SAUR39, a small auxin-up RNA gene, acts as a negative regulator of auxin synthesis and transport in rice

Abstract

The phytohormone auxin plays a critical role for plant growth by regulating the expression of a set of genes. One large auxin-responsive gene family of this type is the small auxin-up RNA (SAUR) genes, although their function is largely unknown. The expression of the rice (Oryza sativa) SAUR39 gene showed rapid induction by transient change in different environmental factors, including auxin, nitrogen, salinity, cytokinin, and anoxia. Transgenic rice plants overexpressing the SAUR39 gene resulted in lower shoot and root growth, altered shoot morphology, smaller vascular tissue, and lower yield compared with wild-type plants. The SAUR39 gene was expressed at higher levels in older leaves, unlike auxin biosynthesis, which occurs largely in the meristematic region. The transgenic plants had a lower auxin level and a reduced polar auxin transport as well as the down-regulation of some putative auxin biosynthesis and transporter genes. Biochemical analysis also revealed that transgenic plants had lower chlorophyll content, higher levels of anthocyanin, abscisic acid, sugar, and starch, and faster leaf senescence compared with wild-type plants at the vegetative stage. Most of these phenomena have been shown to be negatively correlated with auxin level and transport. Transcript profiling revealed that metabolic perturbations in overexpresser plants were largely due to transcriptional changes of genes involved in photosynthesis, senescence, chlorophyll production, anthocyanin accumulation, sugar synthesis, and transport. The lower growth and yield of overexpresser plants was largely recovered by exogenous auxin application. Taken together, the results suggest that SAUR39 acts as a negative regulator for auxin synthesis and transport.

Figure 1.

Identification, auxin response, and expression pattern of the SAUR39 gene. A, Expression of the SAUR39 gene in shoots of wild-type rice plants grown at 1 or 10 mm N for 4 weeks. Two hours before harvest, plants were switched from 1 to 10 mm N (induction) and from 10 to 1 mm N (reduction). B, Expression of SAUR39 after application of NAA in shoots. The wild-type rice plants were grown for 4 weeks, and shoots were harvested at the indicated time points after application of 4 μm NAA. C, Expression of SAUR39 in different tissues of wild-type plants. Inf, Inflorescence; FL, flag leaf; YL, young leaf; ML, mature leaf; Int, internode. D, Expression of SAUR39 in shoots of wild-type (WT) and OX lines, measured by real-time PCR.

Figure 2.

Growth of wild-type (WT) and SAUR39-OX rice plants. A, Three-week-old plants. Arrows indicate start of senescence in OX plants. B, Ten-week-old plants. Arrows indicate wider angle of leaves in OX plants.

Figure 3.

Biochemical analysis in wild-type (WT) and SAUR39-OX rice plants. Free IAA (A), chlorophyll (B), anthocyanin (C), total soluble sugars (D), total starch (E), and ABA (F) contents in shoots of 4-week-old wild-type and OX rice plants. Data are means ± sd (n = 3–5). Bars with different letters indicate significant differences at P < 0.05 (Fisher's protected lsd test). DW, Dry weight; FW, fresh weight.

Figure 4.

Vascular tissue in rice stems. A and B, Stem cross-sections of wild-type (WT; A) and OX (B) plants stained with toluidine blue. C, Number of sieve tube element cells. Data are means ± se; bars with different letters indicate significant differences at P < 0.05 (Fisher's protected lsd test). The photograph and data are representative of sections taken from at least three different plants and eight to 10 sections in each plant from 4-week-old rice plants. BSC, Bundle sheath cells; CC, companion cells; MX, metaxylem; PXL, protoxylem lacuna; STEC, sieve tube element cells; XPC, xylem parenchyma cells.

Figure 5.

Subcellular localization of transiently expressed 2XGFP-SAUR39 (A) and 2XGFP (B) proteins. The plasmids were transformed biolistically into tobacco BY2 cells, and images were observed by epifluorescence microscopy. From left to right are the subcellular localization of 2XGFP-SAUR39 or 2XGFP fusion protein, nuclei stained with 4′,6-diamidino-2-phenylindole (DAPI), overlay of the two images, and differential interference contrast (DIC) images.

Figure 6.

Rice wild-type (WT) and SAUR39-OX plants. A, Plants grown in regular nutrient solution without NAA. B, Plants grown in nutrient solution with 0.04 μm NAA. Photographs were taken when plants were 7 weeks old.