Analysis of the expression of the AGL17-like clade of MADS-box transcription factors in rice.

In plants, MADS-box transcription factors are key regulators of floral and fruit development, organ dehiscence and stress responses. Nevertheless, the functions of most of them are still unknown. In Arabidopsis thaliana, the AGL17-like clade of MADS-box transcription factors comprises four members. AGL17 is involved in floral induction, whereas AGL44/ANR1 is involved in the adaptive development of roots in response to nitrate. AGL21 is primarily expressed in the roots and AGL16 in the leaves, suggesting that these transcription factors may be involved in the control of vegetative development. In Oryza sativa, the AGL17-like clade comprises five members, OsMADS23, OsMADS25, OsMADS27, OsMADS57 and OsMADS61. In a first attempt to characterize their functions, we used promoter::Gus reporter gene fusions and RT-qPCR to study the expression patterns of these genes and their regulation by different external stimuli. The OsMADS23, OsMADS25, OsMADS27 and OsMADS57 promoters were active in the root's central cylinder. In addition, the OsMADS57 promoter was active in leaves, whereas the OsMADS61 promoter was only active in the leaf tips and the stem base. OsMADS25 and OsMADS27 transcripts accumulated in response to osmotic stress, whereas the expression levels of OsMADS25, OsMADS27 and OsMADS57 were slightly induced by nitrate. Each of these five genes was responsive to various hormonal treatments. These distinct expression patterns indicate that these five genes have specific and non-redundant functions that likely differs from those of their A. thaliana homologs.

Regulation of shoot and root development through mutual signaling.

Plants adjust their development in relation to the availability of nutrient sources. This necessitates signaling between root and shoot. Aside from the well-known systemic signaling processes mediated by auxin, cytokinin, and sugars, new pathways involving carotenoid-derived hormones have recently been identified. The auxin-responsive MAX pathway controls shoot branching through the biosynthesis of strigolactone in the roots. The BYPASS1 gene affects the production of an as-yet unknown carotenoid-derived substance in roots that promotes shoot development. Novel local and systemic mechanisms that control adaptive root development in response to nitrogen and phosphorus starvation were recently discovered. Notably, the ability of the NITRATE TRANSPORTER 1.1 to transport auxin drew for the first time a functional link between auxin, root development, and nitrate availability in soil. The study of plant response to phosphorus starvation allowed the identification of a systemic mobile miRNA. Deciphering and integrating these signaling pathways at the whole-plant level provide a new perspective for understanding how plants regulate their development in response to environmental cues.

Genomic and evolutionary features of the SPI-1 type III secretion system that is present in Xanthomonas albilineans but is not essential for xylem colonization and symptom development of sugarcane leaf scald.

Xanthomonas albilineans is the causal agent of sugarcane leaf scald. Interestingly, this bacterium, which is not known to be insect or animal associated, possesses a type III secretion system (T3SS) belonging to the injectisome family Salmonella pathogenicity island 1 (SPI-1). The T3SS SPI-1 of X. albilineans shares only low similarity with other available T3SS SPI-1 sequences. Screening of a collection of 128 plant-pathogenic bacteria revealed that this T3SS SPI-1 is present in only two species of Xanthomonas: X. albilineans and X. axonopodis pv. phaseoli. Inoculation of sugarcane with knockout mutants showed that this system is not required by X. albilineans to spread within xylem vessels and to cause disease symptoms. This result was confirmed by the absence of this T3SS SPI-1 in an X. albilineans strain isolated from diseased sugarcane. To investigate the importance of the T3SS SPI-1 during the life cycle of X. albilineans, we analyzed T3SS SPI-1 sequences from 11 strains spanning the genetic diversity of this species. No nonsense mutations or frameshifting indels were observed in any of these strains, suggesting that the T3SS SPI-1 system is maintained within the species X. albilineans. Evolutionary features of T3SS SPI-1 based on phylogenetic, recombination, and selection analyses are discussed in the context of the possible functional importance of T3SS SPI-1 in the ecology of X. albilineans.