R [751]. The importance with the IPA pathway in plant development and improvement has made

R [751]. The importance with the IPA pathway in plant development and improvement has made it the focus of study in recent years, and it can be regulated by many developmental and environmental signals, like phytohormones [22,48,82]. Early definite proof from the effect of cytokinin on auxin biosynthesis was reported by Jones et al. (2010), displaying that cytokinin can α9β1 medchemexpress induce auxin biosynthesis and that some signaling components of auxin and cytokinin are involved in this method; on the other hand, the molecular mechanism remains unclear [15]. Later, the authors found that cytokinin can induce the expression of both TAA1 and YUC8 genes to enhance auxin production [16,17]. Inside the adventitious root apex of Arabidopsis, cytokinin-mediated up-regulation of YUC6 was discovered to become involved inside the formation in the QC (Quiescent Center) [83]. Cytokinin-induced expression of YUC1 and YUC4 inside the P2Y1 Receptor site gynoecia primordium has been reported to ensure correct domain patterning [39]. Transcriptional activation of TAA and YUC family genes by cytokinin is dependent on cytokinin signaling transduction, as has been shown for TAA1/CKRC1 [16] and YUC8/CKRC2 [17] in Arabidopsis (Figure 1). Yan et al. (2017) identified that type-B ARRs can directly bind two cis elements in the promoter as well as the second intron of TAA1 to activate its transcription [18]. The AHKs-ARR1/12-mediated cytokinin signaling pathway can also be reported to become required for cytokinin-induced up-regulation of TAA1 and YUC8, plus the PIF4 (phytochrome-interacting components four) is expected for this upregulation. Transcription of PIF4 itself is induced by cytokinin via the AHKs-ARR1/12 signaling pathway, indicating that PIF4 plays an crucial part in mediating the regulatory impact of cytokinin around the transcriptions of TAA1 and YUC8 genes in the IPA pathway of auxin biosynthesis [17].Int. J. Mol. Sci. 2021, 22, x FOR PEER REVIEW3 ofInt. J. Mol. Sci. 2021, 22,of PIF4 itself is induced by cytokinin via the AHKs-ARR1/12 signaling pathway, indicat3 of 15 ing that PIF4 plays an necessary part in mediating the regulatory effect of cytokinin around the transcriptions of TAA1 and YUC8 genes inside the IPA pathway of auxin biosynthesis [17].Figure 1. Auxin biosynthesis/metabolism and transport regulated cytokinin. Figure 1. Auxin biosynthesis/metabolism and transportregulated byby cytokinin.Upstream of the IPA pathway, the two subunits of rate-limiting anthranilate synUpstream of your IPA pathway, the two subunits of rate-limiting (anthranilate thase, ASA1 (anthranilate synthase alpha-subunit 1)/WEI2/CKRC6 and ASB1 anthranilate synthase, ASA1 (anthranilate synthase alpha-subunit 1)/WEI2/CKRC6 and are particularly synthase beta-subunit 1)/WEI7 within the Trp biosynthesis pathway [72,84], ASB1 (anthranilate synthase beta-subunit 1)/WEI7 within the Trp biosynthesis pathway [72,84], are particularly expressed inside the root tip [72,73,79,85]. It has been shown that the transcriptional levels of ASA1/WEI2/CKRC6 [72,73,79,85]. can be induced by cytokinin transcriptional levels of expressed in the root tipand ASB1/WEI7It has been shown that the[15,85,86], and ARR1 promotes auxin biosynthesis within the stem cell niche via the up-regulation[15,85,86], and ARR1 ASA1/WEI2/CKRC6 and ASB1/WEI7 may be induced by cytokinin of ASB1/WEI7 in the root meristem (Figure 1) [85]. promotes auxin biosynthesis within the stem cell niche through the up-regulation of ASB1/WEI7 in In a different proposed but nonetheless somewhat obscure TD pathway named the IAOx (indolethe root meristem (Figure 1).