Ehanced Drought Tolerace And Genome-Wide Transcripteome In Transgenic Rice Expressing Xanthomonas Oryzae Hrfl Gene

Drought is the most devastating factor that menaces food production and food security, especially in areas with inadequate agricultural water resources. The estimated annual loss from the national economy caused by drought alone exceeds $25 billion in China. Global warming and increasingly frequent occurrence of drought exacerbate the situation. Warming since 1981 has resulted in annual combined losses of these three crops, wheat, maize and barley, representing roughly 40 Mt or $5 billion per year, as of 2002. Globally, rice is the most important crop in terms of the number of people dependent on it as a direct source of calories. It has been estimated that rice production consumes about half of the total water consumption in China. Drought stress is still the most important constraint in rice production, mostly due to annual variation in the rainfall patterns and uneven distribution of rainfall in the rice growing season. Consequently, with the global shortage of water, enhancing the drought tolerance of crops has a potentially huge impact on annual productivity, in addition to improved water management practices.Harpin proteins, which are secreted through Type III protein secretion system of Gram-negative plant pathogenic bacteria, generally affect virulence in host plants and induce hypersensitive cell death in nonhost plants and elicit multiple plant responses. Exogenous applications of Harpins induce SAR in plants by the activation of defense pathways mediated by salicylic acid, jasmonic acid or ethylene. Transformation of Harpin-encoding genes improves disease resistance in tobacco, rice, rape and cotton, a, a Harpin protein secreted by Erwinia amylovora, induces drought tolerance in Arabidopsis by activating the AB12-dependent ABA signaling. So far it is still unknown whether genetic transformation of Harpin-encoding genes improves plant tolerance to drought. Shao et al. （2008） have reported that expression of hrfl, a Harpin-encoding gene, in rice conferred durable nonspecific resistance to Magnaporthe grisea. In this study, the results showed that overexpression of a Harpin-encoding gene hrfl in rice increased drought tolerance through abscisic acid （ABA） signaling, hrfl-expression induces the increase of ABA content and promotes stomata closure in rice. The hrfl transgenic rice lines exhibited significantly increase in water retention ability, levels of free proline （Pro） and soluble sugars, tolerance to oxidative stress, the reactive oxygen species （ROS）-scavenging ability, and expression levels of four stress-related genes, OsLEA3-1, OsP5CS, Mn-SOD and NM₀01074345, under drought stress. Our studies confirmed that hrfl conferred transgenic crops enhanced tolerance to drought stress. These results suggest that Harpins may offer new opportunities for generating drought resistance in other crops.The transcriptome profile of the transgenic rice line NJH12 expressing hrfl from Xanthomonas oryzae pv. oryzae was analysed using a customized 57 k rice cDNA microarray. A total of 225 cDNA transcripts were differentially expressed in the transgenic line NJH12, in which several differentially expressed genes were related to the rice biotic and abiotic stress, including the transcripts related to defence response, the transcripts related to signal transduction, and the transcripts related to programmed cell death. Furthermore, transcripts encoding various recepter kinases, such as leucine-rich protein kinases, and mitogen-activated protein kinases were up-regulated in the transgenic line NJH12 and expression of transcripts related to the energy producing and consuming pathway was also increased. By analysing the putative functions of these transcripts, it is speculated that the signaling between drought tolerance and disease resistance in hrf1-overexpressing rice might converge on some pathways.