Intestinal colonization by bacteria of oral origin has been correlated with several negative health outcomes, including inflammatory bowel disease. However, a causal role of oral bacteria ectopically colonizing the intestine remains unclear. Using gnotobiotic techniques, we show that strains of Klebsiella spp. isolated from the salivary microbiota are strong inducers of T helper 1 (TH1) cells when they colonize in the gut. These Klebsiella strains are resistant to multiple antibiotics, tend to colonize when the intestinal microbiota is dysbiotic, and elicit a severe gut inflammation in the context of a genetically susceptible host. Our findings suggest that the oral cavity may serve as a reservoir for potential intestinal pathobionts that can exacerbate intestinal disease.

Tight coordination between plastid differentiation and plant development is best evidenced by the synchronized development of photosynthetic tissues and the biogenesis of chloroplasts. Here, we show that Arabidopsis thaliana roots demonstrate accelerated chlorophyll accumulation and chloroplast development when they are detached from shoots. However, this phenomenon is repressed by auxin treatment. Mutant analyses suggest that auxin transported from the shoot represses root greening via the function of INDOLE-3-ACETIC ACID14, AUXIN RESPONSE FACTOR7 (ARF7), and ARF19. Cytokinin signaling, on the contrary, is required for chlorophyll biosynthesis in roots. The regulation by auxin/cytokinin is dependent on the transcription factor LONG HYPOCOTYL5 (HY5), which is required for the expression of key chlorophyll biosynthesis genes in roots. The expression of yet another root greening transcription factor, GOLDEN2-LIKE2 (GLK2), was found to be regulated in opposing directions by auxin and cytokinin. Furthermore, both the hormone signaling and the GLK transcription factors modified the accumulation of HY5 in roots. Overexpression of GLKs in the hy5 mutant provided evidence that GLKs require HY5 to maximize their activities in root greening. We conclude that the combination of HY5 and GLKs, functioning downstream of light and auxin/cytokinin signaling pathways, is responsible for coordinated expression of the key genes in chloroplast biogenesis.

The development of cells specialized for water conduction or support is a striking innovation of plants that has enabled them to colonize land. The NAC transcription factors regulate the differentiation of these cells in vascular plants. However, the path by which plants with these cells have evolved from their nonvascular ancestors is unclear. We investigated genes of the moss Physcomitrella patens that encode NAC proteins. Loss-of-function mutants formed abnormal water-conducting and supporting cells, as well as malformed sporophyte cells, and overexpression induced ectopic differentiation of water-conducting-like cells. Our results show conservation of transcriptional regulation and cellular function between moss and Arabidopsis thaliana water-conducting cells. The conserved genetic basis suggests roles for NAC proteins in the adaptation of plants to land.

Drought causes substantial reductions in crop yields worldwide. Therefore, we set out to identify new chemical and genetic factors that regulate drought resistance in Arabidopsis thaliana. Karrikins (KARs) are a class of butenolide compounds found in smoke that promote seed germination, and have been reported to improve seedling vigor under stressful growth conditions. Here, we discovered that mutations in KARRIKIN INSENSITIVE2 (KAI2), encoding the proposed karrikin receptor, result in hypersensitivity to water deprivation. We performed transcriptomic, physiological and biochemical analyses of kai2 plants to understand the basis for KAI2-regulated drought resistance. We found that kai2 mutants have increased rates of water loss and drought-induced cell membrane damage, enlarged stomatal apertures, and higher cuticular permeability. In addition, kai2 plants have reduced anthocyanin biosynthesis during drought, and are hyposensitive to abscisic acid (ABA) in stomatal closure and cotyledon opening assays. We identified genes that are likely associated with the observed physiological and biochemical changes through a genome-wide transcriptome analysis of kai2 under both well-watered and dehydration conditions. These data provide evidence for crosstalk between ABA- and KAI2-dependent signaling pathways in regulating plant responses to drought. A comparison of the strigolactone receptor mutant d14 (DWARF14) to kai2 indicated that strigolactones also contributes to plant drought adaptation, although not by affecting cuticle development. Our findings suggest that chemical or genetic manipulation of KAI2 and D14 signaling may provide novel ways to improve drought resistance.

Peroxisomes are essential organelles that are characterized by the possession of enzymes that produce hydrogen peroxide (H 2 O 2 ) as part of their normal catalytic cycle. During the metabolic process, peroxisomal proteins are inevitably damaged by H 2 O 2 and the integrity of the peroxisomes is impaired. Here, we show that autophagy, an intracellular process for vacuolar degradation, selectively degrades dysfunctional peroxisomes. Marked accumulation of peroxisomes was observed in the leaves but not roots of autophagy-related (ATG)-knockout Arabidopsis thaliana mutants. The peroxisomes in leaf cells contained markedly increased levels of catalase in an insoluble and inactive aggregate form. The chemically inducible complementation system in ATG5-knockout Arabidopsis provided the evidence that these accumulated peroxisomes were delivered to vacuoles for degradation by autophagy. Interestingly, autophagosomal membrane structures specifically recognized the abnormal peroxisomes at the site of the aggregates. Thus, autophagy is essential for the quality control of peroxisomes in leaves and for proper plant development under natural growth conditions.