Maria-Cecília D. Costa, Department of Molecular and Cell Biology, University of Cape Town, South Africa

Phylogenetic evidence suggests that resurrection plants have appropriated a seed-desiccation tolerance mechanism and adjusted it to the whole-plant context (Costa et al., 2017b). For instance, sucrose and oligosaccharides were shown to confer desiccation tolerance to maturing seeds of soybean (Glycine max), pea (Pisum sativum), and maize (Zea mays) (Koster and Leopold, 1988), while in leaves of the resurrection plant Haberlea rhodopensis, the maintenance of constant high levels of sucrose and raffinose increased the chances of surviving desiccation (Djilianov et al., 2011). The accumulation of dehydrins was highlighted as a major attribute for desiccation tolerance in developing Fagus sylvatica seeds (Kalemba et al., 2009) as well as the leaves of the resurrection plant Craterostigma plantagineum (Giarola et al., 2015). In developing rice (Oryza sativa) seeds and in H. rhodopensis leaves, the activation of an extensive antioxidant gene network was correlated with desiccation tolerance (Gechev et al., 2013; He et al., 2016).

Orthodox seeds represent the ultimate example of desiccation tolerance, but they are not the only life form that tolerates desiccation. We find examples of desiccation tolerance also in yeasts, crustaceans, nematodes, rotifers, tardigrades, mosses, liverworts, lichens, algae, and whole plants. It is very likely that desiccation tolerance arose in the transition from aquatic to terrestrial life forms, in organisms living in seashores and tidal habitats, where surviving cycles of dehydration and rehydration were crucial to cell formation and functioning. Later, as plants expanded their habitat onto dry land, which resulted in the establishment of more complex ecosystems, a drawback became evident: desiccation-tolerant plants grow slowly (Alpert, 2006). The processes involved in desiccation tolerance—shutting down metabolism early during drying and fully recovering it upon rehydration—consume energy; desiccation sensitive species use this energy for growth instead, giving them an advantage. This ultimately led to the loss of desiccation tolerance in leaves of vascular plants (Oliver et al., 2000; Alpert, 2006). Desiccation tolerance-related genes were then recruited for processes such as the response to water stress, improved control of water status, vegetative cellular protection and repair, and desiccation tolerance of spores, pollen grains, and seeds (Oliver et al., 2000). Nowadays, desiccation tolerance is common in pteridophytes, rare in angiosperms, and absent in gymnosperms (Porembski, 2011).

In total, only about 1,300 species of vascular plants, of which 135 are flowering plants (termed “resurrection plants”), display desiccation tolerance in their vegetative tissues (Gaff, 1971; Porembski, 2011). These plants occur predominantly in shallow soils on rocky outcrops in semitropical and tropical regions of Africa (Figure 1), America, and Australia (Gaff, 1971; Gaff, 1977; Gaff, 1987; Farrant et al., 2015). Under the harsh microclimatic and edaphic conditions of these areas, resurrection plants are not outcompeted by faster growing desiccation-sensitive plants (Porembski, 2011; Farrant et al., 2015).

Figure 1   Figure 1: Myrothamnus flabellifolia plants growing in their natural environment in Lydenburg (Mpumalanga, South Africa). (Courtesy of Andri Kruger.)

Phylogenetic evidence suggests that resurrection plants have appropriated a seed-desiccation tolerance mechanism and adjusted it to the whole-plant context (Costa et al., 2017b). For instance, sucrose and oligosaccharides were shown to confer desiccation tolerance to maturing seeds of soybean (Glycine max), pea (Pisum sativum), and maize (Zea mays) (Koster and Leopold, 1988), while in leaves of the resurrection plant Haberlea rhodopensis, the maintenance of constant high levels of sucrose and raffinose increased the chances of surviving desiccation (Djilianov et al., 2011). The accumulation of dehydrins was highlighted as a major attribute for desiccation tolerance in developing Fagus sylvatica seeds (Kalemba et al., 2009) as well as the leaves of the resurrection plant Craterostigma plantagineum (Giarola et al., 2015). In developing rice (Oryza sativa) seeds and in H. rhodopensis leaves, the activation of an extensive antioxidant gene network was correlated with desiccation tolerance (Gechev et al., 2013; He et al., 2016).

The recently published comprehensive desiccation-associated transcriptomes of resurrection plants and of seedlings in which desiccation tolerance has been re-introduced are confirming that the vegetative desiccation tolerance in resurrection plants is an adaptation of developmentally-regulated seed desiccation tolerance mechanisms, thus allowing the identification of the exact mechanisms employed (Figure 2) (Costa et al., 2017b; VanBuren et al., 2017). For example, mechanisms involved in the regulated shutdown of photosynthesis in resurrection plants and developing seeds; protection against water loss and institution of a slow drying rate; maintenance of cell integrity via the accumulation of compounds; modification of cell wall plasticity/elasticity; and longevity in the dry state (Costa et al., 2017b).

Figure 2   Summary of differences between resurrection plants and desiccation-sensitive plants (modified from Costa et al., 2017). Upon dehydration below 80–60% relative water content (RWC), resurrection plants activate a series of protective mechanisms, such as regulated shutdown of photosynthesis, reduction of leaf area exposed to light, accumulation of protective molecules, and modification of cell wall plasticity/elasticity. At similar RWCs, photosynthesis fails to be downregulated in desiccation-sensitive plants and consequently, there is an increase in the levels of reactive oxygen species (ROS), unfolded and damaged proteins, and other cellular toxins, and activation of drought-induced leaf senescence and cell death.

One of the aims of desiccation tolerance studies is the generation of information useful for the development of crops with improved drought tolerance. Some mechanisms identified by these studies have been successfully used to generate more stress-tolerant plants. For instance, the dehydration responsive element binding (DREB) proteins are some of the earliest transcription factors found to be involved in abiotic stress responses in plants (Rehman and Mahmood, 2015). VuDREB2A (from cowpea, Vigna unguiculata) enhanced drought tolerance in transgenic Arabidopsis thaliana by activating a series of stress-responsive genes (Sadhukhan et al., 2014). Tobacco plants transformed with ThDREB (from Tamarix hispida) performed better than non-transformed plants under salt and osmotic stress (Yang et al., 2017). Transgenic rice plants overexpressing both OsPIL1 and OsDREB1A showed improved drought tolerance without compromising growth (Kudo et al., 2017).

Further progress in improving drought-tolerance in sensitive plants can be made by the identification and characterization of desiccation tolerance-related genes in resurrection plants. For instance, the gene XvSAP1 (from the resurrection plant Xerophyta viscosa) enhanced tolerance to salinity and to osmotic and high-temperature stress in transgenic A. thaliana plants, possibly by stabilizing cell membranes (Garwe et al., 2006). Transgenic tobacco plants overexpressing the aldose reductase enzyme ALDRXV4 (from X. viscosa) showed improved photosynthetic efficiency, less electrolyte damage, greater water retention, and higher proline accumulation under osmotic stress conditions (Kumar et al., 2013). Rice plants constitutively expressing an osmotin from the resurrection plant Tripogon loliiformis (TlOsm) showed increased tolerance to drought, cold, and salinity stresses (Le et al., 2017).

However, individual genes identified by desiccation tolerance studies have only a limited effect on improving stress tolerance because they might require coordinated expression with other genes to be functional. In this sense, the identification of key factors in regulatory networks controlling desiccation tolerance is a more promising approach because it enables the simultaneous manipulation of multiple pathways that are required for tolerance of extreme dehydration (Farrant et al., 2015).

The plant hormone abscisic acid (ABA) is a master regulator of plant development and water loss responses (Nakashima and Yamaguchi-Shinozaki, 2013; Okamoto et al., 2013). ABA regulates embryo and seed development (promotion of seed dormancy and desiccation tolerance), germination, seedling establishment, vegetative development, general growth, and accumulation of protective molecules during dehydration (Nakashima and Yamaguchi-Shinozaki, 2013; Okamoto et al., 2013). ABA accumulates in dehydrating tissues of several species, such as A. thaliana (Harb et al., 2010), Citrus sp. (Agustí et al., 2007), maize (Lü et al., 2007), and the resurrection plants C. plantagineum (Bartels, 2005), Boea hygrometrica (VanBuren et al., 2017), Sporobolus stapfianus (Whittaker et al., 2001), and X. viscosa (Costa et al., 2017a). In dehydrating tissues, ABA leads to changes in gene transcription and in transcript processing and stability and induces the accumulation of dehydrins, antioxidants, transcription factors, protein kinases and phosphatases, and enzymes involved in phospholipid signaling. Increasing the knowledge of ABA-dependent drought responses and their core signalling components may open promising possibilities for the development of drought-tolerant crop plants.

Orthodox seeds and resurrection plants can serve as models to understand the numerous interacting factors promoting desiccation tolerance (Farrant et al., 2015; VanBuren et al., 2017; Costa et al., 2017b). Several studies have advanced our knowledge on such factors. One example is the ABA signalosome complex (PYR/PYL-PP2C-SnRK2) that regulates stress responses and seed development. When ABA levels increase due to environmental stimuli, the ABA signalosome complex induces cellular changes that help to maintain cellular water status and protect proteins and cellular organelles from collapsing under water stress. Moreover, the interaction between ABA and ethylene is crucial for desiccation tolerance. The last step in the biosynthesis of ethylene is susceptible to ABA inhibition, while ethylene modulates drought stress signalling by acting antagonistically to ABA.

A last example of interacting factors promoting desiccation tolerance is the raffinose family oligosaccharides (RFOs). RFOs participate in several crucial plant cellular functions including transport and storage of carbon, signal transduction, membrane trafficking, and mRNA export. RFOs also act as compatible solutes, signalling molecules, and antioxidants. In desiccated leaves of Craterostigma sp., de novo biosynthesis of RFOs induced by water deficit provides additional cellular protection (Egert et al., 2015). In maturing Medicago truncatula seeds, the conversion of sucrose into RFOs was linked to seed vigor during germination and seedling establishment (Vandecasteele et al., 2011).

New data from whole genome, transcriptome, metabolome, and proteome studies are revealing new genes and pathways for crop improvement from non-model plants, increasing the tool box available for plant breeders. Desiccation tolerance studies have a potential not completely explored to contribute with this tool box and facilitate efforts to develop new crop varieties better able to tolerate water deficit conditions.

References

Agustí J, Zapater M, Iglesias DJ, Cercós M, Tadeo FR, Talón M (2007) Differential expression of putative 9-cis-epoxycarotenoid dioxygenases and abscisic acid accumulation in water stressed vegetative and reproductive tissues of citrus. Plant Sci 172: 85–94

Alpert P (2006) Constraints of tolerance: Why are desiccation-tolerant organisms so small or rare? J Exp Biol 209: 1575–1584

Bartels D (2005) Desiccation tolerance studied in the resurrection plant Craterostigma plantagineum. Integr Comp Biol 45: 696–700

Costa M-CD, Artur MAS, Maia J, Jonkheer E, Derks MFL, Nijveen H, Williams B, Mundree SG, Jiménez-Gómez JM, Hesselink T, et al (2017a) A footprint of desiccation tolerance in the genome of Xerophyta viscosa. Nat Plants 3: 17038

Costa M-CD, Cooper K, Hilhorst HWM, Farrant JM (2017b) Orthodox seeds and resurrection plants: two of a kind? Plant Physiol pp.00760.2017

Djilianov DL, Ivanov S, Moyankova DP, Miteva L, Kirova E, Alexieva V, Joudi M, Peshev D, Van den Ende W (2011) Sugar ratios, glutathione redox status and phenols in the resurrection species Haberlea rhodopensis and the closely related non-resurrection species Chirita eberhardtii. Plant Biol 13: 767–776

Egert A, Eicher B, Keller F, Peters S (2015) Evidence for water deficit-induced mass increases of raffinose family oligosaccharides (RFOs) in the leaves of three Craterostigma resurrection plant species. Front Physiol 6: 1–6

Farrant JM, Cooper K, Hilgart A, Abdalla KO, Bentley J, Thomson JA, Dace HJW, Peton N, Mundree SG, Rafudeen M (2015) A molecular physiological review of vegetative desiccation tolerance in the resurrection plant Xerophyta viscosa (Baker). Planta 242: 407–426

Gaff DF (1971) Desiccation-tolerant flowering plants in southern Africa. Science (80- ) 174: 1033–1034

Gaff DF (1977) Desiccation tolerant vascular plants of southern Africa. Oecologia 31: 95–109

Gaff DF (1987) Desiccation tolerant plants in South America. Oecologia 74: 133–136

Garwe D, Thomson JA, Mundree SG (2006) XVSAP1 from Xerophyta viscosa improves osmotic-, salinity- and high-temperature-stress tolerance in Arabidopsis. Biotechnol J 1: 1137–1146

Gechev TS, Benina M, Obata T, Tohge T, Sujeeth N, Minkov I, Hille J, Temanni M-R, Marriott AS, Bergström E, et al (2013) Molecular mechanisms of desiccation tolerance in the resurrection glacial relic Haberlea rhodopensis. Cell Mol Life Sci 70: 689–709

Giarola V, Challabathula D, Bartels D (2015) Quantification of expression of dehydrin isoforms in the desiccation tolerant plant Craterostigma plantagineum using specifically designed reference genes. Plant Sci 236: 103–115

Harb A, Krishnan A, Ambavaram MMR, Pereira A (2010) Molecular and physiological analysis of drought stress in Arabidopsis reveals early responses leading to acclimation in plant growth. Plant Physiol 154: 1254–1271

He Y, Cheng J, Li X, Zeng P, Yang B, Zhang H, Wang Z (2016) Acquisition of desiccation tolerance during seed development is associated with oxidative processes in rice. NRC Res Press 11: 1–11

Kalemba EM, Janowiak F, Pukacka S (2009) Desiccation tolerance acquisition in developing beech (Fagus sylvatica L.) seeds: The contribution of dehydrin-like protein. Trees 23: 305–315

Koster KL, Leopold AC (1988) Sugars and desiccation tolerance in seeds. Plant Physiol 88: 829–832

Kudo M, Kidokoro S, Yoshida T, Mizoi J, Todaka D, Fernie AR, Shinozaki K, Yamaguchi-Shinozaki K (2017) Double overexpression of DREB and PIF transcription factors improves drought stress tolerance and cell elongation in transgenic plants. Plant Biotechnol J 15: 458–471

Kumar D, Singh P, Yusuf MA, Upadhyaya CP, Roy SD, Hohn T, Sarin NB (2013) The Xerophyta viscosa aldose reductase (ALDRXV4) confers enhanced drought and salinity tolerance to transgenic tobacco plants by scavenging methylglyoxal and reducing the membrane damage. Mol Biotechnol 54: 292–303

Le TTT, Williams B, Mundree S (2017) An osmotin from the resurrection plant Tripogon loliiformis ( TlOsm ) confers tolerance to multiple abiotic stresses in transgenic rice. Physiol Plant. doi: 10.1111/ppl.12585

Lü B, Gong Z, Wang J, Zhang J, Liang J (2007) Microtubule dynamics in relation to osmotic stress-induced ABA accumulation in Zea mays roots. J Exp Bot 58: 2565–2572

Nakashima K, Yamaguchi-Shinozaki K (2013) ABA signaling in stress-response and seed development. Plant Cell Rep 32: 959–970

Okamoto M, Peterson FC, Defries A, Park S-Y, Endo A, Nambara E, Volkman BF, Cutler SR (2013) Activation of dimeric ABA receptors elicits guard cell closure, ABA-regulated gene expression, and drought tolerance. Proc Natl Acad Sci U S A 110: 12132–7

Oliver MJ, Tuba Z, Mishler BD (2000) The evolution of vegetative desiccation tolerance in land plants. Plant Ecol 151: 85–100

Porembski S (2011) Evolution, diversity, and habitats of poikilohydrous vascular plants. In U Lüttge, E Beck, D Bartels, eds, Plant Desiccation Toler. Springer-Verlag Berlin Heidelberg, pp 139–156

Rehman S, Mahmood T (2015) Functional role of DREB and ERF transcription factors: regulating stress-responsive network in plants. Acta Physiol Plant. doi: 10.1007/s11738-015-1929-1

Sadhukhan A, Kobayashi Y, Kobayashi Y, Tokizawa M, Yamamoto YY, Iuchi S, Koyama H, Panda SK, Sahoo L (2014) VuDREB2A, a novel DREB2-type transcription factor in the drought-tolerant legume cowpea, mediates DRE-dependent expression of stress-responsive genes and confers enhanced drought resistance in transgenic Arabidopsis. Planta 240: 645–664

VanBuren R, Wai J, Zhang Q, Song X, Edger PP, Bryant D, Michael TP, Mockler TC, Bartels D (2017) Seed desiccation mechanisms co-opted for vegetative desiccation in the resurrection grass Oropetium thomeaum. Plant Cell Environ. doi: 10.1111/pce.13027

Vandecasteele C, Teulat-Merah B, Morère-Le Paven MC, Leprince O, Ly Vu B, Viau L, Ledroit L, Pelletier S, Payet N, Satour P, et al (2011) Quantitative trait loci analysis reveals a correlation between the ratio of sucrose/raffinose family oligosaccharides and seed vigour in Medicago truncatula. Plant, Cell Environ 34: 1473–1487

Whittaker A, Bochicchio A, Vazzana C, Lindsey GG, Farrant JM (2001) Changes in leaf hexokinase activity and metabolite levels in response to drying in the desiccation-tolerant species Sporobolus stapfianus and Xerophyta viscosa. J Exp Bot 52: 961–969

Yang G, Yu L, Zhang K, Zhao Y, Guo Y, Gao C (2017) A ThDREB gene from Tamarix hispida improved the salt and drought tolerance of transgenic tobacco and T. hispida. Plant Physiol Biochem. doi: 10.1016/j.plaphy.2017.02.007