Control of Root Cap Formation by MicroRNA-Targeted Auxin Response Factors in Arabidopsis

Abstract

The plant root cap mediates the direction of root tip growth and protects internal cells. Root cap cells are continuously produced from distal stem cells, and the phytohormone auxin provides position information for root distal organization. Here, we identify the Arabidopsis thaliana auxin response factors ARF10 and ARF16, targeted by microRNA160 (miR160), as the controller of root cap cell formation. The Pro 35S :MIR160 plants, in which the expression of ARF10 and ARF16 is repressed, and the arf10-2 arf16-2 double mutants display the same root tip defect, with uncontrolled cell division and blocked cell differentiation in the root distal region and show a tumor-like root apex and loss of gravity-sensing. ARF10 and ARF16 play a role in restricting stem cell niche and promoting columella cell differentiation; although functionally redundant, the two ARFs are indispensable for root cap development, and the auxin signal cannot bypass them to initiate columella cell production. In root, auxin and miR160 regulate the expression of ARF10 and ARF16 genes independently, generating a pattern consistent with root cap development. We further demonstrate that miR160-uncoupled production of ARF16 exerts pleiotropic effects on plant phenotypes, and miR160 plays an essential role in regulating Arabidopsis development and growth.

Evolution of plant microRNA gene families

“…In plants, miRNAs and their target genes have been conserved since the last common ancestor of bryophytes and seed plants more than 400 million years ago [3]. Functionally, plant miRNAs are involved in many fundamental biological processes [4], such as leaf polarity [5,6], floral identity [5,6], stress responses [7], and auxin responses [8]. Although the phenomena related to miRNA and siRNA functions were initially observed in plants [9], the first miRNA ever cloned was from an animal, Caenorhabditis elegans [10][11][12].…”

MicroRNAs and their regulatory roles in animals and plants

“…This approach has been used to identify several miRNA targets that play an important role in worm development (Lee et al, 1993;Wightman et al, 1993;Reinhart et al, 2000;Johnston and Hobert, 2003). For all known miRNA targets, they have conserved perfect or near-perfect complementary sites of miRNAs (Llave et al, 2002;Pasquinelli and Ruvkun, 2002;Saxena et al, 2003;Bartel, 2004;Mallory et al, 2004a;Meister et al, 2004b;Ota et al, 2004;Vella et al, 2004;Bagga et al, 2005;Brown and Sanseau, 2005;Millar and Waterhouse, 2005), especially for plant miRNAs (Aukerman and Sakai, 2003;Wang et al, 2005a;Williams et al, 2005b). This suggests a powerful strategy to predict miRNA targets by computational approaches.…”

Fine tuning of auxin signaling by miRNAs

“…The target is specified by the perfect or near-perfect sequence complementarity between the miRNA and its respective binding site on the targeted mRNA (Aukerman and Sakai, 2003;Wang et al, 2005a;Williams et al, 2005b). Experimental evidence indicates that there is usually a single miRNA binding site anywhere along the target mRNA.…”

“…GH3 encodes auxin conjugated enzymes, which act to reduce free auxin levels (Staswick et al, 2005) and AUX/IAA encode rapidly turned over transcriptional repressors of auxin-inducible gene (Tiwari et al, 2004). The Arabidopsis genome contains 23 ARF genes a few of which have been characterized (Wang et al, 2005a). ARFs are characterized by an amino terminal DNA-binding domain, a mid domain and two carboxyterminal dimerisation domains (Tiwari et al, 2003).…”

“…ARF10 and ARF16 dictate the root cap cell formation to mediate the direction of root tip growth. In absence of miR160 mediated regulation of ARF10 and ARF16 severe developmental abnormalities occur (Wang et al, 2005a). These include abnormal roots, leaf curling, serrated laminas, early flowering with altered floral morphology, defective embryos and reduced fertility (Liu et al, 2007).…”