Auxin receptor OsTIR1 mediates auxin signaling during seed filling in rice.

Cereal endosperm represents the most important source of the world's food. Nevertheless, the molecular mechanisms behind sugar import into rice (Oryza sativa) endosperm and their relationship with auxin signaling are poorly understood. Here, we report that auxin transport inhibitor response 1 (TIR1) plays an essential role in rice grain yield and quality via modulating sugar transport into endosperm. The fluctuations of OsTIR1 transcripts parallel to the early stage of grain expansion among those of the 5 TIR1/AFB (auxin-signaling F-box) auxin co-receptor proteins. OsTIR1 is abundantly expressed in ovular vascular trace, nucellar projection, nucellar epidermis, aleurone layer cells, and endosperm, providing a potential path for sugar into the endosperm. Compared to wild-type (WT) plants, starch accumulation is repressed by mutation of OsTIR1 and improved by overexpression of the gene, ultimately leading to reduced grain yield and quality in tir1 mutants but improvement in overexpression lines. Of the rice AUXIN RESPONSE FACTOR (ARF) genes, only the OsARF25 transcript is repressed in tir1 mutants and enhanced by overexpression of OsTIR1; its highest transcript is recorded at 10 d after fertilization, consistent with OsTIR1 expression. Also, OsARF25 can bind the promoter of the sugar transporter OsSWEET11 (SWEET, sugars will eventually be exported transporter) in vivo and in vitro. arf25 and arf25/sweet11 mutants exhibit reduced starch content and seed size (relative to the WTs), similar to tir1 mutants. Our data reveal that OsTIR1 mediates sugar import into endosperm via the auxin signaling component OsARF25 interacting with sugar transporter OsSWEET11. The results of this study are of great significance to further clarify the regulatory mechanism of auxin signaling on grain development in rice.

The D14-SDEL1-SPX4 cascade integrates the strigolactone and phosphate signalling networks in rice.

Modern agriculture needs large quantities of phosphate (Pi) fertilisers to obtain high yields. Information on how plants sense and adapt to Pi is required to enhance phosphorus-use efficiency (PUE) and thereby promote agricultural sustainability. Here, we show that strigolactones (SLs) regulate rice root developmental and metabolic adaptations to low Pi, by promoting efficient Pi uptake and translocation from roots to shoots. Low Pi stress triggers the synthesis of SLs, which dissociate the Pi central signalling module of SPX domain-containing protein (SPX4) and PHOSPHATE STARVATION RESPONSE protein (PHR2), leading to the release of PHR2 into the nucleus and activating the expression of Pi-starvation-induced genes including Pi transporters. The SL synthetic analogue GR24 enhances the interaction between the SL receptor DWARF 14 (D14) and a RING-finger ubiquitin E3 ligase (SDEL1). The sdel mutants have a reduced response to Pi starvation relative to wild-type plants, leading to insensitive root adaptation to Pi. Also, SLs induce the degradation of SPX4 via forming the D14-SDEL1-SPX4 complex. Our findings reveal a novel mechanism underlying crosstalk between the SL and Pi signalling networks in response to Pi fluctuations, which will enable breeding of high-PUE crop plants.

Cytokinin catabolism and transport are involved in strigolactone-modulated rice tiller bud elongation fueled by phosphate and nitrogen supply.

Phosphate (P) and nitrogen (N) fertilization affect rice tillering, indicating that P- and N-regulated tiller growth has a crucial effect on grain yield. Cytokinins and strigolactones (SLs) promote and inhibit tiller bud outgrowth, respectively; however, the underlying mechanisms are unclear. In this study, tiller bud outgrowth and cytokinin fractions were evaluated in rice plants fertilized at different levels of P and N. Low phosphate or nitrogen (LP or LN) reduced rice tiller numbers and bud elongation, in line with low cytokinin levels in tiller buds and xylem sap as well as low TCSn:GUS expression, a sensitive cytokinin signal reporter, in the stem base. Furthermore, exogenous cytokinin (6-benzylaminopurin, 6-BA) administration restored bud length and TCSn:GUS activity in LP- and LN-treated plants to similar levels as control plants. The TCSn:GUS activity and tiller bud outgrowth were less affected by LP and LN supplies in SL-synthetic and SL-signaling mutants (d17 and d53) compared to LP- and LN-treated wild-type (WT) plants, indicating that SL modulate tiller bud elongation under LP and LN supplies by reducing the cytokinin levels in tiller buds. OsCKX9 (a cytokinin catabolism gene) transcription in buds and roots was induced by LP, LN supplies and by adding the SL analog GR24. A reduced response of cytokinin fractions to LP and LN supplies was observed in tiller buds and xylem sap of the d53 mutant compared to WT plants. These results suggest that cytokinin catabolism and transport are involved in SL-modulated rice tillering fueled by P and N fertilization.

Strigolactone and gibberellin signaling coordinately regulate metabolic adaptations to changes in nitrogen availability in rice.

Modern semi-dwarf rice varieties of the "Green Revolution" require a high supply of nitrogen (N) fertilizer to produce high yields. A better understanding of the interplay between N metabolism and plant developmental processes is required for improved N-use efficiency and agricultural sustainability. Here, we show that strigolactones (SLs) modulate root metabolic and developmental adaptations to low N availability for ensuring efficient uptake and translocation of available N. The key repressor DWARF 53 (D53) of the SL signaling pathway interacts with the transcription factor GROWTH-REGULATING FACTOR 4 (GRF4) and prevents GRF4 from binding to its target gene promoters. N limitation induces the accumulation of SLs, which in turn promotes SL-mediated degradation of D53, leading to the release of GRF4 and thus promoting the expression of genes associated with N metabolism. N limitation also induces degradation of the DELLA protein SLENDER RICE 1 (SLR1) in an D14- and D53-dependent manner, effectively releasing GRF4 from competitive inhibition caused by SLR1. Collectively, our findings reveal a previously unrecognized mechanism underlying SL and gibberellin crosstalk in response to N availability, advancing our understanding of plant growth-metabolic coordination and facilitating the design of the strategies for improving N-use efficiency in high-yield crops.

Leaf cytokinin accumulation promotes potato growth in mixed nitrogen supply by coordination of nitrogen and carbon metabolism.

The source and sink balance determines crop growth, which is largely modulated by nitrogen (N) supplies. The use of mixed ammonium and nitrate as N supply can improve plant growth, however mechanisms involving the coordination of carbon and N metabolism are not well understood. Here, we investigated potato plants responding to N forms and confirmed that, compared with sole nitrate supply, mixed N (75 %/25 % nitrate/ammonium) enhanced leaf area, photosynthetic activity and N metabolism and accordingly resulted in outgrowth of stolons and shoot axillary buds. Cytokinin transportation in xylem sap and local cytokinin synthesis in leaves were up-regulated in mixed-N-treated potato plants relative to sole nitrate provision; and exogenous application of 6-benzylaminopurine in addition to sole nitrate restored leaf area, photosynthetic capacity and N content in leaves to the similar as those under mixed-N treatment. Partial defoliation, an effective method to enhance the sink strength, induced more cytokinin content in leaflets under two treatments relative to their respective controls and ultimately resulted in larger photosynthesis capacity and leaf area. These results suggest that mixed-N-enhanced plant growth through the coordination of carbon and N metabolism largely depends on the signal molecule cytokinin modulated by N supplies.

Higher nitrogen content and auxin export from rice tiller enhance low-ammonium-dependent tiller outgrowth.

In the early growth stage, nutrient uptake by rice roots is weak. However, rice tillering at this stage would require high N input. Thus, it is vital to clarify the mechanism involved in tillering capacity with low N inputs. In this report, two widely-planted japonica cultivars (cvs Yangyujing 2 and Nanjing 45) were selected mainly because, unlike cv. Nanjing 45, cv. Yangyujing 2 shows low-N-induced tiller outgrowth. Responses of tillers in two rice cultivars to mixture of N forms versus sole NH4+ supply were similar, suggesting that NH4+ plays a pivotal role in N-modulated rice tillering. Under low NH4+ supply, higher expression of OsAMT1.2, OsAMT1.3, OsGS1;2, and OsGS2 was recorded in the roots of cv. Yangyujing 2 in comparison with cv. Nanjing 45, ultimately resulting in higher N content and dry weight in cv. Yangyujing 2. Stronger 3H-IAA export from tiller stems was observed in cv. Yangyujing 2, mainly due to higher expression level of auxin efflux transporters. Moreover, tillers in auxin efflux transporter mutant ospin9 did not respond to NH4+ supply relative to wild-type plants. These findings can be used in the molecular breeding of rice varieties to simultaneously improve rice population productivity and reduce N fertilizer input.