An increase in root hair length and density and the development of arbuscular mycorrhiza symbiosis are two alternative strategies of most plants to increase the root–soil surface area under phosphorus (P) deficiency. Across many plant species, root hair length and mycorrhization density are inversely correlated. Root architecture, rooting density and physiology also differ between species. This study aims to understand the relationship among root hairs, arbuscular mycorrhizal fungi (AMF) colonization, plant growth, P acquisition and mycorrhizal-specific P_i transporter gene expression in maize. Using nearly isogenic maize lines, the B73 wild type and the rth3 root hairless mutant, we quantified the effect of root hairs and AMF infection in a calcareous soil under P deficiency through a combined analysis of morphological, physiological and molecular factors. Wild-type root hairs extended the rhizosphere for acid phosphatase activity by 0.5 mm compared with the rth3 hairless mutant, as measured by in situ zymography. Total root length of the wild type was longer than that of rth3 under P deficiency. Higher AMF colonization and mycorrhiza-induced phosphate transporter gene expression were identified in the mutant under P deficiency, but plant growth and P acquisition were similar between mutant and the wild type. The mycorrhizal dependency of maize was 33 % higher than the root hair dependency. The results identified larger mycorrhizal dependency than root hair dependency under P deficiency in maize. Root hairs and AMF inoculation are two alternative ways to increase P_i acquisition under P deficiency, but these two strategies compete with each other.

Root hairs are single-cell extensions of the epidermis that face into the soil and increase the root–soil contact surface. Root hairs enlarge the rhizosphere radially and are very important for taking up water and sparingly soluble nutrients, such as the poorly soil-mobile phosphate. In order to quantify the importance of root hairs for maize, a mutant and the corresponding wild type were compared. The rth2 maize mutant with very short root hairs was assayed for growth and phosphorus (P) acquisition in a slightly alkaline soil with low P and limited water supply in the absence of mycorrhization and with ample P supply. Root and shoot growth was additively impaired under P deficiency and drought. Internal P concentrations declined with reduced water and P supply, whereas micronutrients (iron, zinc) were little affected. The very short root hairs in rth2 did not affect internal P concentrations, but the P content of juvenile plants was halved under combined stress. The rth2 plants had more fine roots and increased specific root length, but P mobilization traits (root organic carbon and phosphatase exudation) differed little. The results confirm the importance of root hairs for maize P uptake and content, but not for internal P concentrations. Furthermore, the performance of root hair mutants may be biased by secondary effects, such as altered root growth.

Plants face a constantly changing environment, requiring fine tuning of their growth and development. Plants have therefore developed numerous mechanisms to cope with environmental stress conditions. One striking example is root response to water deficit. Upon drought (which causes osmotic stress to cells), plants can among other responses alter locally their root system architecture (hydropatterning) or orientate their root growth to optimize water uptake (hydrotropism). They can also modify their hydraulic properties, metabolism and development coordinately at the whole root and plant levels. Upstream of these developmental and physiological changes, plant roots must perceive and transduce signals for water availability. Here, we review current knowledge on plant osmotic perception and discuss how long distance signaling can play a role in signal integration, leading to the great phenotypic plasticity of roots and plant development.

Plants can develop root systems with distinct anatomical features and morphological plasticity to forage nutrients distributed heterogeneously in soils. Lateral root proliferation is a typical nutrient-foraging response to a local supply of nitrate, which has been investigated across many plant species. However, the underlying mechanism in maize roots remains largely unknown. Here, we report on identification of a maize truncated MIKC-type MADS-box transcription factor (ZmTMM1) lacking K- and C-domains, expressed preferentially in the lateral root branching zone and induced by the localized supply of nitrate. ZmTMM1 belongs to the AGL17-like MADS-box transcription factor family that contains orthologs of ANR1, a key regulator for root nitrate foraging in Arabidopsis. Ectopic overexpression of ZmTMM1 recovers the defective growth of lateral roots in the Arabidopsis anr1 agl21 double mutant. The local activation of glucocorticoid receptor fusion proteins for ZmTMM1 and an artificially truncated form of AtANR1 without the K- and C-domains stimulates the lateral root growth of the Arabidopsis anr1 agl21 mutant, providing evidence that ZmTMM1 encodes a functional MADS-box that modulates lateral root development. However, no phenotype was observed in ZmTMM1-RNAi transgenic maize lines, suggesting a possible genetic redundancy of ZmTMM1 with other AGL17-like genes in maize. A comparative genome analysis further suggests that a nitrate-inducible transcriptional regulation is probably conserved in both truncated and non-truncated forms of ZmTMM1-like MADS-box transcription factors found in grass species.

Root proliferation is a response to a heterogeneous nutrient distribution. However, the growth of root hairs in response to heterogeneous nutrients and the relationship between root hairs and lateral roots remain unclear. This study aims to understand the effects of heterogeneous nutrients on root hair growth and the trade-off between root hairs and lateral roots in phosphorus (P) acquisition. Near-isogenic maize lines, the B73 wild type (WT) and the rth3 root hairless mutant, were grown in rhizoboxes with uniform or localized supply of 40 (low) or 140 (high) mg P kg⁻¹ soil. Both WT and rth3 had nearly two-fold greater shoot biomass and P content under local than uniform treatment at low P. Significant root proliferation was observed in both WT and rth3 in the nutrient patch, with the WT accompanied by an obvious increase (from 0.7 to 1.2 mm) in root hair length. The root response ratio of rth3 was greater than that of WT at low P, but could not completely compensate for the loss of root hairs. This suggests that plants enhanced P acquisition through complementarity between lateral roots and root hairs, and thus regulated nutrient foraging and shoot growth. The disappearance of WT and rth3 root response differences at high P indicated that the P application reduced the dependence of the plants on specific root traits to obtain nutrients. In addition to root proliferation, the root response to a nutrient-rich patch was also accompanied by root hair elongation. The genotypes without root hairs increased their investment in lateral roots in a nutrient-rich patch to compensate for the absence of root hairs, suggesting that plants enhanced nutrient acquisition by regulating the trade-off of complementary root traits.

Improved management and breeding increased maize (Zea mays L.) yields over the last century, but nutritional efficiency was usually not the focus. This study investigates whether old and recently released flint and dent maize seedlings vary in the phosphorus (P) acquisition and utilization. P use efficiency (PUE) and related traits were measured and compared at two P levels in a calcareous soil. PUE and P acquisition efficiency (PAE) from founder flints to elite flints declined over the last decades. This was associated with smaller root systems, reduced ability to exploit external P, decreased rhizosphere pH and shorter root hairs in low P. Comparing flints with doubled haploid landraces (DH_LR), old and elite dents and hybrids revealed that dents started to acquire exogenous P earlier and had improved PUE. Most DH_LRs had similar PUE as elite flints. When evaluating root traits associated with P efficiency, seed P was also critical, and it is important to stack different root traits to optimize PUE, P utilization efficiency (PUtE) and PAE in breeding programmes. The root hair length, the ability to acidify the rhizosphere and the root diameter in flint and dent pools may be utilized to improve P use in maize germplasm.