Auxin overproduction in shoots cannot rescue auxin deficiencies in Arabidopsis roots

Abstract

Auxin plays an essential role in root development. It has been a long-held dogma that auxin required for root development is mainly transported from shoots into roots by polarly localized auxin transporters. However, it is known that auxin is also synthesized in roots. Here we demonstrate that a group of YUCCA (YUC) genes, which encode the rate-limiting enzymes for auxin biosynthesis, plays an essential role in Arabidopsis root development. Five YUC genes (YUC3, YUC5, YUC7, YUC8 and YUC9) display distinct expression patterns during root development. Simultaneous inactivation of the five YUC genes (yucQ mutants) leads to the development of very short and agravitropic primary roots. The yucQ phenotypes are rescued by either adding 5 nM of the natural auxin, IAA, in the growth media or by expressing a YUC gene in the roots of yucQ. Interestingly, overexpression of a YUC gene in shoots in yucQ causes the characteristic auxin overproduction phenotypes in shoots; however, the root defects of yucQ are not rescued. Our data demonstrate that localized auxin biosynthesis in roots is required for normal root development and that auxin transported from shoots is not sufficient for supporting root elongation and root gravitropic responses.

Keywords: Auxin; Biosynthesis; Hormone; Root development; Transport.

Fig. 1

Identification of auxin biosynthesis genes in Arabidopsis roots. (A) Cluster analysis of the expression patterns of YUC genes in root cells. Microarray data from the public domain were clustered using methods previously described in (Birnbaum et al. (2003) and Toufighi et al. (2005). Note that YUC3, YUC5, YUC7, YUC8 and YUC9 are expressed in root cells whereas other YUC genes have low or no expression in roots. (B) Arabidopsis has 11 YUC flavin monooxygenases that can be divided into five clades (Cheng et al. 2006). The YUC genes with expression in roots form two close clades in the phylogenetic tree. (C) The patterns of GUS expression in roots of Arabidopsis transgenic plants that harbor YUC promoter::GUS constructs.

Fig. 2

A group of YUC genes are important for auxin biosynthesis and root development. (A) Overexpression of the YUC genes causes auxin overproduction phenotypes. (B) Simultaneous inactivation of YUC3, YUC5, YUC7, YUC8 and YUC9 leads to dramatic defects in Arabidopsis root development. The yucQ mutants develop short and agravitropic primary roots. (C). Roots of yucQ are smaller in diameter and have a smaller meristem. (D) The yucQ roots have a smaller root cap and an abnormal meristem.

Fig. 3

The root defects in yucQ are caused by partial auxin deficiency in roots. (A) Lower auxin levels are detected in yucQ roots than in wild-type roots. (B) The expression level of the auxin reporter DR5–GFP is dramatically decreased in yucQ. (C) The root defects of yucQ are partially rescued by 5 nM IAA.

Fig. 4

Shoot-produced auxin is not sufficient for rescuing the root defects of yucQ. (A) A construct for inducible expression of YUC3 in Arabidopsis shoots. The synthetic transcription factor XVE is under the control of a shoot-specific promoter FRO6p. Upon estradiol treatment, YUC3 is specifically expressed in the shoots. (B) Overexpression of YUC3 in wild-type shoots causes auxin overproduction in shoots. Note that the estradiol-treated seedling has long hypocotyls, epinastic cotyledons and elevated expression of the auxin reporter DR5–GUS in cotyledons. (C) DR5–GFP expression in roots in the mock-treated plants in the wild-type background and in the estradiol-treated plants. Although induction of YUC3 expression in shoots caused auxin overproduction in shoots, the expression of DR5–GFP was similar to that of the mock-treated plants. (D) DR5–GFP levels were reduced in yucQ, and overexpression of YUC3 in shoots did not lead to an increase in expression of DR5–GFP in roots. (E) Overproduction of auxin in shoots in yucQ did not rescue the root defects of yucQ. (F) The root phenotypes of yucQ (3 days old) were reversed when YUC3 was expressed using a root-specific promoter RCH1p. (G) Hypocotyl length of wild-type/FRO6::XVE::YUC3 and yucQ/FRO6::XVE::YUC3 plants treated with DMSO (dimethylsulfoxide) or estradiol. (H) The effects of induction of YUC3 expression in wild-type/FRO6::XVE::YUC3 and yucQ/FRO6::XVE::YUC3 plants on root length.