Nitrogen (N) fertilization is one of the most crucial factors that contribute to increasing food production requiring the generation of rice cultivars with improved N use efficiency (NUE) to maintain yield during low N fertilizer application. To assay NUE extent, we developed a screening system to evaluate shoot growth of each rice cultivar under gradient changes in N concentrations. This system comprises a gradient hydroponic culture and growth visualization systems. The former allows gradient changes in ammonium concentrations, while the latter records the increment in shoot length of individual rice seedlings at given time periods using a fixed-point camera. We chose 69 cultivars including two controls (Oryza sativa L. cv. Nipponbare [WRC01] and Kasalath [WRC02]) from the World Rice Core Collection to investigate shoot growth responses under ammonium-sufficient, ammonium-limited, and low ammonium concentration gradients without transplanting stress. We observed three growth patterns in response to different ammonium concentrations. Subsequently, we selected three representative cultivars (Kasalath, WRC03, and WRC05) for the characteristic responses under the different ammonium environments. Distinct expression patterns of glutamine synthetase 1;2 (OsGS1;2) but OsGS1;1 were observed in response to varying ammonium concentration regimes, indicating that the expression patterns of OsGS1;2 may be a growth marker in terms of shoot growth when transitioning from ammonium-limited to low ammonium concentrations. This system with the level of OsGS1;2 allows us to screen for candidate cultivars that return high NUE in low N environments.
A group of anti-nutritional specialized metabolites called steroidal glycoalkaloids (SGAs) are produced in Solanum species such as tomato, potato, and eggplant. The transcription factor JASMONATE-RESPONSIVE ETHYLENE RESPONSE FACTOR 4 (JRE4) regulates many SGA biosynthesis genes in tomato and potato. Here we report that the expression of a cluster of genes encoding nitrate transporter 1/peptide transporter family (NPF) members is downregulated in the jre4-1 loss-of-function tomato mutant, which has a low-SGA phenotype compared to the wild type. NPFs are a large family of plant membrane transporters that transport a wide range of substrates, including specialized metabolites. We found that the JRE4-regulated NPF genes are induced by the defense-related phytohormone jasmonate. Conversely, jasmonate-mediated induction of gene expression was attenuated by ethylene treatment of the leaves. The co-regulation of the NPF genes with SGA biosynthesis genes by JRE4 suggests that NPF transporters are involved in the SGA pathway.
In plants, mitogen activated protein kinases (MPKs) are involved in various signaling pathways that lead to biotic and abiotic responses as well as that regulate developmental processes. Among them, MPK6 and its closely related homologue, MPK3, act redundantly and are known to be involved in asymmetric cell divisions of meristemoid mother cells in stomata development and of zygotes in Arabidopsis. Loss-of-function mutants of GLE4/OsMPK6, which is an orthologue of MPK6 in rice, showed a defect in polarity establishment in early stage of embryogenesis. However, because of the embryo lethality of the mutations, the function of GLE4/OsMPK6 in post-embryonic development is not clarified. Here, we report the analysis of post embryonic function of GLE4/OsMPK6 in vegetative stage of rice using regenerated gle4/osmpk6 homozygous plants from tissue culture. The regenerated plants are dwarf and produce multiple shoots with small leaves. These shoots never develop into reproductive stage, instead, proliferate vegetative shoots repeatedly. Leaves of gle4/osmpk6 have small leaf blade at the tip and blade-sheath boundary become obscure. Stomata arrangement is also disturbed in gle4/osmpk6 leaf blade. The shape of shoot apical meristem of gle4/osmpk6 become disorganized. Thus, GLE4/OsMPK6 functions in shoot organization and stomata patterning in the post embryonic development in rice.
Target of rapamycin (TOR) regulates essential processes associated with plant growth, development, and cell death by modulating metabolic activities and translation in response to environmental signals. The ATP-competitive TOR inhibitor AZD8055 suppressed the hypersensitive response (HR) cell death in Nicotiana benthamiana infected with the incompatible Ralstonia solanacearum. The induced expression of the HR marker gene hin1 was also inhibited by the AZD8055 treatment. To further clarify the mechanisms underlying TOR-regulated HR cell death, we focused on TOR-related ErbB3-binding protein 1 (EBP1) in N. benthamiana (NbEBP1). We found four EBP1 orthologs in the N. benthamiana genome. The expression levels of all four EBP1 orthologs in N. benthamiana were up-regulated by the R. solanacearum infection. The silencing of the four NbEBP1 orthologs suppressed the induction of HR cell death, hin1 expression, and the production of reactive oxygen species. These results suggest that the TOR signaling pathway helps regulate HR cell death along with reactive oxygen species-related signaling in N. benthamiana.