction, starch and sucrose metabolism, metabolites transportation, as well as transcription regulation, were identified as involved in C4 photosynthesis. These findings will provide important insights into the differential gene regulation between C3 and C4 species, and a good genetic resource for establishing C4 pathways in C3 crops.

Activation of plant immune responses is often associated with an inhibition of plant growth. The molecular mechanisms underlying this fitness cost are unknown. Here, we utilize the autoimmune response mutant suppressor of npr1, constitutive1 (snc1) resulting from an activated form of the Disease Resistance (R) gene to dissect the genetic component mediating growth inhibition in Arabidopsis (Arabidopsis thaliana). The radical-induced cell death1 (rcd1) mutant defective in responses to reactive oxygen species (ROS) was isolated as an enhancer of the snc1 mutant in growth inhibition but not in defense response activation. Similarly, the vitamin C2 (vtc2) and vtc3 mutants defective in ROS detoxification enhanced the growth defects of snc1. Thus, perturbation of ROS status by R gene activation is responsible for the growth inhibition, and this effect is independent of defense response activation. This was further supported by the partial rescue of growth defects of rcd1 snc1 by the respiratory burst oxidase homolog D (rbohD) and rbohF mutations compromising the generation of ROS burst. Collectively, these findings indicate that perturbation of ROS homeostasis contributes to the fitness cost independent of defense activation.