Maize (Zea mays
) is not only a key human food and animal feed crop throughout the world but also an important raw material for the food industry and energy production plants [1
]. Low phosphate concentrations are frequently a constraint for maize growth and development, and therefore, enormous quantities of phosphate fertilizer are expended in maize cultivation, which increases the cost of planting. Although the total amount of phosphorus (P) in the soil may be high, plants mainly absorb P in the inorganic form (Pi), which is present at a low concentration, limiting plant growth and development [2
Phosphorus is an essential macronutrient for all living organisms and plays important roles in energy metabolism; biosynthesis of nucleic acids, phospholipids and membranes; cellular signal transduction and the regulation of many enzymes [3
]. Plants have evolved two broad strategies to cope with phosphate starvation, which involve changes in physiology, biochemistry and root morphology that enhance their ability to activate, assimilate and transport insoluble phosphate in soils [5
Low-phosphate stress not only increases root biomass but can also cause significant changes in root morphology, including altering the root-to-shoot ratio, total root length and lateral root length and numbers, to increase the contact area with the soil; these changes improve the absorptive capacity of roots. Low phosphate availability has been found to favor lateral root growth over primary root growth by dramatically reducing primary root length and increasing lateral root elongation and lateral root density in Arabidopsis
]. In white lupine, the rate of proteoid root formation is found to be greatest in solutions with a Pi concentration of 1–10
and is suppressed at concentrations of 25
Pi and higher [7
]. The effect of low phosphate levels on the rice root system is found to be induction of adventitious root growth and increases in the number and length of lateral roots, which increase the surface area available for nutrient uptake by roots [8
]. The root-to-shoot ratio has been observed to increase in maize when plants were subjected to Pi deficiency, although the reported effects on root length and biomass are inconsistent. Anghinoni and Barber [9
] reported that root length was increased in 12-day-old seedlings when they were cultured under low-phosphate conditions. In contrast, Mollier and Pellerin [10
] observed that the elongation rate of axile roots was maintained throughout the experimental period, whereas the emergence of new axile roots and the elongation of first-order lateral roots were dramatically reduced. These differences may be due to differences in the experimental conditions and genotypes involved, and they suggest that there is a high degree of plasticity in maize roots.
In recent years, several components involved in Pi starvation signaling in plants have been identified and characterized in detail. PHR1, a conserved MYB transcription factor, is shown to regulate microRNA399 (miR399) family members and a number of other Pi starvation response genes [11
]. Recently, the OsPHR1
genes of rice have been cloned, both of which are involved in the Pi starvation response. Overexpression of OsPHR2
induces a Pi starvation response, suggesting that this gene plays a conserved upstream role in Pi signaling regulation [13
]. Three additional transcription factors, OsPTF1/ZmPTF1, WRKY75 and ZAT6 (zinc finger of Arabidopsis
6, a cysteine-2/histidine-2 zinc finger protein), are found to participate in the regulation of plant adaptation to phosphate starvation [14
]. Moreover, miR399 has been found to be involved in the posttranscriptional regulation of Pi homeostasis [12
]. Recently, Pant et al.
] reported that miR399 is a long-distance signal for the regulation of Pi homeostasis in the phloem. Several Pi phosphate transporters and key enzymes that produce organic acids have been identified, the upregulation of which could increase the uptake and use efficiency of Pi within a plant [21
]. Although certain components of Pi starvation signaling in plants have been identified, the overall pathway is still poorly understood and requires further investigation.
High-throughput macro/microarray technologies have contributed enormously to demonstrating the transcriptional regulation associated with abiotic stresses, including low levels of inorganic Pi. Hammond et al.
genes for which the expression in the leaves increases specifically in response to P starvation when the P content in plant tissues begins to decline, but before the lack of P affects growth, and these researchers identified marker genes to monitor P deficiency in plants [22
]. Based on the expression analysis of plants during a 3-d period after the removal of Pi from the growth medium, Wu et al.
suggested that a significant fraction of regulatory genes exhibit distinct or even contrasting expression patterns in the leaves and roots of Arabidopsis
plants in response to Pi starvation, supporting the idea that distinct strategies are used in different plant organs in response to a shortage of Pi in growth media [23
]. This hypothesis was confirmed by Misson et al.
using Affymetrix gene chips [24
]. In rice, Wasaki et al.
found that sulfoquinovosyl diacylglycerol (SQDG) synthesis-related genes and polysaccharide metabolism were affected by Pi levels [25
]. Calderon-Vazquez et al.
examined transcript profiles of Zea mays
roots and revealed gene responses to phosphate deficiency at the plant- and species-specific levels [27
]. Gene expression analyses of responses to phosphorus deficiency are also performed in proteoid roots of white lupin [28
] and roots of the common bean [29
]. A large number of differentially expressed genes have been discovered using macro/microarrays. A proteomics approach was used to identify proteins that are differentially expressed under low-phosphate conditions and among different inbred lines [30
]. Taken together, these findings suggest four main changes when plants are subjected to low-phosphate conditions: 1) phosphorus absorption and utilization-related genes, such as phosphate transporters, acid phosphatases, organic acid synthases and nucleases, which could improve Pi absorption and release Pi from internal and external environments, are induced when plants are subjected to low-phosphate conditions; 2) lipid metabolism and membrane components are altered by the substitution of P with sulfur in various types of lipids; 3) primary modes of metabolism, such as carbon metabolism and nitrogen metabolism, are affected by a lack of phosphate; and 4) there are changes in gene expression related to the response to metallic elements and other abiotic stresses. The results of high-throughput analysis give us a better understanding of plant responses to phosphate starvation, but little is known regarding the plant root modifications that occur under low-phosphate conditions and their regulatory mechanisms. The available evidence suggests that auxin plays an important role in mediating the effects of Pi starvation on root system architecture. Phosphate availability alters lateral root development in Arabidopsis
at least partly by modulating auxin sensitivity via a mechanism involving the TIR1 auxin receptor [32
In this study, the response to phosphate starvation of the roots of maize plants from the inbred line Q319 was analyzed. The numbers of lateral roots and lateral root primordia decreased after 6
days of culture in a low-phosphate solution (LP) compared with those of plants grown under normal conditions (sufficient phosphate, SP), and these differences increased in association with the stress caused by phosphate starvation. However, the growth of primary roots appeared not to be sensitive to low phosphate levels. This finding differed from what is observed in Arabidopsis
; when the root tip of an Arabidopsis
plant comes into contact with low-phosphate media, primary root growth ceases. To elucidate how low phosphate levels regulate root modifications, especially lateral root development, a transcriptomic analysis of the 1.0-1.5
cm lateral root primordium zone (LRZ) of maize Q319 roots was completed. The data analysis showed that auxin signaling participated in the response to low-phosphate conditions and the modification of root morphology, and LOB (Lateral organ boundaries) domain proteins might represent an intermediary between auxin signaling and root morphology. The retardation of lateral root development may be caused by the coordinated downregulation of the genes involved in DNA replication, gene expression, protein synthesis and degradation and cell growth. These findings enrich our understanding of plant responses to low-phosphate conditions and maize root morphogenesis.