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A search for growth inhibitors in rice root exudates was undertaken in order to clarify the allelopathic system in rice (Oryza sativa L.). Rice seedlings inhibited the growth of cress (Lepidium sativum L.) and lettuce (Lactuca sativa L.) seedlings when the cress and lettuce were grown with rice seedlings. The putative compound causing the inhibitory effect of rice seedlings was isolated from their culture solution, and the chemical structure of the inhibitor was determined by spectral data as momilactone B. Momilactone B inhibited the growth of cress and lettuce seedlings at concentrations greater than 3 and 30 microM, respectively. The concentration of momilactone B was 3.4 and 1.1 nmol per seedling in the culture solutions of husked and non-husked rice seedlings, respectively. These results suggest that rice seedlings may release momilactone B into the environment and the stress caused by the husk-treatment may increase the amount of momilactone B released. Thus, momilactone B may play an important role in rice allelopathy.

The distribution of the endogenous auxin-inhibiting substance, 8-epixanthatin, was determined in the lighted and shaded sides of phototropically-stimulated, de-etiolated sunflower (Helianthus annuus L. cv. Taiyo) hypocotyls. From 40 min after the onset of phototropic stimulation, the growth rate at the lighted side was inhibited, whereas that at the shaded side showed no change. In the lighted side, 8-epixanthatin increased by 20 min after the onset of unilateral illumination and, after 40 min, reached a 3-fold larger concentration than that in the shaded side. Unilateral application of 8-epixanthatin suppressed the growth of etiolated hypocotyls at the applied side only, causing the hypocotyls to bend at the site of application. It is concluded that phototropic curvature in sunflower hypocotyls is caused by a lateral gradient of the auxin-inhibiting substance 8-epixanthatin.

Auxin-binding protein (ABP1) is a putative receptor for auxin in the plasma membrane. We isolated a full-length cDNA encoding ABP1 from radish by screening a cDNA library with its partial cDNA fragment generated by the reverse transcriptase-polymerase chain reaction (RT-PCR) method. Radish abpl mRNA was highly expressed in cotyledons, hooks and hypocotyls, and less so in roots of radish seedlings. The deduced amino acid sequence of radish abp1 consisted of conserved auxin-binding motifs, a signal peptide and ER-retention signal, and contained two potentially N-linked glycosylation signals. To analyze the biochemical properties of the radish abpl product, this cDNA was expressed in insect cells by the baculovirus vector system. The result of tunicamycin-treatment showed that the signal peptide was cleaved and that the radish abpl product was glycosylated at both target sites in insect cells.