Over-expression of OSRIP18 increases drought and salt tolerance in transgenic rice plants.

Both drought and high salinity stresses are major abiotic factors that limit the yield of agricultural crops. Transgenic techniques have been regarded as effective ways to improve crops in their tolerance to these abiotic stresses. Functional characterization of genes is the prerequisite to identify candidates for such improvement. Here, we have investigated the biological functions of an Oryza sativa Ribosome-inactivating protein gene 18 (OSRIP18) by ectopically expressing this gene under the control of CaMV 35S promoter in the rice genome. We have generated 11 independent transgenic rice plants and all of them showed significantly increased tolerance to drought and high salinity stresses. Global gene expression changes by Microarray analysis showed that more than 100 probe sets were detected with up-regulated expression abundance while signals from only three probe sets were down-regulated after over-expression of OSRIP18. Most of them were not regulated by drought or high salinity stresses. Our data suggested that the increased tolerance to these abiotic stresses in transgenic plants might be due to up-regulation of some stress-dependent/independent genes and OSRIP18 may be potentially useful in further improving plant tolerance to various abiotic stresses by over-expression.

Genome-wide survey of the RIP domain family in Oryza sativa and their expression profiles under various abiotic and biotic stresses.

Ribosome-inactivating proteins (RIPs) are N-glycosidases that inhibit protein synthesis by depurinating rRNA. Despite their identification more than 25 years ago, little is known about their biological functions. Here, we report a genome-wide identification of the RIP family in rice based on the complete genome sequence analysis. Our data show that rice genome encodes at least 31 members of this family and they all belong to type 1 RIP genes. This family might have evolved in parallel to species evolution and genome-wide duplications represent the major mechanism for this family expansion. Subsequently, we analyzed their expression under biotic (bacteria and fungus infection), abiotic (cold, drought and salinity) and the phytohormone ABA treatment. These data showed that some members of this family were expressed in various tissues with differentiated expression abundances whereas several members showed no expression under normal growth conditions or various environmental stresses. On the other hand, the expression of many RIP members was regulated by various abiotic and biotic stresses. All these data suggested that specific members of the RIP family in rice might play important roles in biotic and abiotic stress-related biological processes and function as a regulator of various environmental cues and hormone signaling. They may be potentially useful in improving plant tolerance to various abiotic and biotic stresses by over-expressing or suppressing these genes.

Large-scale systematic study on stability of the Ds element and timing of transposition in rice.

Activator/Dissociation (Ac/Ds) transposon mutagenesis is a widely used tool for gene identification; however, several reports on silencing of the Ac/Ds element in starter lines and in stable transposants question the applicability of such an approach in later generations. We have performed a systematic analysis on various aspects of the silencing phenomenon in rice (Oryza sativa ssp. japonica cv. Nipponbare). High somatic and germinal transposition frequencies observed in earlier generations were maintained as late as T4 and T5 generations; thus the propagation of parental lines did not induce transposon silencing. Moreover, the stably transposed Ds element was active even at the F5 generation, since Ac could remobilize the Ds element as indicated by the footprint analysis of several revertants. Expression of the bar gene was monitored from F3 to F6 generations in >1,000 lines. Strikingly, substantial transgene silencing was not observed in any of the generations tested. We analyzed the timing of transposition during rice development and provide evidence that Ds is transposed late after tiller formation. The possibility, that the independent events could be the result of secondary transposition, was ruled out by analyzing potential footprints by reciprocal PCR. Our study validates the Ac/Ds system as a tool for large-scale mutagenesis in rice, since the Ds elements were active in the starter and insertion lines even in the later generations. We propose that harvesting rice seeds using their panicles is an alternative way to increase the number of independent transposants due to post-tillering transposition.