Although N-acetylglucosamine-1-P uridylyltransferase (GlcNAc1pUT) that catalyzes the final step of the hexosamine biosynthetic pathway and is conserved among, organisms, produces UDP-N-acetylglucosamine (UDP-GlcNAc), an essential sugar moiety involved in protein glycosylation and structural polymers, its biological function in plants remains unknown. In this study, two GlcNA.UT genes were characterized in Arabidopsis thaliana. The single mutants glcna.ut1 and glcna.ut2 revealed no obvious phenotype, but their homozygous double mutant was lethal, reflecting the functional redundancy of these genes in being essential for plant growth. Mutant plants, GlcNA.UT1/glcna.ut1 glcna.ut2/ glcna.ut2, obtained from an F2-segregating population following reciprocal crosses of glcna.ut1 with glcna.ut2, displayed shorter siliques and fewer seed sets combined with impaired pollen viability and unfertilized ovules. Genetic analyses further demonstrated that the progeny of the GlcNA.UT1/glcna.ut1 glcna.ut2/glcna.ut2 mutant plants, but not those of the glcna.ut1/glcna.ut1 GlcNA.UT2/glcna.ut2 mutant plants, suffer from the aberrant transmission of (glcna.ut1 glcna.ut2) gametes. In parallel, cell biology analyses revealed a substantial defect in male gametophytes appearing during the late vacuolated or pollen mitosis I stages and that the female gametophyte is arrested during the uninucleate embryo sac stage in GlcNA.UT1/glcna.ut1 glcna.ut2/glcna.ut2 mutant plants. Nevertheless, although the glcna.ut1/glcna.ut1 GlcNA.UT2/glcna.ut2 mutant plants exhibited a normal transmission of (glcna.ut1 glcna.ut2) gametes and gametophytic development, the development of numerous embryos was arrested during the early globular stage within the embryo sacs. Collectively, despite having overlapping functions, the GlcNA.UT genes play an indispensable role in the unique mediation of gametogenesis and embryogenesis.

Ralstonia solanacearum causes a deadly wilting disease on a wide range of crops. To elucidate pathogenesis of this bacterium in different host plants, we set out to identify R. solanacearum genes involved in pathogenesis by screening random transposon insertion mutants of a highly virulent strain, Pss190, on tomato and Arabidopsis thaliana. Mutants exhibiting various decreased virulence levels on these two hosts were identified. Sequence analysis showed that most, but not all, of the identified pathogenesis genes are conserved among distinct R. solanacearum strains. A few of the disrupted loci were not reported previously as being involved in R. solanacearum pathogenesis. Notably, a group of mutants exhibited differential pathogenesis on tomato and Arabidopsis. These results were confirmed by characterizing allelic mutants in one other R. solanacearum strain of the same phylotype. The significantly decreased mutants' colonization in Arabidopsis was found to be correlated with differential pathogenesis on these two plants. Differential requirement of virulence genes suggests adaptation of this bacterium in different host environments. Together, this study reveals commonalities and differences of R. solanacearum pathogenesis on single solanaceous and nonsolanaceous hosts, and provides important new insights into interactions between R. solanacearum and different host plants.

N-acetylglucosamine (GlcNAc) is the fundamental amino sugar moiety that is essential for protein glycosylation. UDP-GlcNAc, an active form of GlcNAc, is synthesized through the hexosamine biosynthetic pathway (HBP). Arabidopsis N-acetylglucosamine-1-P uridylyltransferases (GlcNAc1pUTs), encoded by GlcNA.UTs, catalyze the last step in the HBP pathway, but their biochemical and molecular functions are less clear. In this study, the GlcNA.UT1 expression was knocked down by the double-stranded RNA interference (dsRNAi) in the glcna.ut2 null mutant background. The RNAi transgenic plants, which are referred to as iU1, displayed the reduced UDP-GlcNAc biosynthesis, altered protein N-glycosylation and induced an unfolded protein response under salt-stressed conditions. Moreover, the iU1 transgenic plants displayed sterility and salt hypersensitivity, including delay of both seed germination and early seedling establishment, which is associated with the induction of ABA biosynthesis and signaling. These salt hypersensitive phenotypes can be rescued by exogenous fluridone, an inhibitor of ABA biosynthesis, and by introducing an ABA-deficient mutant allele nced3 into iU1 transgenic plants. Transcriptomic analyses further supported the upregulated genes that were involved in ABA biosynthesis and signaling networks, and response to salt stress in iU1 plants. Collectively, these data indicated that GlcNAc1pUTs are essential for UDP-GlcNAc biosynthesis, protein N-glycosylation, fertility, and the response of plants to salt stress through ABA signaling pathways during seed germination and early seedling development.

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