Cells of the zona fasciculata and zona reticularis lack aldosterone synthase (CYP11B2) that converts corticosterone to aldosterone, and thus these tissues produce only the weak mineralocorticoid corticosterone. However, both these zones do contain the CYP17A1 missing in zona glomerulosa and thus produce the major glucocorticoid, cortisol. Zona fasciculata and zona reticularis cells also contain CYP17A1, whose 17,20-lyase activity is responsible for producing the androgens, dehydroepiandosterone (DHEA) and androstenedione. Thus, fasciculata and reticularis cells can make corticosteroids and the adrenal androgens, but not aldosterone.
Previous studies have demonstrated that auxin regulates LR initiation through activation of auxin-inducible LBD16/ASL18 and LBD29/ASL16 genes by ARF7 and ARF19 ( Okushima et al. , 2007 ). To examine the relationship between RLF and ARF7/19-mediated auxin signaling, we investigated whether the rlf-1 mutation affects ARF7 , ARF19 and auxin-inducible LBD16/ASL18 and LBD29/ASL16 gene expression. As shown in Figure 3(a,b) , the expression levels of ARF7 and ARF19 mRNAs and auxin-inducible accumulation of LBD16/ASL18 and LBD29/ASL16 mRNAs were not affected in the rlf-1 roots compared to wild-type roots. We also examined tissue-specific expression of both pLBD16::GUS and pLBD29::GUS reporters in the roots of rlf-1 and wild-type plants. In wild-type roots, the reporters are mainly expressed at the LR initiation sites and LR primordia, respectively, and exogenous auxin treatment induces GUS activity both in the LR primordia and in the distal region of the primary roots ( Figure 3c ) ( Okushima et al. , 2007 ). In the rlf-1 roots, the expression patterns of both pLBD16::GUS and pLBD29::GUS in the mutant are similar to those in the wild-type in the absence or presence of exogenous auxin ( Figure 3c ). These results indicate that RLF is not required for auxin-induced expression of LBD16/ASL18 and LBD29/ASL16 , which are direct targets of ARF7/19 .