Search

This paper reports an on-chip analytical method for studying physical mechanisms of plant root growth in soil environments. To quantitatively evaluate physical interaction between root and soil, we developed a silicon-based microchannel device integrated with force displacement sensor which mimics a barrier in soil. By using developed microsystem, we successfully characterized the driving forces of root growth in three kinds of plants including Arabidopsis thaliana, which is known as a model organism. This analytical method allows us to efficiently characterize potential adaptability of root system to soil environments at early growth stage. The gaining knowledge might contribute for breed improvement, in terms of increasing crop productivity and plant biomass.

Phloem is a conductive tissue that allocates nutrients from mature source leaves to sinks such as young developing tissues. Phloem also delivers proteins and RNA species, such as small RNAs and mRNAs. Intensive studies on plant systemic signaling revealed the essential roles of proteins and RNA species. However, many of their functions are still largely unknown, with the roles of transported mRNAs being particularly poorly understood. A major difficulty is the absence of an accurate and comprehensive list of mobile transcripts. In this study, we used a hetero-graft system with Nicotiana benthamiana as the recipient scion and Arabidopsis as the donor stock, to identify transcripts that moved long distances across the graft union. We identified 138 Arabidopsis transcripts as mobile mRNAs, which we collectively termed the mRNA mobilome. Reverse transcription-PCR, quantitative real-time PCR and droplet digital PCR analyses confirmed the mobility. The transcripts included potential signaling factors and, unexpectedly, more general factors. In our investigations, we found no preferred transcript length, no previously known sequence motifs in promoter or transcript sequences and no similarities between the level of the transcripts and that in the source leaves. Grafting experiments regarding the function of ERECTA, an identified transcript, showed that no function of the transcript mobilized. To our knowledge, this is the first report identifying transcripts that move over long distances using a hetero-graft system between different plant taxa.

In this paper, we present a novel microsystem for investigating mechanisms of plant-root growth. To measure a generative force by plant-root growth efficiently, we developed a silicon-based microchannel integrated with MEMS force sensors. We have also developed a protocol for sowing a plant seed onto the micro device and experimentally confirmed that plant-roots can grow through the microchannel filled with agarose-gel medium. By using the microsystems, we successfully estimated the generative force by plant-root growth.

In plants, the phloem is the component of the vascular system that delivers nutrients and transmits signals from mature leaves to developing sink tissues. Recent studies have identified proteins, mRNA, and small RNA within the phloem sap of several plant species. It is now of considerable interest to elucidate the biological functions of these potential long-distance signal agents, to further our understanding of how plants coordinate their developmental programs at the whole-plant level. In this study, we developed a strategy for the functional analysis of phloem-mobile mRNA by focusing on IAA transcripts, whose mobility has previously been reported in melon (Cucumis melo cv. Hale's Best Jumbo). Indoleacetic acid (IAA) proteins are key transcriptional regulators of auxin signaling, and are involved in a broad range of developmental processes including root development. We used a combination of vasculature-enriched sampling and hetero-grafting techniques to identify IAA18 and IAA28 as phloem-mobile transcripts in the model plant Arabidopsis thaliana. Micro-grafting experiments were used to confirm that these IAA transcripts, which are generated in vascular tissues of mature leaves, are then transported into the root system where they negatively regulate lateral root formation. Based on these findings, we present a model in which auxin distribution, in combination with phloem-mobile Aux/IAA transcripts, can determine the sites of auxin action.

Long-distance signaling via phloem tissues is an important mechanism for inter-organ communication. Such communication allows plants to integrate environmental information into physiological and developmental responses. Grafting has provided persuasive evidence of long-distance signaling involved in various processes, including flowering, tuberization, nodulation, shoot branching, post-transcriptional gene silencing, and disease resistance. A micro-grafting technique to generate two-shoot grafts is available for young seedlings of Arabidopsis thaliana and was adapted for use in the study of flowering. Histological analysis using transgenic plants expressing beta-glucuronidase (GUS) in phloem tissues showed that phloem continuity between a stock and a scion was established between 7 and 10 days after grafting. Experiments using tracer dyes and enhanced green fluorescent protein (EGFP) showed that the phloem connection was functional and capable of effecting macromolecular transmission. Successful grafts can be obtained at high frequency (10-30%) and selected after 2-3 weeks of post-surgery growth. This method was applied successfully to the study of flowering, one of the important events regulated by long-distance signaling. This grafting technique will facilitate the study of the long-distance action of genes involved in various aspects of growth and development, and in transport of signal molecules.

FLOWERING LOCUS T ( FT ) is a conserved promoter of flowering that acts downstream of various regulatory pathways, including one that mediates photoperiodic induction through CONSTANS ( CO ), and is expressed in the vasculature of cotyledons and leaves. A bZIP transcription factor, FD, preferentially expressed in the shoot apex is required for FT to promote flowering. FD and FT are interdependent partners through protein interaction and act at the shoot apex to promote floral transition and to initiate floral development through transcriptional activation of a floral meristem identity gene, APETALA1 ( AP1 ). FT may represent a long-distance signal in flowering.