The auxin response factor, OsARF19, controls rice leaf angles through positively regulating OsGH3-5 and OsBRI1.

Auxin and brassinosteroid (BR) are important phytohormones for controlling lamina inclination implicated in plant architecture and grain yield. But the molecular mechanism of auxin and BR crosstalk for regulating lamina inclination remains unknown. Auxin response factors (ARFs) control various aspects of plant growth and development. We here report that OsARF19-overexpression rice lines show an enlarged lamina inclination due to increase of its adaxial cell division. OsARF19 is expressed in various organs including lamina joint and strongly induced by auxin and BR. Chromatin immunoprecipitation (ChIP) and yeast one-hybrid assays demonstrate that OsARF19 binds to the promoter of OsGH3-5 and brassinosteroid insensitive 1 (OsBRI1) directing their expression. OsGH3-5-overexpression lines show a similar phenotype as OsARF19-O1. Free auxin contents in the lamina joint of OsGH3-5-O1 or OsARF19-O1 are reduced. OsGH3-5 is localized at the endoplasmic retieulum (ER) matching reduction of the free auxin contents in OsGH3-5-O1. osarf19-TDNA and osgh3-5-Tos17 mutants without erected leaves show a function redundancy with other members of their gene family. OsARF19-overexpression lines are sensitive to exogenous BR treatment and alter the expressions of genes related to BR signalling. These findings provide novel insights into auxin and BR signalling, and might have significant implications for improving plant architecture of monocot crops.

Phosphate transporters OsPHT1;9 and OsPHT1;10 are involved in phosphate uptake in rice.

We characterized the function of two rice phosphate (Pi) transporters: OsPHT1;9 (OsPT9) and OsPHT1;10 (OsPT10). OsPT9 and OsPT10 were expressed in the root epidermis, root hairs and lateral roots, with their expression being specifically induced by Pi starvation. In leaves, expression of the two genes was observed in both mesophyll and vasculature. High-affinity Km values for Pi transport of OsPT9 and OsPT10 were determined by yeast experiments and two-electrode voltage clamp analysis of anion transport in Xenopus oocytes expressing OsPT9 and OsPT10. Pi uptake and Pi concentrations in transgenic plants harbouring overexpressed OsPT9 and OsPT10 were determined by Pi concentration analysis and (33) P-labelled Pi uptake rate analysis. Significantly higher Pi uptake rates in transgenic plants compared with wild-type plants were observed under both high-Pi and low-Pi solution culture conditions. Conversely, although no alterations in Pi concentration were found in OsPT9 or OsPT10 knockdown plants, a significant reduction in Pi concentration in both shoots and roots was observed in double-knockdown plants grown under both high- and low-Pi conditions. Taken together, our results suggest that OsPT9 and OsPT10 redundantly function in Pi uptake.

Can exploiting natural genetic variation in leaf photosynthesis contribute to increasing rice productivity? A simulation analysis.

Rice productivity can be limited by available photosynthetic assimilates from leaves. However, the lack of significant correlation between crop yield and leaf photosynthetic rate (A) is noted frequently. Engineering for improved leaf photosynthesis has been argued to yield little increase in crop productivity because of complicated constraints and feedback mechanisms when moving up from leaf to crop level. Here we examined the extent to which natural genetic variation in A can contribute to increasing rice productivity. Using the mechanistic model GECROS, we analysed the impact of genetic variation in A on crop biomass production, based on the quantitative trait loci for various photosynthetic components within a rice introgression line population. We showed that genetic variation in A of 25% can be scaled up equally to crop level, resulting in an increase in biomass of 22-29% across different locations and years. This was probably because the genetic variation in A resulted not only from Rubisco (ribulose 1,5-bisphosphate carboxylase/oxygenase)-limited photosynthesis but also from electron transport-limited photosynthesis; as a result, photosynthetic rates could be improved for both light-saturated and light-limited leaves in the canopy. Rice productivity could be significantly improved by mining the natural variation in existing germ-plasm, especially the variation in parameters determining light-limited photosynthesis.

Tissue-specific and light-dependent regulation of phytochrome gene expression in rice.

Phytochromes are red- and far red light photoreceptors in higher plants. Rice (Oryza sativa L.) has three phytochromes (phyA, phyB and phyC), which play distinct as well as cooperative roles in light perception. To gain a better understanding of individual phytochrome functions in rice, expression patterns of three phytochrome genes were characterized using promoter-GUS fusion constructs. The phytochrome genes PHYA and PHYB showed distinct patterns of tissue- and developmental stage-specific expression in rice. The PHYA promoter-GUS was expressed in all leaf tissues in etiolated seedlings, while its expression was restricted to vascular bundles in expanded leaves of light-grown seedlings. These observations suggest that light represses the expression of the PHYA gene in all cells except vascular bundle cells in rice seedlings. Red light was effective, but far red light was ineffective in gene repression, and red light-induced repression was not observed in phyB mutants. These results indicate that phyB is involved in light-dependent and tissue-specific repression of the PHYA gene in rice.

Light induces jasmonate-isoleucine conjugation via OsJAR1-dependent and -independent pathways in rice.

The bioactive form of jasmonate is the conjugate of the amino acid isoleucine (Ile) with jasmonic acid (JA), which is biosynthesized in a reaction catalysed by the GH3 enzyme JASMONATE RESISTANT 1 (JAR1). We examined the biochemical properties of OsJAR1 and its involvement in photomorphogenesis of rice (Oryza sativa). OsJAR1 has a similar substrate specificities as its orthologue in Arabidopsis. However, osjar1 loss-of-function mutants did not show as severe coleoptile phenotypes as the JA-deficient mutants coleoptile photomorphogenesis 2 (cpm2) and hebiba, which develop long coleoptiles in all light qualities we examined. Analysis of hormonal contents in the young seedling stage revealed that osjar1 mutants are still able to synthesize JA-Ile conjugate in response to blue light, suggesting that a redundantly active enzyme can conjugate JA and Ile in rice seedlings. A good candidate for this enzyme is OsJAR2, which was found to be able to catalyse the conjugation of JA with Ile as well as with some additional amino acids. In contrast, if plants in the vegetative stage were mechanically wounded, the content of JA-Ile was severely reduced in osjar1, demonstrating that OsJAR1 is the most important JA-Ile conjugating enzyme in the wounding response during the vegetative stage.

Comparative chemical screening and genetic analysis reveal tentoxin as a new virulence factor in Cochliobolus miyabeanus, the causal agent of brown spot disease on rice.

Brown spot disease, caused by Cochliobolus miyabeanus, is currently considered to be one of the most important yield reducers of rice (Oryza sativa L.). Despite its agricultural importance, little is known about the virulence mechanisms deployed by the fungus. Therefore, we set out to identify novel virulence factors with a role in disease development. This article reports, for the first time, the production of tentoxin by C. miyabeanus as a virulence factor during brown spot disease and the identification of the non-ribosomal protein synthetase (NRPS) CmNps3, responsible for tentoxin biosynthesis. We compared the chemical compounds produced by C. miyabeanus strains differing in virulence ability using ultra-high-performance liquid chromatography (UHPLC) coupled to high-resolution Orbitrap mass spectrometry (HRMS). The production of tentoxin by a highly virulent strain was revealed by principal component analysis of the detected ions and confirmed by UHPLC coupled to tandem-quadrupole mass spectrometry (MS/MS). The corresponding NRPS was identified by in silico genome analysis and confirmed by gene deletion. Infection tests with wild-type and Cmnps3 mutants showed that tentoxin acts as a virulence factor and is correlated with chlorosis development during the second phase of infection. Although rice has previously been classified as a tentoxin-insensitive plant species, our data demonstrate that tentoxin production by C. miyabeanus affects symptom development.