Abstract
Plant roots are the first parts of plants to face drought stress (DS), and thus root modification is important for plants to adapt to drought. We hypothesized that the roots of arbuscular mycorrhizal (AM) plants exhibit better adaptation in terms of morphology and phytohormones under DS. Trifoliate orange seedlings inoculated with Diversispora versiformis were subjected to well-watered (WW) and DS conditions for 6 weeks. AM seedlings exhibited better growth performance and significantly greater number of 1st, 2nd, and 3rd order lateral roots, root length, area, average diameter, volume, tips, forks, and crossings than non-AM seedlings under both WW and DS conditions. AM fungal inoculation considerably increased root hair density under both WW and DS and root hair length under DS, while dramatically decreased root hair length under WW but there was no change in root hair diameter. AM plants had greater concentrations of indole-3-acetic acid, methyl jasmonate, nitric oxide, and calmodulin in roots, which were significantly correlated with changes in root morphology. These results support the hypothesis that AM plants show superior adaptation in root morphology under DS that is potentially associated with indole-3-acetic acid, methyl jasmonate, nitric oxide, and calmodulin levels.
Highlights
Plant roots are the first parts of plants to face drought stress (DS), and root modification is important for plants to adapt to drought
The potential mechanisms are: (1) direct water absorption by extraradical hyphae[9,10], (2) increase in nutrient uptake, especially P11, (3) biochemical changes in osmotic adjustment and antioxidant defence systems[12,13,14,15,16], (4) improvement of the soil structure by glomalin-related soil proteins of arbuscular mycorrhizal (AM) fungi[17], and (5) changes in molecule levels, including aquaporins[7,18,19], genes involved in oxidative stress[20] and proline synthesis[21], dehydrin proteins[22], and genes involved in stomata development and abscisic acid (ABA) responses or encoding proteins involved in plant response pathways[23,24]
Root colonization by D. versiformis ranged from 37.5% to 55.3% in AM seedlings, and Drought stress (DS) treatment significantly (P < 0.05) decreased root colonization by 32.2% compared with WW (Table 1)
Summary
Plant roots are the first parts of plants to face drought stress (DS), and root modification is important for plants to adapt to drought. AM seedlings exhibited better growth performance and significantly greater number of 1st, 2nd, and 3rd order lateral roots, root length, area, average diameter, volume, tips, forks, and crossings than non-AM seedlings under both WW and DS conditions. AM plants had greater concentrations of indole-3-acetic acid, methyl jasmonate, nitric oxide, and calmodulin in roots, which were significantly correlated with changes in root morphology. Plant roots are usually the first plant parts to encounter DS They are highly sensitive and responsive to DS conditions, which makes them intimately linked with the drought adaptation of the entire plant[2]. AM fungi modulated greater root hair modification to improve aboveground growth[31] These root modifications under mycorrhization could be linked to the improved uptake of nutrients[32], sugar allocation to roots[33], and polyamine metabolism[28], independent of common symbiosis signalling[34]. Little is known about the response of phytohormones to mycorrhization under DS and the subsequent regulation of root morphology by these phytohormones
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