Neighboring plants divergently modulate effects of loss-of-function in maize mycorrhizal phosphate uptake on host physiology and root fungal microbiota

Abstract

Maize, a main crop worldwide, establishes a mutualistic symbiosis with arbuscular mycorrhizal (AM) fungi providing nutrients to the roots from soil volumes which are normally not in reach of the non-colonized root. The mycorrhizal phosphate uptake pathway (MPU) spans from extraradical hyphae to root cortex cells housing fungal arbuscules and promotes the supply of phosphate to the mycorrhizal host in exchange for photosynthetic carbon. This symbiotic association with the mycobiont has been shown to affect plant host nutritional status and growth performance. However, whether and how the MPU affects the root microbial community associated with mycorrhizal hosts in association with neighboring plants, remains to be demonstrated. Here the maize germinal Mu transposon insertion mutant pht1;6, defective in mycorrhiza-specific Pi transporter PHT1;6 gene, and wild type B73 (wt) plants were grown in mono- and mixed culture and examined under greenhouse and field conditions. Disruption of the MPU in pht1;6 resulted in strongly diminished growth performance, in reduced P allocation to photosynthetic source leaves, and in imbalances in leaf elemental composition beyond P. At the microbial community level a loss of MPU activity had a minor effect on the root-associated fungal microbiome which was almost fully restricted to AM fungi of the Glomeromycotina. Moreover, while wt grew better in presence of pht1;6, pht1;6 accumulated little biomass irrespective of whether it was grown in mono- or mixed culture and despite of an enhanced fungal colonization of its roots in co-culture with wt. This suggested that a functional MPU is prerequisite to maintain maize growth and that neighboring plants competed for AM fungal Pi in low P soil. Thus future strategies towards improving yield in maize populations on soils with low inputs of P fertilizer could be realized by enhancing MPU at the individual plant level while leaving the root-associated fungal community largely unaffected.

Fig 1. Physiological parameters of maize plants.

A) Shoot dry weight (DW) in GH 2014 experiment and shoot fresh weight (FW) in Field 2015 experiment. Different letters indicate significant differences between the treatments. B) Concentration of phosphorus (P) in source leaves of pht1;6 and wt plants in GH 2014 and in Field 2015 experiments. Different letters indicate significant differences between the treatments (ANOVA followed by Tukey’s HSD test, P < 0.05, n = 5 in GH 2014 or n = 8–10 in Field 2015 experiment).

Fig 2. Fungal colonization degree in the roots of wt and pht1;6 (mu) plants growing in an agricultural soil (+[NPK]) in the greenhouse and under the +[NPK],–[P] +[NK] and–[NPK] soil management type under field conditions.

A) Percentage of plant roots with observable fungal hyphae (H) or hyphae with well-developed arbuscules (H+A) or hyphae with well-developed arbuscules and vesicles (H+A+V). ‘Colonization’ indicates the sum of ‘H’ plus ‘A’ plus ‘V’ representing the overall percentage of plant roots colonized by fungal structures (n = 5). B) Percentage of plant roots with hyphae with well-developed arbuscules in plants sampled in Field 2015 experiment. Different letters indicate significant differences between the treatments (ANOVA followed by Tukey’s HSD test, P < 0.05, applied on each category of fungal structures separately, n = 5 or 8–10, in GH 2014 and Field 2015 experiments, respectively).

Fig 3. Comparison of fungal communities colonizing roots and rhizosphere of B73 wild-type (wt), and pht1;6 plants (mu) visualized by principal coordinates analysis (PCoA) on Bray-Curtis dissimilarities of fungal communities in root and rhizosphere samples in GH 2014 experiment performed in +[NPK] soil and in field 2015 performed under the +[NPK],–[P] +[NK] and–[NPK] soil management types.

Samples were colored according to the pot design.

Fig 4. Relative abundance (percentage of total ITS2 reads) of fungal orders in rhizosphere and root samples in GH 2014 and Field 2015 experiments.

The orders which were significantly affected by soil type in the Field 2015 in root or rhizosphere compartments are indicated with symbols (ANOVA P < 0.05)

Fig 5. Average relative abundance (RA) of fungal OTUs.

A) RA of fungal OTUs differing between wt and mutant roots identified by SIMPER analysis. The stripes on the right side of the plot indicate the P-value of binary comparisons (Wilcoxon test, black indicates significant P < 0.05, white non-significant P-value). Different letters indicate the comparisons performed: A: wt from wt_wt pot vs. mutant from mu_mu pot in GH 2014, B: mutant from mu_mu pot vs. mu from mu_wt pot in GH 2014, C: mutant vs. wt in +[NPK], D: mutant vs. wt in–P +[NK], E: mutant vs wt in–[NPK] field. B) Summarized RA of Glomeromycota OTUs and the community structure at the family level of Glomeromycota fungi in in root samples. Different letters indicate significant differences between the treatments (ANOVA followed by Tukey’s HSD test, P < 0.05).