From Biological Warfare to the Brighter Side of Rice Research

Abstract

Rice (Orzya sativa L.) is an important staple food crop in Asia providing nourishment to half of the world’s population. With the global human population set to rise to over nine billion by 2050 (according to United Nations predictions, see http://esa. un.org/wpp/Other-Information/Press_Release_WPP2010.pdf ), there are increasing pressures to secure the production and productivity of this crop for future generations. In addition, because rice is also considered a model organism for monocotyledonous plants due to its relatively small and sequenced genome, considerable efforts are underway to better understand the biology of this crop plant. To celebrate some of these excellent advances Plant and Cell Physiology is proud to present a special collection of articles featuring the latest research on rice, covering topics such as disease resistance and trait biology, to the latest technologies being developed to further facilitate in-depth research into this agronomically important species. Due to its growth in moist, temperate climates, rice is susceptible to pathogen attack. With incidences of rice blast fungus (Magnaporthe grisea) infection significantly decreasing annual global yields, research into rice-pathogen interactions is becoming central to stabilising the productivity of this crop food. Innate immunity in rice can be triggered by two main pathways either involving recognition of pathogen-associated molecular patterns (PAMPS) such as chitin and sphingolipids, or highly variable microbial effector molecules (Chen and Ronald 2011). The small plasma membrane bound Rac/Rop GTPase, OsRac1 is pivotal for both PAMP-triggered immunity and effector-triggered immunity, which have been shown to lead to a range of responses including cell death, ROS production, activation of pathogen-related gene expression and phytoalexin biosynthesis (Chen et al. 2010, Kawasaki et al. 1999). On pages 740–754, Kim et al. investigate the downstream transcriptional signalling cascade involved in triggering the OsRac1 mediated immune response in rice. The authors identify a novel bHLH transcription factor, Rac Immunity 1 (RAI1), that acts as a key downstream component of OsRac1 signalling following M. grisea attack to regulate expression of the elicitor-responsive genes PAL1 and OsWRKY19. Further, RAI1 interacts in vivo with OsMAPK3/6 in the OsRac1 complex, thereby expanding the battalion of plant signalling components responding to the relentless attack by microbial and pathogenic microorganisms. In an effort to sustain rice as a reliable food crop, trait research studies designed to ameliorate agronomically important traits such as grain size, vivipary, uniform germination, and seedling vigour and establishment have become a hot focus in recent years (Martinez-Andujar et al. 2012, Nambara and Nonogaki, 2012). Rice grains are complex structures that are formed following double fertilisation. Intriguingly, as vital carriers of genetic information, they are able to remain dormant for required periods of time before germinating under favourable conditions to ensure plant survival to the next generation. Therefore, grains ought to seemingly possess a mechanism that enables them to perform such a task and successfully develop into an established seedling. In this issue, Sano et al. (see pages 687–698) applied transcriptional and translational inhibitors to rice seeds, then performed a proteomic analysis of these chemically modified materials to determine the stability and extent to which seed proteins can be synthesised from long-stored mRNAs. Their findings indicate that while mature grains contained levels of stored mRNAs sufficient to initiate germination events, de novo translation was essential for successful completion of rice seed germination. Furthermore, this study provides novel insight into how plants have the ability to select discrete long-lived mRNAs to be translated during early germination when others are targeted for degradation. Flowering is a key agronomic trait in plants with wider significant biological implications, as it represents the onset of the mature reproductive phase in the plant life-cycle. Like animals, plants also respond to seasonal cues, such as exposure to day length, and adjust their time to flowering accordingly such that double fertilisation and seed production occurs when stressful conditions are least likely to be encountered. In contrast to Arabidopsis thaliana, flowering time in rice is promoted by exposure to short days. In addition, rice photoperiodicity is also mediated by two regulators of flowering under long days: Grain number, plant height and heading date locus 7 (Ghd7) and Early heading date 1 (Ehd1), for which there are no clear orthologues present in Arabidopsis (Itoh et al. 2010). Despite these apparent differences, the Flowering Time (FT) orthologues appears to play a central role in flowering regulation (Simpson and Dean 2002) in both species. Another key player in controlling flowering time is Early flowering 3 (ELF3), which in Arabidopsis is required for maintaining circadian rhythms (Dixon et al. 2011). In this issue, two papers report work on an ELF3 homologue in rice, Ef7/Heading date 17 (HD17) (Matsubara et al. 2008, Yuan et al. 2009). The Editor-in-chief’s choice article by Saito et al. on pages 717–728, presents work on the characterisation of the ef7 mutant, which exhibits delayed flowering. Genetic