Medicago SPX1 and SPX3 regulate phosphate homeostasis, mycorrhizal colonization, and arbuscule degradation

Abstract

To acquire sufficient mineral nutrients such as phosphate (Pi) from the soil, most plants engage in symbiosis with arbuscular mycorrhizal (AM) fungi. Attracted by plant-secreted strigolactones, the fungi colonize the roots and form highly-branched hyphal structures called arbuscules inside inner cortex cells. The host plant must control the different steps of this interaction to maintain its symbiotic nature. However, how plants sense the amount of Pi obtained from the fungus, and how this determines the arbuscule lifespan, are far from understood. Here, we show that Medicago truncatula SPX-domain containing proteins SPX1 and SPX3 regulate root Pi starvation responses, in part by interacting with PHOSPHATE RESPONSE REGULATOR2, as well as fungal colonization and arbuscule degradation. SPX1 and SPX3 are induced upon Pi starvation but become more restricted to arbuscule-containing cells upon the establishment of symbiosis. This induction in arbuscule-containing cells is associated with the presence of cis-regulatory AW-boxes and transcriptional regulation by the WRINKLED1-like transcription factor WRI5a. Under Pi-limiting conditions, SPX1 and SPX3 facilitate the expression of the strigolactone biosynthesis gene DWARF27, which could help explain the increased fungal branching in response to root exudates. Later, in arbuscule-containing cells, SPX1 and SPX3 redundantly control arbuscule degradation. Thus, SPX proteins play important roles as phosphate sensors to maintain a beneficial AM symbiosis.

PHOSPHATE STARVATION RESPONSE enables arbuscular mycorrhiza symbiosis

“…In contrast, Medicago truncatula SPX1 and SPX3 were shown to positively affect root length colonization by promoting the expression of SL biosynthesis genes. However, they also seemed to promote arbuscule degeneration and turnover, indicating a dual role of these proteins in regulating AM in M. truncatula 44 . The contrasting findings in rice and M. truncatula raise the possibility that different SPX family members play divergent roles in AM or that they differently affect AM in grasses vs legumes.…”

Molecular Regulation of Arbuscular Mycorrhizal Symbiosis

“…These results indicate that MYB1 accelerates arbuscule degeneration when the fungal arbuscule is not providing Pi to the plant, and suggest a functional redundancy at the MYB1 level [ 141 ]. Recent research carried out by Wang, et al [ 142 ] in M. truncatula also shows that the SPX-domain containing proteins SPX1 and SPX3 regulate arbuscule degradation, probably by sensing the delivered Pi at the arbuscule level. The spx1 spx3 double mutant has a significant lower ratio of degrading arbuscules, and exhibits a reduced expression of hydrolase genes induced by MYB1, such as Cysteine Protease 3 (CP3) and Chitinase [ 141 , 142 ].…”

“…Recent research carried out by Wang, et al [ 142 ] in M. truncatula also shows that the SPX-domain containing proteins SPX1 and SPX3 regulate arbuscule degradation, probably by sensing the delivered Pi at the arbuscule level. The spx1 spx3 double mutant has a significant lower ratio of degrading arbuscules, and exhibits a reduced expression of hydrolase genes induced by MYB1, such as Cysteine Protease 3 (CP3) and Chitinase [ 141 , 142 ]. The opposite phenotype has been observed in composite M. truncatula plants overexpressing SPX1 and SPX3 genes [ 142 ], supporting the idea that SPX1/3 play a role in regulating arbuscule degradation.…”

“…The spx1 spx3 double mutant has a significant lower ratio of degrading arbuscules, and exhibits a reduced expression of hydrolase genes induced by MYB1, such as Cysteine Protease 3 (CP3) and Chitinase [ 141 , 142 ]. The opposite phenotype has been observed in composite M. truncatula plants overexpressing SPX1 and SPX3 genes [ 142 ], supporting the idea that SPX1/3 play a role in regulating arbuscule degradation. Curiously, alternative roles of SPX proteins on AM regulation have been observed.…”

Primary nutrient sensors in plants

“…More recent literature furthermore suggests that SPX proteins may also control Pi influx through mycorrhizal fungi; expression of SPX1 and SPX3 in Medicago trunculata is restricted to arbuscule-containing cells of the root after the establishment of the mycorrhizal symbiosis. In these specific cells, biosynthesis of the hormone strigolactone is upregulated, which promotes fungal branching and increased root colonization ( Wang et al., 2021b ).…”