Strigolactones (SLs) were originally isolated from plant root exudates as germination stimulants for root parasitic plants and thus were regarded as detrimental to the producing plants. Their role as indispensable chemical signals for root colonization by symbiotic arbuscular mycorrhizal (AM) fungi was subsequently demonstrated, and SLs then became recognized as beneficial plant metabolites. In addition to these functions as communication chemicals for both symbiosis and parasitism in the rhizosphere, it has been recently shown that SLs or their metabolites are a novel class of plant hormones that inhibit shoot branching. Furthermore, SLs are suggested to have other biological functions in rhizosphere communications and in plant growth and development. In this issue, 4 review papers describing SLs as germination stimulants for root parasitic plants, the chemistry, structure-activity relationships and molecular design of SL analogues for practical use, physiological functions of SLs, and development of SL-biosynthetic inhibitors are included.

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Strigolactones (SLs) were originally isolated from plant root exudates as germination stimulants for root parasitic plants of the family Orobanchaceae, including witchweeds (Striga spp.), broomrapes (Orobanche and Phelipanche spp.), and Alectra spp., and so were regarded as detrimental to the producing plants. Most of these root parasites cause devastating damages to agricultural production in particular in developing countries, and their effective and economically feasible control measures have not yet been established. In this review, the unique host-dependent lifecycle of, discovery of SLs as germination stimulants for, and possible contribution of SLs to the host recognition of root parasitic plants are summarized.

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In this review, structural requirements of strigolactones (SLs) for seed germination of root parasitic plants and induction of hyphal branching in arbuscular mycorrhizal (AM) fungi are summarized. In addition, based on these results together with other available data, molecular design of SL analogs for practical application in agriculture is discussed. It appears that chemical modifications of SL may provide new SL analogs that are active seed germination stimulants but inactive in AM hyphal branching and vice-versa. Some of chemically stable SL analogs are promising leads for new plant growth regulators.

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Strigolactones (SLs) are released from plant roots to soil and function as communication signals for symbiosis and parasitism in the rhizoshere. In addition, SLs are a new class of hormones that inhibit shoot branching of the host plants. It is an interesting question why SLs play an important role in both communication with other organisms and inhibition of shoot branching. Auxin and cytokinin are also essential hormones to regulate shoot branching. It is important to understand how auxin, cytokinin, and SLs interact each other. Although two carotenoid cleavage dioxygenases (CCDs) and a cytochrome P450 monooxygenase contribute to SL biosynthesis and an F-box protein is involved in the perception or signal transduction in the SL pathway, more genes are likely to be required to produce SLs and transmit the signals. Furthermore, SLs may have other function in host plants in addition to the inhibition of shoot branching. Many researchers address these questions and contribute to new findings. Here, I introduce recent advances on the physiological function of SLs in the host plants.

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[in Japanese]

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For the last fifty years, unique signal molecule, ppGpp, has been received broad attention by a number of molecularbiologists. Recently, it has been revealed that the ppGpp biosynthesis system also exists in chloroplasts of photosynthetic eukaryotes including land plants. For elucidating physiological functions of the ppGpp in plants, currently some different approaches such as biochemical analysis of synthetic enzymes, genetic analysis of mutant plants, and in vitro reconstitution analysis of ppGpp-signaling in chloroplast extracts, have been utilized. This review overviews recent progress and perspectives in understanding of plant ppGpp-signaling systems.

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[in Japanese]

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The use of small molecule for perturbing protein function have potential advantages to classic genetics, while genetics is still valuable in complementing disadvantages of small molecule approach. Although the theoretical framework of small molecule-approach have been established, practical aspect of this strategy still needs more examples to be discussed. Here, I discuss some practices from our current work related to small molecules, as with an attempt to show what experiments should be prioritized for small molecule approach.

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