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.

Auxin transport sites are visualized in planta using fluorescent auxin analogs

“…Petersson et al demonstrated that de novo synthesized IAA accumulated at the root apex, thereby indicating that a source of IAA exists in the most apical region of Arabidopsis primary roots (10). Recent studies have shown that the expression of IAA biosynthesis genes is mainly localized near the QC region (25,28,42). Our fluorescent NBD-auxins were not accumulated in the QC region in our experimental conditions.…”

“…Recent findings regarding auxin biosynthesis through the IPA pathway have indicated an important contribution of locally synthesized IAA to the formation of auxin maxima (10,25). The DR5 reporter system allows for the visualization of auxin levels as the sum of transported and locally synthesized auxin, whereas our system using NBD-auxins selectively displays the flow of auxin.…”

Neighbor Detection Induces Organ-Specific Transcriptomes, Revealing Patterns Underlying Hypocotyl-Specific Growth

“…These data also suggest that YUC-mediated auxin production in cotyledons triggers elongation of the hypocotyl. To test whether induction of a YUC gene in cotyledons can induce hypocotyl elongation, we used an inducible YUC3 line in which this gene is selectively expressed in photosynthetic tissues (Chen et al, 2014). Interestingly, following YUC3 induction, this line showed marked hypocotyl elongation, confirming that cotyledon-derived auxin is sufficient to trigger hypocotyl elongation (Supplemental Figure 4D).…”

“…We used the following Arabidopsis thaliana genotypes (cv Columbia-0): pin3 pin4 pin7, d6pk01, D6PK-OX (Willige et al, 2013), sav3-2 (Tao et al, 2008), pif7-1 (Li et al, 2012), DII-VENUS (Brunoud et al, 2012), pif4 pif5 pif7 (de Wit et al, 2015), amiD2/5/7/8/9 (Spartz et al, 2014), amiSAUR19/23/24 (Spartz et al, 2012), and iYUC3 (FRO6:XVE:YUC3; Chen et al, 2014). To generate the yuc2 yuc5 yuc8 yuc9 quadruple mutant, T-DNA and transposon insertion lines were obtained from the ABRC, the Cold Spring Harbor Laboratory, or GABI-Kat.…”

“…To generate the yuc2 yuc5 yuc8 yuc9 quadruple mutant, T-DNA and transposon insertion lines were obtained from the ABRC, the Cold Spring Harbor Laboratory, or GABI-Kat. Mutant yucca lines from Yunde Zhao were previously described (Chen et al, 2014). The yuc quadruple mutant was obtained by repeated crossing and PCR genotyping using the described primers.…”