Abstract
Arbuscular mycorrhizal (AM) colonization of plant roots causes the down-regulation of expression of phosphate (Pi) or nitrogen (N) transporter genes involved in direct nutrient uptake pathways. The mechanism of this effect remains unknown. In the present study, we sought to determine whether the expression of Pi or N transporter genes in roots of winter wheat colonized by AM fungus responded to (1) Pi or N nutrient signals transferred from the AM extra-radical hyphae, or (2) carbon allocation changes in the AM association. A three-compartment culture system, comprising a root compartment (RC), a root and AM hyphae compartment (RHC), and an AM hyphae compartment (HC), was used to test whether the expression of Pi or N transporter genes responded to nutrients (Pi, NH4+ and NO3-) added only to the HC. Different AM inoculation density treatments (roots were inoculated with 0, 20, 50 and 200 g AM inoculum) and light regime treatments (6 hours light and 18 hours light) were established to test the effects of carbon allocation on the expression of Pi or N transporter genes in wheat roots. The expression of two Pi transporter genes (TaPT4 and TaPHT1.2), five nitrate transporter genes (TaNRT1.1, TaNRT1.2, TaNRT2.1, TaNRT2.2, and TaNRT2.3), and an ammonium transporter gene (TaAMT1.2) was quantified using real-time polymerase chain reaction. The expression of TaPT4, TaNRT2.2, and TaAMT1.2 was down-regulated by AM colonization only when roots of host plants received Pi or N nutrient signals. However, the expression of TaPHT1.2, TaNRT2.1, and TaNRT2.3 was down-regulated by AM colonization, regardless of whether there was nutrient transfer from AM hyphae. The expression of TaNRT1.2 was also down-regulated by AM colonization even when there was no nutrient transfer from AM hyphae. The present study showed that an increase in carbon consumption by the AM fungi did not necessarily result in greater down-regulation of expression of Pi or N transporter genes.
Highlights
More than two-thirds of plant species can form mutualistic associations with arbuscular mycorrhizal (AM) fungi [1]
To estimate the effect of Pi or N nutrient signals transferred by AM extraradical hyphae on the expression of Pi or N transporter genes involved in the direct nutrient uptake pathway in AM-wheat association, a three-compartment culture system was used in experiment I (Fig 1A)
The AM extra-radical hyphae successfully extended into hyphae compartment (HC), and the hyphal length density (HLD) of HCs in F.m-nutrient and F.m+NO3- treatments were higher than those in the other two treatments (Table 2)
Summary
More than two-thirds of plant species can form mutualistic associations with arbuscular mycorrhizal (AM) fungi [1]. [5] speculated that colonization by AM fungi may inactivate the direct Pi uptake pathway via down-regulation of the expression of Pi transporter genes in the epidermis and root hairs. The high affinity Pi transporters responsible for the direct Pi uptake pathway are often Pi-starvation inducible and are expressed in root hairs and epidermal cells [6]. Since the late 1990s, various studies have shown that colonization by AM fungi results in the down-regulation of expression of Pi-starvation inducible Pi transporter genes in Medicago truncatula, maize (Zea mays L.), and wheat (Triticum aestivum L.) [7, 8]. A split-root study found that AM colonization regulated the expression of the Pi-starvation inducible Pi transporter genes in winter wheat in a localized manner but not systemically [10]. Some Pi transporters in the Pht gene family do not respond to AM colonization [6]
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