Phosphate starvation signaling in rice

Abstract

Phosphorus is one of the most essential and limiting macronutrients for plants. Phosphate (Pi) deficiency could affect crop productivity seriously in agriculture. How to cope with this problem? Unveiling the molecular mechanism behind the Pi starvation responses of plants will be helpful to solve this issue. Rice is one of the most important crops, which feeds over one-third of the people in the world. In this review, we summarize the recent progress on Pi starvation signaling in rice with the intention to provide a further insight into the molecular mechanism of Pi starvation responses in rice and to give a new research direction to design transgenic plants with high Pi efficiency.

Too Little? Too Much?

Plants need to take large amounts of phosphorus to meet the demand of growth and development. Because of the low contents of Pi in the natural soil, the Pi fertilizers are widely applied in agriculture to increase the Pi availability. However, only a very small portion of the Pi fertilizers can be assimilated and utilized by plants, which is due to easy chelation of Pi with cations or organic compounds to form insoluble complexes¹ and the relative low Pi uptake efficiency by plants. The increasing application of Pi fertilizers not only increases the costs, even worse, it results in severe soil and water pollution because of the large amounts of rudimental Pi fertilizers in the soil. Thus comes a dilemma: on one hand, the available Pi contents are very low in the natural soil, on the other hand, to increase crop productivity, the Pi fertilizers are over-applied in agriculture, which inevitably leads to a series of negative effects. How to make a balance between too little natural available Pi contents and too much Pi fertilizer application? Developing transgenic crops with high Pi uptake and utilization efficiency would be a feasible strategy to solve this problem.

How Plants Response to Pi Starvation

To cope with Pi deficiency, plants have evolved multiple strategies to increase Pi availability.² Root architecture alteration is the most important morphology adaption in response to Pi depletion, which enables plants to increase surface contact with the soil and to acquire Pi more efficiently. In rice, the elongation of primary and adventitious roots is the typical root architecture alteration under Pi starvation.^3,4 The upregulated expression of Pi transporter (PT) is another significant manner to increase the Pi uptake and transport under Pi starvation. Besides, the stimulation of acid phosphatase and ribonuclease activities can recycle Pi from organic phosphorus compounds to increase Pi availability under Pi deficient conditions. As an essential component of metabolic intermediates, Pi is involved in multiple metabolism processes. Therefore, Pi starvation will trigger multiple metabolism adjustments such as lipid composition alteration, glycolysis acceleration, nitrogen assimilation repression and increased metal uptake, by which plants can enhance Pi scavenging and decrease the Pi consumption. Taken together, the Pi starvation triggers multiple adaptive responses which finally intend to maximize acquisition of external Pi and reprioritize utilization of internal Pi to adapt to low Pi environments. Thus, understanding the molecular mechanism of Pi starvation responses will provide useful information to design transgenic crops with high Pi uptake and utilization efficiency.

Major Players in Regulation of Pi Starvation Signaling

The Pi starvation signaling pathway has been well studied in Arabidopsis. The major players including SIZ1, PHR1, miR399 and PHO2, comprise the most important Pi starvation signal transduction pathway.⁵ PHR1, a Myb transcription factor, is the key regulator in this signal transduction pathway.^6,7 PHR1 binds to P1BS element (an imperfect palindromic sequence, GNATATNC) in the promoter region of some Pi starvation induced (PSI) genes. Overexpression of PHR1 leads to excessive Pi accumulation in the shoot and constitutive activation of a series of PSI genes.⁸ SIZ1 is a small plant ubiquitin-like modifier (SUMO) E3 ligase that controls PHR1 sumoylation.⁹ miR399, the target gene of PHR1, is specifically induced by Pi starvation and can negatively regulate PHO2 expression through mRNA degradation.^10,11 Both miR399 overexpression and mutations of PHO2 result in over-accumulation of Pi in the shoot. Expression of several PSI genes such as Pht1;8, Pht1;9, AtIPS1 and AT4 is increased in pho2 mutants even under Pi sufficient conditions, suggesting that PHO2 is involved in the regulation of Pi starvation responses.⁷

As one of the most important crops, rice feeds over one-third the population in the world. Different from other crops, the majority of rice is grown in the paddy field, which makes it more accessible to pollute the water source by rudimental Pi fertilizers. Thus, improving the Pi uptake efficiency in rice and decrease the application of Pi fertilizers would be more meaningful. In comparison with Arabidopsis, the Pi starvation signaling is still poorly understood in rice. Increasing evidences suggested that the Pi starvation signal transduction pathway is highly conserved between rice and Arabidopsis. Similar with PHR1, OsPHR2 is shown to play a crucial role in Pi starvation signaling in rice.¹² OsPHR2 overexpression leads to Pi over-accumulation in the shoot and activation the expression of PSI genes and PT genes. It is further proved that upregulation of OsPT2 is mainly responsible for the excessive Pi accumulation in OsPHR2 overexpression plants.¹³ OsSPX1, the member of SPX (SYG/PHO81/XPR1) domain genes, is also involved in OsPHR2-mediated Pi starvation signaling. It is thought that OsSPX1 can suppress the function of OsPHR2 to regulate Pi uptake and the expression of PSI genes,¹³ suggesting OsSPX1 functions as a negative regulator up-stream of OsPHR2. Interestingly, OsSPX1 is upregulated significantly in OsPHR2 overexpression plants, implies that OsSPX1 regulates OsPHR2 possibly in a feed-back manner. However, the regulation mechanism of OsSPX1 is still needed to elucidate. Consistent with miR399 in Arabidopsis, OsmiR399 is also activated by OsPHR2 and induced by Pi starvation in rice.^7,12 Moreover, our work was shown that both OsmiR399 overexpression and mutation in LTN1 (OsPHO2) resulted in Pi over-accumulation in the shoot, and the transcript of LTN1 is significantly decreased in OsmiR399 overexpression plants.¹⁴ These results demonstrate that OsmiR399 works down-stream of OsPHR2 to regulate Pi starvation signaling in rice and its target gene is LTN1. LTN1, the homolog of Arabidopsis PHO2, plays a pivotal role in regulation of multiple Pi starvation responses in rice.¹⁴ Loss-of-function of LTN1 leads to constitutive activation of multiple Pi starvation responses such as upregulation of PTs, stimulation of acid phosphatase and ribonuclease activities, lipid composition transformation, nitrogen assimilation repression and increased metal uptake even under Pi-sufficient conditions, demonstrating LTN1 is a key negative regulator of Pi starvation responses in rice. The expression of the genes responsible for these Pi starvation responses are also altered accordingly, suggesting LTN1 regulates Pi starvation responses by controlling the expression of related genes at transcription level. LTN1 encodes a putative ubiquitin-conjugating enzyme (E2) which plays a key role in 26S proteasome dependent protein degradation, suggesting that LTN1 probably regulates Pi starvation signaling by mediating its target protein degradation. However, the target protein of LTN1 is still completely unknown. In addition, the expression of OsSPX1 is also increased significantly in ltn1 (ospho2) mutant,^14,15 indicating OsSPX1 also participates in the regulation of Pi starvation signaling down-stream of LTN1.

Root architecture alteration is the most significant morphology adaption to Pi starvation, which can largely increase the Pi uptake under Pi deficient conditions. However, the knowledge of regulation mechanism of root architecture alteration in response to Pi starvation is still very limited in rice. By now, only several components involved in this process have been identified. Overexpression of OsPHR2 leads to the elongation of roots and proliferation of root hairs, which mimics Pi starvation-triggered root architecture alteration.¹² This demonstrates the involvement of OsPHR2 in the regulation of Pi starvation induced root architecture alteration. It is also shown that LTN1 can regulate Pi starvation-dependent root morphology alteration, which is represented by the enhanced elongation of the primary and adventitious roots in ltn1 mutant under Pi deficient conditions.¹⁴ However, the regulating mechanism of these two genes (OsPHR2 and LTN1) in root architecture alteration is still unclear. The following work, such as identification of the down-stream genes, will be necessary to explain how these two genes regulate root architecture alteration in response to Pi starvation.

Conclusion and Perspectives

More and more evidence has proved that the Pi starvation signaling is highly conserved between Arabidopsis and rice. The major players of Pi starvation signaling in rice include OsSPX1, OsPHR2, OsmiR399 and LTN1. The Pi starvation signal is transferred orderly by OsPHR2, OsmiR399 and LTN1 and finally activates multiple Pi starvation responses. Possibly, OsSPX1 works as a feedback negative regulator, which tunes Pi starvation signaling in a subtle manner. However, to further understand the Pi starvation signaling in rice there still are two key questions which need to be clarified. How OsPHR2 response to Pi starvation? Because the induction of OsPHR2 at transcriptional level is not obvious under Pi starvation, thus we speculate that it is regulated at the post-translational level. OsPHR2 probably responses to Pi starvation via protein accumulation or protein modification, which activates its function to upregulate the expression of PSI genes. Similar with PHR1, the most possible modification manner of OsPHR2 in responses to Pi starvation might be protein sumoylation. Following work on the identification of rice homolog of AtSIZ1 and sumoylation of OsPHR2 will be helpful to elucidate this question. Another crucial question is how LTN1 regulates the downstream genes. Apparently, LTN1 target/interacted protein screening would be essential. To approach this purpose, the work including yeast two-hybrid screening for LTN1-interacted proteins, ltn1 suppressor screening and isolation of Pi starvation response defective mutants are needed.