Taking Hormone Crosstalk to a New Level: Brassinosteroids Regulate Gibberellin Biosynthesis.

Given that both brassinosteroid (BR) and gibberellin (GA) phytohormones regulate plant growth (reviewed in Schwechheimer, 2011; Fridman andSavaldi-Goldstein, 2013), it is perhaps not surprising that there is extensive crosstalk between the BR and GA signaling networks, withDELLAproteins and the BZR1 transcription factor playing particularly important roles to integrate the signals. However, it had long been thought that the effects of BR were not mediated directly through GA, for instance, based on the inability of exogenous GA to rescue the phenotypes of BR mutants in various species. Recent findings have brought this question back to the forefront. Stewart Lilley et al. (2013) reported that expression of the GA biosynthetic gene GA20ox in Arabidopsis thaliana is responsive to exogenous BR, and Tong et al. (2014) found that GA biosynthesis is regulated by BRs in rice (Oryza sativa).Now,Unterholzneretal. (2015)have established that some downstream effects of BRs in Arabidopsis are in fact mediated by their function in GA production. Unterholzner and coworkers found that GA was decreased in an Arabidopsis BR biosynthetic mutant (cpd) and in BR signaling mutants (including the receptor mutant bri1-1).These resultswere instrikingcontrast to the decade-old findings that GA levels are not changed in pea (Pisum sativum) BR mutants (Jager et al., 2005). To explore this potential interaction between BR andGA biosynthesis, Unterholzner et al. used bri1-1 and the milder BR receptor mutant allele bri1-301 aswellastheBRbiosynthesismutantcpd.For each mutant, germination on water agar was reduced compared with that of the wild type. Importantly, germination of these BRmutants was rescued by exogenous GA and by stratification, which is known to induce GA biosynthesis. These results demonstrate that downstream effects of BR on germination in the wild type are mediated by GA. Furthermore, GA partially restored hypocotyl elongation and plant height phenotypes of the BR mutants.Growthconditionsgreatly influenced these effects of GA, which likely explains why they have not been observed previously. Unterholzner et al. went on to elucidate how the effects of BR on GA biosynthesis are mediated. They found that the expression of the GA biosynthetic genes GA20ox andGA3oxwas reduced in BR biosynthetic and signalingmutants. Treatmentwith exogenous BR in the cpd background further demonstrated that BR regulates GA biosynthesis by influencing the expressionof genes responsible for the rate-limiting step of GA production. The authors elegantly showed that defects in bri1-301 could be rescued by expression of GA20ox1 under the BRI1 promoter (Figure 1), which indicates that a portion of the bri1-301 phenotypes are related to GA deficiency. They found that the BES1 and BZR1 transcription factors likely mediate this BRresponsive expression through binding a motif in thepromotersofGA20ox1andother GA biosynthetic genes. This nice work from Unterholzner et al. establishes that BR influencesGAbiosynthesis indicotsaswell as in monocots and that the two hormones function in the same pathway for at least somedownstreameffects. Italsomakesclear crosstalk between these phytohormones occurs not just at the level of signals converging on the same transcription factors, but also at the level of hormone biosynthesis. Nancy R. Hofmann Science Editor [email protected] ORCID ID: 0000-0001-9504-1152