Insights into plant annexins function in abiotic and biotic stress tolerance

ABSTRACT

Crop productivity depends heavily on several biotic and abiotic factors. Plant annexins are a multigene family of calcium-dependent phospholipid-binding proteins that function in response to environmental stresses and signaling during growth and development of plants. We recently isolated and characterized a Triticum durum annexin, called TdANN12, which is upregulated by different abiotic stresses. Overexpression of TdANN12 in transgenic tobacco improves stress tolerance through ROS removal. This mini-review outlines the functional characterization of plant annexin genes and suggests how these features could be exploitated to improve stress tolerance in plants. Furthermore, transgenic overexpression of plant annexin genes in crops (tobacco, tomato, rice, alfalfa, cotton, and potato) will be discussed as a promising approach to acquire abiotic and biotic stress tolerance.

Introduction

Calcium signaling is typically involved in early responses to stress in plants, and the transduction of these calcium signals into adaptive responses requires calcium-binding proteins.^1,2 These proteins include annexins, which are commonly found in animal and in most eukaryotes.³ Since the first plant annexins were identified in tomato plants,⁴ numerous annexins have been reported in monocot (rice, maize, Medicago, and wheat) and eudicot (Arabidopsis, pea, cotton, potato, and tobacco) species.⁵ Plant annexins are characterized by a shorter N-terminal region and only one or two conserved annexin repeats.⁶ Most of plants annexin are located in the cytosol, but some of them can be present in the plasma membrane, endomembranes, or nuclear envelope.⁷ A few studies reported that annexins have peroxidase and ATPase/GTPase activities as well as Ca²⁺ channel-regulating activity, and regulate diverse aspects of plant growth, development, and stress responses.⁸ Mortimer et al.⁹ demonstrated that Maize annexin peroxidase activity appears independent of heme and persists after membrane association, the latter suggesting a role in reactive oxygen species signaling.

Despite the complex nature of abiotic stress tolerance, plant-annexin genes can have a potential role in biotechnological crop improvement. This mini-review will summarize the most important developments in our understanding of the function of annexin proteins and their ability to introduce enhanced stress tolerance in crops.

Annexins: key players for abiotic stress tolerance

The role of annexins in abiotic stress tolerance has been extensively analyzed and numerous transgenic studies have revealed a positive impact of the expression of annexin genes on plant stress tolerance (Table 1). Studies performed by Kovacs et al.¹³ have shown that MsANN2 from Medicago sativa is expressed when exposed to saline stress. In Arabidopsis, AtANN1 and AtANN4 interact with each other and regulate the tolerance toward salt- and drought-induced stresses in a light-dependent manner.^18,19 Similarly, Yadav et al.¹⁵ concluded that overexpression of AtANN8 alleviates the stresses induced by salt and dehydration in transgenic Arabidopsis. Moreover, Qiao et al.¹² reported that OsANN1 from rice is upregulated under heat stress and rice plants overexpressing OsANN1 show better heat stress tolerance at seedling stages. Li et al.¹⁴ demonstrated that the expression of OsANN3 is upregulated by ABA and drought stress and that the overexpression of OsANN3 in rice confers tolerance to drought by promoting stomatal closure and/or ABA accumulation. Ijaz et al.³ discovered that the expression of SpANN2 in tomato (Solanum pennellii) enhances of its tolerance toward drought and salt stresses. In addition, many cotton annexins have been reported to regulate cotton fiber development and abiotic stress responses.¹⁷ Ben Saad et al.¹⁶ observed that the expression of the durum wheat TdANN12 in transgenic tobacco increases salt and osmotic stress tolerance. Similarly, Szalonek et al.¹¹ found that the overexpression of StANN1 from potato (Solanum tuberosum) in transgenic potato plants results in higher tolerance to drought stress and light. Jami et al.¹⁰ also reported that the transgenic tobacco plants with constitutively expressed mustard annexin in (BjANN1) are more tolerant to different abiotic stress treatments. Furthermore, Ahmed et al.²⁰ reported that the constitutive expression of Brassica juncea annexin, BjANN2, confers salt tolerance and glucose and ABA insensitivity to mustard transgenic plants by enhancing proline accumulation and maintaining ion homeostasis.

Table 1.

List of the miscellaneous annexin genes from plants involved in abiotic and biotic stress tolerance mechanism.

Gene	Isolated from	Expressed in	Observations	Reference
BjAnn2	Brassica juncea	Brassica juncea	Expression in transgenic mustard confers salt tolerance, ABA and glucose insensitivity by enhanced proline accumulation an maintaining ion homeostasis	10
BjAnn1	Brassica juncea	Tobacco	Ectopic expression in transgenic tobacco confers tolerance to abiotic and biotic stress treatments	11
SpAnn2	Solanum pennellii	Tomato	Overexpression in transgenic tomato improves salt and drought tolerance through ABA synthesis and the elimination of ROS	3
OsANN3	Oryza sativa	Rice	Positive regulator of ABA-dependent stress tolerance in rice	12
MsAnn2	Alfalfa	Alfalfa	Plants overexpressing MsAnn2 are tolerant to drought and ABA	9
AnnAt1	Arabidopsis thaliana	Arabidopsis thaliana	Upregulated by oxidative stress, osmotic stress and its overexpression in Arabidopsis leads to drought tolerance	13
GhAnn1	Cotton	Cotton	Plays a role in fiber elongation and conferred tolerance to drought and salt stresses in transgenic cotton	14
OsAnn1	Oryza sativa	Rice	Enhance tolerance to heat stress by modulating H2O2 accumulation	15
StAnn1	Solanum tuberosum	Potato	Light and drought stress tolerance	16
TdAnn12	Triticum durum	Tobacco	Differentially expressed in different tissues, up-regulated by various stress and confers tolerance to salt and osmotic stress in tobacco	17

Annexins responsive to biotic stress

In addition to the functions described above, plant annexins play roles in biotic stress (Table 1). In B. dioica, when the internodes undergo mechanical stress, annexins present in the cytosol of the parenchyma cells are redistributed to the plasma membrane.²¹ Additionally, the stress mechanism is known to elevate the concentration of cytosolic free Ca²⁺ ([Ca²⁺]cyt), which could trigger an association between annexins and the plasma membrane. Although the significance of annexin relocation is not fully understood, studies suggest that it regulates growth in the stressed tissues by restraining the radial expansion resulting from mechanical stress. Annexin relocation has also been suggested to be important in ‘conditioning’ the plasma membrane for further stress responses. The genes encoding plant annexins have been previously induced in the early stages of nodulation in Medicago truncatula during its symbiotic interaction with the bacterium Rhizobium meliloti,^7,22 but annexins have not yet been associated with plant–phytopathogen interactions. Gidrol et al.²³ observed that AtANN1 expression is induced by salicylic acid, which implicates this annexin in pathogen defense responses. Similarly, the expression of AtANN4 in Arabidopsis, LeANN34 in tomato and NtANN12 in tobacco also increases during pathogen attacks. However, the functional significance of the expression of annexin genes in these events is unclear.^24,25 Zhao et al.²⁶ reported that AtANN1- and AtANN4-overexpressing lines are more resistant to Meloidogyne incognita and that, on the contrary, the AtAnn1 or AtAnn4 knockout lines are more sensitive. This demonstrates the positive role of annexins in plant defense.

Annexins and reactive oxygen species

The production of reactive oxygen species (ROS) has been linked to plant growth regulation and [Ca²⁺]cyt regulation under certain conditions.²⁷ The stress circumstances that cause ROS generation are also known to prompt annexin accumulation or induce their relocation to plasma membrane. However, whether ROS and elevated [Ca²⁺]cyt can trigger annexin responses remains unknown. A survey of the literature reveals that membrane oxidation increases the binding of animal annexins to cell membranes.²⁸ In vitro studies report that peroxides can induce insertion of the channel-forming vertebrate ANNA5 into membranes, and in vivo experiments show that peroxide-induced Ca²⁺ influx in DT40 pre-B cells requires ANNA5.²⁹ This suggests that channel-forming plant annexins (such as AtANN1) are candidates for the ROS-activated channels identified in several plant cells.³⁰ Several studies have also suggested that plant annexins are involved in the oxidative stress response.^16,31 Chen et al.³² supported that plant annexins have a key role in the cross-talk between calcium regulation and ROS production under stress signaling. They showed that annexin detected in the proteomes of onion scales from post‐thaw recovered (REC) treatments could facilitate tissue recovery by restoring the membrane calcium and ion homeostasis (especially related to Ca²⁺ and K⁺) and reducing oxidative injury.³² For example, OsANN1 from rice exhibits Ca²⁺-binding ability and ATPase activity. Heat stress induces overproduction of H₂O₂, resulting in the upregulation of OsAnn1, SOD, and CAT to pump ROS as a stress defense mechanism.¹² Zhang et al.³³ indicated that EpANN from lichenized fungus (Endocarpon pusillum) could regulate ROS levels through its peroxidase activity.

Conclusions

Examining annexin genes is of great value in determining the role of annexin signaling in abiotic and biotic stress tolerance, both of which are largely unexplored. Constant enrichment of our knowledge on signaling processes might help assign roles for different plant annexins. With the availability of genome sequences for different species, identification of annexin families across the plant kingdom is being achieved rapidly. Thus, genetic engineering using annexin genes might offer an excellent platform to develop crops with improved abiotic or biotic tolerance.

Funding Statement

This work was supported by the Ministère de l’Enseignement Supérieur et de la Recherche Scientifique.

Acknowledgments

We are grateful to Dr. Frantisek Baluska for kindly inviting this review. This work was supported by a grant from Ministry of Higher Education and Scientific Research of Tunisia (Contrat Programme 2019–2022).

Disclosure of Potential Conflicts of Interest

No potential conflicts of interest were disclosed.