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. 2018 Dec 13;7(4):109. doi: 10.3390/plants7040109

Figure 3.

Figure 3

Xylem differentiation in the primary root. HD-ZIP IIIs are important promotors of xylem differentiation. Their level is regulated by a gradient of inhibitory miRNA165/6. miRNA165/6 levels are defined by SHORTROOT (SHR)/SCARECROW (SCR) dimer presence. The gradient is established by SHR diffusion from its production area in the procambium to the endodermis, where it forms the dimer with SCR that promotes miRNA165/6 expression. This results in a miRNA165/6 gradient with highest levels in the endodermis and an inverse gradient for HD-ZIP IIIs that promote xylem differentiation. The HD-ZIP IIIs induce MP and IAA20/30. They also enhance ACL5 expression and CK (cytokinin) production. ACL5 induces translation of SUPRESSOR OF ACAULIS LIKE (SACL) genes that inhibit LHW–TMO5 dimerization and thus LOG expression, lowering the CK levels. The dimer also induces the CK signalling inhibitor AHP6, inhibiting the negative effect of CK on xylem differentiation. In the procambium, the CK inhibitory effects mediated by the HD-ZIP IIIs are not present, which leads to higher CK levels and signalling, resulting in cell division rather than xylem differentiation. CK induces PIN activity, pumping auxin out of the procambium. This causes an auxin maximum in the xylem axis, which subsequently induces MP expression.