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Endogenous levels of bioactive gibberellins (GAs) are controlled by both biosynthetic and inactivation processes, and some cytochrome P450s are involved in this control mechanism. We have previously reported that CYP714B1 and CYP714B2 encode the enzyme GA 13-oxidase, which is required for GA1 biosynthesis, and that CYP714D1 encodes GA 16α,17-epoxidase, which inactivates the non-13-hydroxy GAs in rice. Arabidopsis has two CYP714 members, CYP714A1 and CYP714A2. To clarify the possible role of these genes in GA metabolism, enzymatic activities of their recombinant proteins were analyzed using a yeast expression system. We found that the recombinant CYP714A1 protein catalyzes the conversion of GA12 to 16-carboxylated GA12 (16-carboxy-16β,17-dihydro GA12), a previously unidentified GA metabolite. Bioassays of this GA product showed that CYP714A1 is an inactivation enzyme in Arabidopsis. This was confirmed by the extreme GA-deficient dwarf phenotype shown by CYP714A1-overexpressing plants. Intriguingly, the recombinant CYP714A2 protein catalyzed the conversion of ent-kaurenoic acid into steviol (ent-13-hydroxy kaurenoic acid). When GA12 was used as a substrate for CYP714A2, 12α-hydroxy GA12 (GA111) was produced as a major product and 13-hydroxy GA12 (GA53) as a minor product. Transgenic Arabidopsis plants overexpressing the CYP714A2 gene showed semi-dwarfism. GA analysis showed that the levels of non-13-hydroxy GAs, including GA4, were decreased, whereas those of 13-hydroxy GAs, including GA1 (which is less active than GA4), were increased in the transgenic plants. Our results suggest that the CYP714 family proteins contribute to the production of diverse GA compounds through various oxidations of C and D rings in both monocots and eudicots.

Strigolactones (SLs) or SL-derived metabolite(s) have recently been shown to act as endogenous inhibitors of axillary bud outgrowth. SLs released from roots induce hyphal branching of arbuscular mycorrhizal (AM) fungi that facilitate the uptake of inorganic nutrients, such as phosphate (Pi) and nitrate, by the host plants. Previous studies have shown that SL levels in root exudates are highly elevated by Pi starvation, which might contribute to successful symbiosis with AM fungi in the rhizosphere. However, how endogenous SL levels elevated by Pi starvation contribute to its hormonal action has been unknown. Here, we show that tiller bud outgrowth in wild-type rice seedlings is inhibited, while root 2'-epi-5-deoxystrigol (epi-5DS) levels are elevated, in response to decreasing Pi concentrations in the media. However, the suppression of tiller bud outgrowth under Pi deficiency does not occur in the SL-deficient and -insensitive mutants. We also show that the responsiveness to exogenous SL is slightly increased by Pi deficiency. When Pi-starved seedlings are transferred to Pi-sufficient media, tiller bud outgrowth is induced following a decrease in root epi-5DS levels. Taken together, these results suggest that elevated SL levels by Pi starvation contribute to the inhibition of tiller bud outgrowth in rice seedlings. We speculate that SL plays a dual role in the adaptation to Pi deficiency; one as a rhizosphere signal to maximize AM fungi symbiosis for improved Pi acquisition and the other as an endogenous hormone or its biosynthetic precursor to optimize shoot branching for efficient Pi utilization.