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We investigated the physiological and molecular basis of lower fecundity of massugu2 (msg2), which is a dominant mutant of an auxin primary response gene, IAA19, in Arabidopsis thaliana. By measuring the length of all stamens and pistils in inflorescences and the reference growth rate of pistils, we constructed growth curves of pistils and stamens between stages 12 and 15 of flower development. Pistil growth was found to consist of a single exponential growth, while stamen growth consisted of three exponential phases. During the second exponential phase, the growth rate of stamen filaments was ～10 times greater than the growth rates in the other two phases. Consequently, stamens whose growth was initially retarded grew longer than the pistil, putting pollen grains on the stigma. msg2-1 stamens, on the other hand, exhibited a less obvious growth increase, resulting in less frequent contact between anthers and stigma. MSG2 was expressed in the stamen filaments and its expression almost coincided with the second growth phase. Stamen filaments appeared to elongate by cell elongation rather than cell division in the epidermal cell file. Considering that MSG2 is likely to be a direct target of the auxin F-box receptors, MSG2 may be one of the master genes that control the transient growth increase of stamen filaments.

In rice (Oryza sativa) seedlings, continuous white-light irradiation inhibited the growth of seminal roots but promoted the growth of crown roots. Here, we examined the mechanisms of photoinhibition of seminal root growth. Photoinhibition occurred in the absence of nitrogen, but increased with increasing nitrogen concentrations. In the presence of nitrogen, photoinhibition was correlated with coiling of the root tips. The seminal roots were most photosensitive 48-72 h after germination during the 7-d period after germination. White-light irradiation for at least 6 h was required for photoinhibition, and the Bunsen-Roscoe law of reciprocity was not observed. Experiments with phytochrome mutants showed that far-red light was perceived exclusively by phyA, that red light was perceived by both phyA and phyB, and phyC had little or no role in growth inhibition or coiling of the seminal roots. Fluence-response curve analyses also showed that phyA and phyB control very low fluence response and low fluence response, respectively, in the seminal roots. This was essentially the same as the growth inhibition previously observed at the late stage of coleoptile development (80 h after germination). These results also suggest that other blue-light photoreceptors are involved in growth inhibition of the seminal roots. The photoperceptive site for the root growth inhibition appeared to be the roots themselves. All three phytochrome species of rice were detected immunochemically in roots.

We have examined the expression pattern of an auxin primary response gene, MSG2/IAA19, during photo- and gravitropic responses of hypocotyls using a transgenic Arabidopsis harboring MSG2/IAA19 promoter::GUS. The upper portion of most etiolated hypocotyls showed uniform b-glucuronidase (GUS) staining with the strongest activity in the pericycle. When hypocotyls were irradiated with unilateral blue light, GUS activity on the concave side of hypocotylswas decreased, resulting in differentialGUSstaining with a stronger signal on the convex side. The number of differentially stained hypocotyls peaked at 24 h after the onset of the phototropic stimuli, while hypocotyl curvature continued to increase for the entire 36-h experimental period. This result suggests that the MSG2/IAA19 expression precedes the phototropic responses. When seedlings were grown under dim white light, their hypocotyls displayed almost no GUS activity. The light-grown hypocotyls also showed differential GUS staining after phototropic stimuli as result of the increase in GUS activity on the convex side of hypocotyls, especially in the epidermis, the outer cortex and pericycle, although GUS activity was much weaker than that observed in etiolated hypocotyls. Similar but less obvious differential staining was obtained for gravitropic response of hypocotyls. Considering the recent finding thatAux/IAA proteins are immediate targets of the auxin F box receptors, MSG2/IAA19 is likely to act as one of master genes for tropic responses.

rcd1 is a mutant of Arabidopsis thaliana that is more resistant to methyl viologen, but more sensitive to ozone than the wild type. rcd1-2 is caused by a single nucleotide substitution that results in a premature stop codon at Trp-332. The rcd1-2 mRNA level does not change significantly with the mutation. Since overexpression of rcd1-1 cDNA has been shown to bring about an rcd1-like phenotype, we created and examined the overexpression lines of RCD1 by the use of the cauliflower mosaic virus 35S promoter. The transgenic lines exhibited a weak rcd1-like phenotype, although no resistance to methyl viologen was observed. Further, they fully complemented the aberrant rcd1-2 phenotype. Subcellular localization of RCD1 was examined by transiently expressing green fluorescent protein (GFP) fused with RCD1 in onion epidermal cells. GFP signals are observed as aggregated foci in the inner nuclear matrix-like region.