Arbuscular mycorrhizal symbiosis-mediated tomato tolerance to drought

ABSTRACT

A multidisciplinary approach, involving eco-physiological, morphometric, biochemical and molecular analyses, has been used to study the impact of two different AM fungi, i.e. Funneliformis mosseae and Rhizophagus intraradices, on tomato response to water stress. Overall, results show that AM symbiosis positively affects the tolerance to drought in tomato with a different plant response depending on the involved AM fungal species.

KEYWORDS:

• AM fungi
• aquaporins
• abiotic stress tolerance
• abscisic acid (ABA)
• iWUE
• stomatal density
• water stress

 

During the last decades, a variety of strategies have been deployed to improve stress tolerance in crops, including traditional selection methods and genetic engineering. The use of root-associated microbial communities to enhance plant tolerance to abiotic stresses has been explored in recent years.¹ An important role as bio-fertilizing microorganisms is played by arbuscular mycorrhizal (AM) fungi thanks to their ability to establish mutualistic symbioses with the roots of most crops.² These symbiotic fungi are considered to be essential elements for plant nutrition, while they receive the products of photosynthesis in turn. Their hyphae can extend for many meters in the soil, helping the host plant to reach water and nutrients.² Furthermore, these fungi have been described to improve plant tolerance to important abiotic environmental conditions such as drought, salt stress and cold.^3,4,5,6,7 We have used a multidisciplinary approach, involving eco-physiological, morphometric, biochemical and molecular analyses, to study the impact of 2 different AM fungi, i.e., Funneliformis mosseae and Rhizophagus intraradices, on tomato response to water stress.⁸ The cooperation in AM interactions is related to the partners involved in the symbiosis, and depends on several factors, including environmental conditions and functional diversity.⁹ Overall, our experimental results show that AM symbiosis positively affects the tolerance to drought in tomato (e.g., improving intrinsic water use efficiency, iWUE) and that R. intraradices seems to be more efficient in the induction of resilience to water stress (WS), at least in the considered tomato cultivar. One noteworthy result regards the role of abscisic acid (ABA), which is an important hormone that regulates plant growth and development and responses to abiotic stresses, such as drought and high salinity.¹⁰ We observed that under severe WS (-1.3 MPa), AM-colonized tomato (AM+) showed a significant lower level of ABA both in roots and leaves compared with uncolonized plants (AM-), suggesting that non-mycorrhizal plants probably faced more intense drought stress than mycorrhizal ones, and produce/accumulate more ABA. To deeply understand the role of ABA in our conditions, the expression of ABA-related genes has also been analyzed, i.e., the ABA-biosynthetic gene LeNCED1 and 3 genes (LePYL9, LePP2C, LeSnR2K) related to ABA signaling mechanisms (Fig. 1). Although this aspect has to be further investigated, taken together, our results suggest that in AM+ plants upon WS the stomata closing is probably not regulated by ABA-mediated mechanisms, but it could be rather induced by passive hydraulic-mediated mechanisms. In woody plants, such as grapevine, it has been demonstrated that stomata closure can be driven by both active (ABA-mediated) and/or passive (hydraulic-mediated) mechanisms.¹¹ Moreover, in addition to opening and closing the stomata, plants may exert control over their gas exchange rates by varying stomata density in new leaves.¹² We have demonstrated that the presence of AM fungi determines a higher stomatal density, especially in plants colonized by R. intraradices.⁸ Since a higher stomatal density can increase the plant CO₂ absorption capacity, these data are in agreement with the significantly higher photosynthetic rates (A_N) measured in AM+ plants both under not stressed (NS) and WS conditions in respect to uncolonized plants, which are also directly correlated with their relative iWUE values.⁸ In order to further clarify these aspects, we also evaluated the expression in new developing tomato leaves, of genes putatively involved in stomatal development regulation (Fig. 1), showing that they are differentially expressed in the presence of the AM fungi. Since the physiological responses of plants to drought stress are also regulated by the expression of genes encoding aquaporins (AQPs),¹³ the expression of several plants and fungal AQPs was considered. It has been already reported that AM symbiosis may regulate the expression of AQP genes, improving root hydraulic conductivity, plant water status and drought tolerance.^{14,15,16,17,18} Under WS, our recent results show that LeNIP3.1 was significantly more expressed in AM fungal-colonized roots (Fig. 1), with higher values in the presence of R. intraradices. This results is in agreement with the specifically expression of a NIP aquaporin gene (LjNIP1) in the root cells containing arbuscules, which represent the key structure of a functional symbiosis, as previously reported in Lotus japonicus.¹⁶ Since AM fungal aquaporin can also have a role in drought tolerance stimulated by AM symbiosis,^19,20 the expression of 2 R. intraradices AQP genes have also been evaluated, showing a significant up-regulation for one of the 2 considered genes (GintAQPF2), in agreement with previous data.²⁰ This contemporaneous induction of both fungal and plant AQP genes in these experiments is a confirmation that the 2 symbionts strictly cooperate to regulate the mycorrhizal drought-stress response. Furthermore, a role of host transpiration and fungal AQP in long-distance fungal polyphosphate (polyP) translocation, and consequently on fungal phosphate (Pi) delivery, has been recently proposed.²¹ The data obtained in our recent publication,⁸ showing a change in plant performance in the presence of the AM fungi under WS, offer new insights for understanding the molecular and physiological mechanisms underlying the tomato tolerance to drought as mediated by the AM fungi. Together with experiments performed by several researchers in the last years, they also open new perspectives in the exploitation of AM symbiosis to enhance crop tolerance to abiotic stress in a scenario of climate change. The evaluation of the nutrient use efficiency, and the regulation of the genes involved in nutrient transport, under drought conditions could represent further steps of this research.

Figure 1. Modulation of the expression of selected genes induced by AM symbiosis in tomato plants subjected to water stress conditions with respect to uncolonised (AM-) plants (adapted from Chitarra et al. 2016).

Disclosure of potential confllicts of interest

No potential conflicts of interest were disclosed.

Funding

This work was supported by the AQUA Project (Progetto Premiale CNR).