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During the last decade, significant research progress in the study of Arabidopsis thaliana has been made in defining the molecular mechanism by which the plant circadian clock regulates flowering time in response to changes in photoperiod. It is generally accepted that the clock-controlled CONSTANS (CO)-FLOWERING LOCUS T (FT)-mediated external coincidence mechanism underlying the photoperiodic control of flowering time is conserved in higher plants, including A. thaliana and Oryza sativa. However, it is also assumed that the mechanism differs considerably in detail among species. Here we characterized the clock-controlled CO-FT pathway in Lotus japonicus (a model legume) in comparison with that of A. thaliana. L. japonicus has at least one FT orthologous gene (named LjFTa), which is induced specifically in long-days and complements the mutational lesion of the A. thaliana FT gene. However, it was speculated that this legume might lack the upstream positive regulator CO. By employing L. japonicus phyB mutant plants, we showed that the photoreceptor mutant displays a phenotype of early flowering due to enhanced expression of LjFTa, suggesting that LjFTa is invovled in the promotion of flowering in L. japonicus. These results are discussed in the context of current knowledge of the flowering in crop legumes such as soybean and garden pea.

Plant circadian clock generates rhythms with a period close to 24 h, and it controls a wide variety of physiological and developmental events, including the transition to reproductive growth (or flowering). During the last decade, significant research progress in Arabidopsis thaliana has been made in defining the molecular mechanism by which the circadian clock regulates flowering time in response to changes in photoperiod. In Lotus japonicus, we have found that LjFTa, which encodes a ortholog of the Arabidopsis FLOWERING LOCUS T (FT), plays an important role in the promotion of flowering, but it is not clear how the expression of LjFTa is regulated in L. japonicus. Based on current knowledge of photoperiodic control of flowering time in A. thaliana, here we examined whether a microRNA is involved in the activation of LjFTa in L. japonicus. Two putative L. japonicus genes that are responsible for the production of miR172 (designated LjmiR172a and LjmiR172b) were cloned. Overexpression of LjmiR172a/b in A. thaliana resulted in markedly accelerated flowering through enhancement of the expression of FT, concomitantly reducing the expression level of TARGET OF EARLY ACTIVATION TAGGED 1 (TOE1) transcripts, the protein product of which functions as a transcriptional repressor of FT. These results suggest that LjmiR172 genes play a positive role in the LjFTa-mediated promotion of flowering in L. japonicus.

Recent intensive studies of the model plant Arabidopsis thaliana have revealed the molecular mechanisms underlying circadian rhythms in detail. Results of phylogenetic analyses indicated that some of core clock genes are widely conserved throughout the plant kingdom. For another model plant the legume Lotus japonicus, we have reported that it has a set of putative clock genes highly homologous to A. thaliana. Taking advantage of the L. japonicus hairy root transformation system, in this study we characterized the promoter activity of A. thaliana core clock genes CCA1 and PRR5 in heterologous L. japonicus cells and found that the molecular mechanism of circadian rhythm in L. japonicus is compatible with that of A. thaliana.

Sessile plants must continuously adjust their growth and development to optimize photosynthetic activity under ever-fluctuating light conditions. Among such light responses in plants, one of the best-characterized events is the so-called shade avoidance, for which a low ratio of the red (R):far-red (FR) light intensities is the most prominent stimulus. Such shade avoidance responses enable plants to overtop their neighbors, thereby enhancing fitness and competitiveness in their natural habitat. Considerable progress has been achieved during the last decade in understanding the molecular mechanisms underlying the shade avoidance responses in the model rosette plant, Arabidopsis thaliana. We characterize here the fundamental aspects of the shade avoidance responses in the model legume, Lotus japonicus, based on the fact that its phyllotaxis (or morphological architecture) is quite different from that of A. thaliana. It was found that L. japonicus displays the characteristic shade avoidance syndrome (SAS) under defined laboratory conditions (a low R:FR ratio, low light intensity, and low blue light intensity) that mimic the natural canopy. In particular, the outgrowth of axillary buds (i.e., both aerial and cotyledonary shoot branching) was severely inhibited in L. japonicus grown in the shade. These results are discussed with special emphasis on the unique aspects of SAS observed with this legume.

Here we report the phenotypic characteristics of a novel hypernodulation mutant, Ljrdh1 (root-determined hypernodulation 1) of Lotus japonicus. At 12 weeks after rhizobial inoculation, there were no differences between the growth of Ljrdh1 and, wild-type. However, Ljrdh1 showed 2 to 3 times higher nitrogen-fixing activity, and seed and pod yields, were approximately 50% higher than the wild-type. This is the first report of a legume hypernodulation mutant showing normal growth and a high-yielding characteristic under optimal cultivation conditions.