Arbuscular mycorrhizal fungi improving drought tolerance of maize plants by up-regulation of aquaporin gene expressions in roots and the fungi themselves

doi:10.3724/SP.J.1258.2012.00973

Abstract

Abstract:

Aims It has been well demonstrated that arbuscular mycorrhizal (AM) fungi can improve water balance and drought tolerance of host plants under drought stress. However, controversy still exists in mechanisms underlying the mycorrhizal functions. For example, in different experiments AM fungi could up- or down-regulate plant aquaporin gene expression. Furthermore, little information is available on the expression of aquaporin genes in AM fungi under drought stress and its contribution to plant drought tolerance. We investigated the effects of an AM fungus, Glomus intraradices, on expression of a plasma membrane intrinsic protein (PIP) gene family containing 13 PIP genes in maize roots and one aquaporin gene from the AM fungus under simulated drought conditions. Our objectives were to systematically investigate the aquaporin gene expression in the mycorrhizal association in response to drought stress and to help understand the molecular basis for drought tolerance of AM symbiosis.
Methods Maize plants inoculated with/without AM fungus G. intraradices were grown under different water regimes in a controlled-environment climate chamber for 42 days. At harvest, the leaf water potential (Ψ) was determined with an SKPM 1400 pressure chamber, and then shoots and roots were collected and carefully cleaned with tap water. Three grams of mixed roots were used to estimate the percentage root colonization after clearing with 10% KOH and staining with 0.05% (v/v) trypan blue in lactic acid. One gram of mixed roots was used to extract total RNA by using TRIZOL to synthesize cDNA. Real-time PCR analysis was performed to estimate the expression of Zm PIP genes and GintAQP1. Shoots and the rest of roots were dried at 105 °C for 10 min and 80 °C for 48 h to obtain dry weights. About 0.2 g of dried shoots or roots was digested by HNO₃for 12 h. The P concentration was recorded by using microplate reader spectra at the wavelength of 820 nm. Experimental data were subjected to statistical analysis using SPSS 13.0.
Important findings Under drought stress, inoculation with AM fungus significantly enhanced expression of eight PIP genes except for Zm PIP1;3, Zm PIP1;4 Zm PIP1;5 and Zm PIP2;2 (Zm PIP2;7 expression not detected), and drought stress also enhanced expression of GintAQP1 cloned from hyphae of G. intraradices. Enhanced aquaporin gene expression was beneficial to improvement of plant water status and increase of leaf water potential.

Key words: aquaporin, arbuscular mycorrhizal (AM) fungus, drought tolerance, leaf water potential, maize

LI Tao, CHEN Bao-Dong. Arbuscular mycorrhizal fungi improving drought tolerance of maize plants by up-regulation of aquaporin gene expressions in roots and the fungi themselves[J]. Chin J Plant Ecol, 2012, 36(9): 973-981.

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Figures/Tables 8

Table 1 Mycorrhizal colonization rate of maize roots under different water regimes (mean ± SD, n = 4)

接种处理 Inoculation treatment	菌根侵染率 Mycorrhizal colonization rate (%)
接种处理 Inoculation treatment	正常水分 Well-watered	干旱 Drought
不接种 Non-inoculated	0.0	0.0
接种 Inoculated	35.1^*	38.5^{* #}

Fig. 1 Effects of inoculation with arbuscular mycorrhizal (AM) fungus Glomus intraradices on maize dry weights under different water regimes (mean ± SD, n = 4). Two-way ANOVA and t-test were performed for shoots and roots, respectively. * indicates significant difference (p < 0.05) between inoculation treatments under the same water treatment.

Fig. 2 Effects of inoculation with arbuscular mycorrhizal (AM) fungus Glomus intraradices on P concentration of maize under different water regimes (mean ± SD, n = 4). Two-way ANOVA and t-test were performed for shoots and roots, respectively. * indicates significant difference (p < 0.05) between inoculation treatments under the same water treatment.

Table 2 Two-way ANOVA of shoot and root dry weight, P concentration and leaf water potential as influenced by inoculation and water treatments

	地上部干重 Shoot dry weight	根部干重 Root dry weight	地上部磷含量 Shoot P concentration	根部磷含量 Root P concentration	叶片水势 Leaf water potential
接种处理 Inoculation treatment (I)	*	ns	*	ns	*
水分处理 Water treatment (W)	ns	ns	ns	ns	**
交互作用 I × W interaction	ns	ns	ns	ns	ns

Fig. 3 Effects of inoculation with arbuscular mycorrhizal (AM) fungus Glomus intraradices on leaf water potential of maize under different water regimes (mean ± SD, n = 4). # indicates significant difference (p < 0.05) between water treatments under the same inoculation treatment.

Fig. 4 Effects of arbuscular mycorrhizal (AM) fungus Glomus intraradices on expression of plasma membrane aquaporin gene in maize roots under different water regimes (mean ± SD, n = 4). # indicates significant difference (p < 0.05) between water treatments under the same inoculation treatment. * indicates significant difference (p < 0.05) between inoculation treatments under the same water treatment. Statistical analysis was performed for each aquaporin gene respectively. D, drought treatment; W, well-watered treatment.

Table 3 Two-way ANOVA of gene expression of 12 Zm PIPs as influenced by inoculation and water treatments.

	Zm PIP1;1	Zm PIP1;2	Zm PIP1;3	Zm PIP1;4	Zm PIP1;5	Zm PIP1;6	Zm PIP2;1	Zm PIP2;2	Zm PIP2;3	Zm PIP2;4	Zm PIP2;5	Zm PIP2;6
接种处理 Inoculation treatment (I)	*	**	ns	ns	ns	**	**	**	**	*	**	**
水分处理 Water treatment (W)	**	**	ns	ns	*	ns	*	ns	ns	ns	**	ns
交互作用 I ×W interaction	ns	ns	ns	ns	ns	ns	ns	ns	ns	ns	ns	ns

Fig. 5 Effects of drought stress on expression of arbuscular mycorrhizal (AM) fungus Glomus intraradices aquaporin gene (mean ± SD, n = 4). # indicates significant difference (p < 0.05) between water treatments under the same inoculation treatment.

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