Rice auxin influx carrier OsAUX1 facilitates root hair elongation in response to low external phosphate

Abstract

Root traits such as root angle and hair length influence resource acquisition particularly for immobile nutrients like phosphorus (P). Here, we attempted to modify root angle in rice by disrupting the OsAUX1 auxin influx transporter gene in an effort to improve rice P acquisition efficiency. We show by X-ray microCT imaging that root angle is altered in the osaux1 mutant, causing preferential foraging in the top soil where P normally accumulates, yet surprisingly, P acquisition efficiency does not improve. Through closer investigation, we reveal that OsAUX1 also promotes root hair elongation in response to P limitation. Reporter studies reveal that auxin response increases in the root hair zone in low P environments. We demonstrate that OsAUX1 functions to mobilize auxin from the root apex to the differentiation zone where this signal promotes hair elongation when roots encounter low external P. We conclude that auxin and OsAUX1 play key roles in promoting root foraging for P in rice.

Fig. 1

OsAUX1 controls rice root angle. a Schematic representation of T-DNA insertion sites in OsAUX1 gene. b Time course images of root angle in WT, osaux1-1;1 and osaux1-1;3 T-DNA mutants. Images were taken after 3 days after seed germination (3DAG) to 8 days post germination (8DAG). White bars represent 0.5 cm

Fig. 2

MicroCT imaging reveals OsAUX1 controls root angle in soil. Comparison of root angles from X-ray CT images of soil grown wild-type (WT), osaux1-1;1 and osaux1-1;3 roots at 1-, 2-, 3-, and 4-week-old stages (denoted W1–4). Scale bar represents 2 cm

Fig. 3

OsAUX1 promotes root hair growth at low external P levels. a 9-day-old WT, osaux1-1;1and osaux1-1;3 seedlings were grown for 6 days in hydroponics at three different P concentrations. Scale bar 1 mm. b Quantitation of RH length in WT, osaux1-1;1and osaux1-1;3 mutants reveal low P. Each bar represents the average length of 30–60 fully elongated RH on >10 nodal roots. *, **, and *** indicate significant difference p value <0.05, 0.001, and 0.0001, respectively. Error bars mean ± SE, n = three biological replicates and p values were calculated by Student’s t test

Fig. 4

Low P increases root hair zone auxin response via AUX1. a, b Two photon laser scanning microscopy images of auxin response reporter DR5:VENUS (green) fluorescence in transgenic rice seedlings grown at either low (a) or high P levels (b). Inset shows close-up of the distal elongation zone. c–f Maximum projection confocal images of Z-stacks of DR5::VENUS fluorescence in the roots of wild type (c, d) or osaux1-1;3 (e, f) seedlings grown in either low (c, e) or high P (d, f). g AUX1_pro:GUS lines reveal OsAUX1 root apical expression. Scale bar represents 100 µm

Fig. 5

Low P root auxin response is independent of plant P status. a Maximum projection confocal images of Z-stacks of DR5::VENUS fluorescence in the seedlings grown initially in high P medium for 7 days and then transferred to high P (i) for a further 6 days. (ii) and (iii) show DR5::VENUS fluorescence of split P experiment roots, where 7-day-old high P roots were split into two halves: one half was grown in high (ii) and the other in low P medium (iii) for a further 6 days. (iv) Maximum projection confocal image of 13-day-old low P grown rice root. b Raw integrated fluorescence intensity quantification of DR5::VENUS roots (from Fig. 5a and Supplemetary Figure 11). Each bar represents the average raw integral density of fluorescence intensity of DR5::VENUS under high P, low P to high P, high P to low P, and low P conditions. Fluorescence intensity of at least 19 roots under low P and high P grown DR5::VENUS seedlings and 10 roots of split P conditions were used for fluorescence intensity measurement in three independent replicates. Scale bar represents 50 µm. Student’s t test was performed to calculate p values