**4. Seed priming and agriculture**

Pre-sowing priming induces a particular physiological status in seeds and has emerged as a promising strategy to improve plant behavior in the field. There is a strong interest for farmers and seed companies to find suitable cheap priming treatments but also to precisely identify the agronomical properties improved as a result of priming in cultivated species.

#### **4.1. Hastening and synchronization of germination**

Primed seeds often exhibit an increased germination rate and greater germination uniformity. An enhanced and uniform seedling emergence may contribute to regular crop establishment. Priming may enhance events taking place at the beginning of the germination, but the whole process is interrupted at a given state, which is the same for all concerned seeds. Priming may also induce structural and ultrastructural modifications that could facilitate subsequent water uptake and attenuate initial differences between the seeds in terms of imbibition, thus resulting in a more uniform germination [47].

A faster emergence may help to improve competitivity of cultivated plants against weed species as recently demonstrated by Jalali and Salehi [73] for sugar beets. In mung bean plants, a faster seedling establishment resulting from priming may contribute to a total increase in yield up to 45% [74].

Priming-induced increase in germination may be associated to a change in plant hormone biosynthesis and signaling. Priming has been reported to increase gibberellins (GA)/abscisic acid (ABA) ratio [75], and this may be a direct consequence of a priming impact in gene expression pattern [76]. A more uniform GA endogenous concentration in primed seeds may help to synchronize endosperm weakening, embryo cell elongation, and reserve mobilization [77]. Ethylene also directly influences germination speed and percentage. Increase in ethylene production during priming may promote endo-β-mannase activity facilitating endosperm weakening and post-priming germination [78]. Priming has been reported to initiate repair and reactivation of pre-existing mitochondria and to initiate the biogenesis of new ones [79]. It may thus afford a higher level of energy over a short time to sustain final germination [80].

#### **4.2. Plant growth**

Plants issued from primed seeds often exhibit a faster growth than those issued from unprimed ones. Determine whether such growth stimulation is the consequence of a more rapid seedling establishment or result from a long-term specific physiological status induced by priming still remains an unresolved question. In numerous cases, the beneficial impact of priming on plant growth is more obvious under nonoptimal than under optimal conditions, leading to the global concept that a major advantage of priming consists in an increase in stress resistance (point 4.10). Thus, in direct relation to memory events, the main question is related to the remanence of priming-induced modifications. Imram et al. [71] showed that such modifications remain intact several weeks after germination in maize.

In rice, priming with 5-aminolevulinic acid improved shoot elongation [81] while priming with picomolar rutin augmented both root and shoot length in relation to an increase in photosyn‐ thetic pigments, phenolic and flavonoid contents [82]. In wheat, priming with sodium prusside stimulated plant growth as a consequence of improved capacity to scavenge free radicals by antioxidants [83], and a similar observation was reported for rice as a result of an increase in glutathione peroxidase (GPX) activity [24] and other antioxidant enzyme activities [84].

The beneficial impact of priming on plant growth may be due to an improved nutrient use efficiency allowing a higher relative growth rate [85] and to an improved regulation of the plant water status [86]. Jisha and Puthur [65] confirmed that the priming effect of β-aminobu‐ tyric acid on seeds of *Vigna radiata* further get carried over the seedlings. A higher growth of seedlings issued from primed seeds may also be analyzed in relation to a direct impact of pretreatment on cell cycle regulation and cell elongation processes (point 7) [77, 78].

#### **4.3. Mineral nutrition**

and seed companies to find suitable cheap priming treatments but also to precisely identify

Primed seeds often exhibit an increased germination rate and greater germination uniformity. An enhanced and uniform seedling emergence may contribute to regular crop establishment. Priming may enhance events taking place at the beginning of the germination, but the whole process is interrupted at a given state, which is the same for all concerned seeds. Priming may also induce structural and ultrastructural modifications that could facilitate subsequent water uptake and attenuate initial differences between the seeds in terms of imbibition, thus resulting

A faster emergence may help to improve competitivity of cultivated plants against weed species as recently demonstrated by Jalali and Salehi [73] for sugar beets. In mung bean plants, a faster seedling establishment resulting from priming may contribute to a total increase in

Priming-induced increase in germination may be associated to a change in plant hormone biosynthesis and signaling. Priming has been reported to increase gibberellins (GA)/abscisic acid (ABA) ratio [75], and this may be a direct consequence of a priming impact in gene expression pattern [76]. A more uniform GA endogenous concentration in primed seeds may help to synchronize endosperm weakening, embryo cell elongation, and reserve mobilization [77]. Ethylene also directly influences germination speed and percentage. Increase in ethylene production during priming may promote endo-β-mannase activity facilitating endosperm weakening and post-priming germination [78]. Priming has been reported to initiate repair and reactivation of pre-existing mitochondria and to initiate the biogenesis of new ones [79]. It may thus afford a higher level of energy over a short time to sustain final germination [80].

Plants issued from primed seeds often exhibit a faster growth than those issued from unprimed ones. Determine whether such growth stimulation is the consequence of a more rapid seedling establishment or result from a long-term specific physiological status induced by priming still remains an unresolved question. In numerous cases, the beneficial impact of priming on plant growth is more obvious under nonoptimal than under optimal conditions, leading to the global concept that a major advantage of priming consists in an increase in stress resistance (point 4.10). Thus, in direct relation to memory events, the main question is related to the remanence of priming-induced modifications. Imram et al. [71] showed that such modifications remain

In rice, priming with 5-aminolevulinic acid improved shoot elongation [81] while priming with picomolar rutin augmented both root and shoot length in relation to an increase in photosyn‐ thetic pigments, phenolic and flavonoid contents [82]. In wheat, priming with sodium prusside stimulated plant growth as a consequence of improved capacity to scavenge free radicals by

the agronomical properties improved as a result of priming in cultivated species.

8 New Challenges in Seed Biology - Basic and Translational Research Driving Seed Technology

**4.1. Hastening and synchronization of germination**

in a more uniform germination [47].

yield up to 45% [74].

**4.2. Plant growth**

intact several weeks after germination in maize.

Modification of nutrient uses efficiency by young seedlings may be a consequence of priminginduced overexpression of genes encoding for specific transporters, although only few transporters appear specifically induced by priming itself [36]. An efficient strategy to improve mineral nutrition of young seedling is to use nutrient-based seed priming strategy. Phospho‐ rous seed priming supported crop development at early stages and may compensate for P deficiency in the soil [87, 88]. Jamil et al. [89] demonstrated that improvement of mineral status of P-primed cereals reduced strigolactone exudation and thus sensitivity to the parasite weed *Striga hermonthica*. Muhammad et al. [85] recently performed experiments using Zn, Mn, B, and P priming. These authors demonstrated that nutrient seed priming allowed maize plants to maintain Zn and Mn supply for at least 3 weeks in highly calcareous soils characterized by a low nutrient availability. Similarly, Pame et al. [90] showed that P accumulation in rice may be increased by using P-primed seeds, which is of special interest in Asia where about onethird of the area of rainfed rice is situated on P-deficient soils. Such a higher absorption could not be explained only by nutrient accumulation in the seeds during the primed phase since it is still observed in plants several weeks after sowing. It may therefore be hypothesized that priming interferes with regulation of acquisition mechanisms and further research is crucially needed to identify the molecular mechanisms involved in these processes. Priming with boron improves seedling emergence in rice and, on a long-term basis, increases panicle fertility in relation to an improvement in stigma receptivity [91]. Seed priming may also contribute to improve N nutrition, mainly through an enhanced nitrate reductase activity in plants [40]. Priming with nonessential beneficial elements, such as Si, leads to an increase in Si content of cultivated plants and has a protective impact on plant development [86].

Beside the improvement of essential elements uptake, priming also helps to reduce accumu‐ lation of putatively toxic elements. Chromium (VI) accumulation is reduced in maize seedlings issued from salicylic acid primed seeds and cultivated in the presence of this toxic element [82]. Osmopriming with PEG and hormopriming with GA improved germination and early seedling growth of white clover maintained on a heavy metal-contaminated soil, but the impact on Cd accumulation by plants may differ according to the considered treatment since GA3 increased Cd accumulation while PEG reduced it [47]. Liu et al. [92] demonstrated that PEG increases Ca2+ cytosolic concentration through hyperpolarization-activated calcium permeable channels, which could explain a lower Cd accumulation as a consequence of an improved selectivity toward calcium.

Numerous data are also available concerning the priming effect on the plant behavior exposed to salinity. It is frequently reported that priming-induced stress resistance may be a conse‐ quence of an improved discrimination for K+ over Na+ nutrition. Both osmo- and hydropriming were efficiently used to influence K+ selectivity of seedlings, but the underlying molecular basis of this improvement still needs to be identified, especially in terms of regulation of monovalent cation transporters.

#### **4.4. Yield-related parameters**

A huge amount of studies is devoted to the impact of seed priming on the seed germination phase and early seedling growth. Most of those studies are conducted under controlled environmental conditions in plant growth chambers or greenhouses. Data reporting a real improvement under field conditions remain rare. Yield effect may be linked to a faster plant establishment allowing a longer growth period. Khan et al. [93] reported that plant issued from primed seed benefits from a longer period of assimilates accumulation in sugar beet. Con‐ versely, in some cases, phenological evolution of cultivated plants may be modified by priming: in chickpea, plants issued from priming encountered an earlier seed maturity allowing them to escape disease or heat terminal stress in the season [94]. Yield increase may also result from a higher plant density observed as a consequence of priming-induced increase in germination percentage [95].

Since less than a decade, several data started to be available for priming-induced yield improvement in rice. Shah et al. [96] demonstrated that priming had a positive effect on the weight of 1000 grains in this species. Boron priming induced an obvious decrease in panicle sterility and consequently improved the number of grains per inflorescence [91]. Binang et al. [97] also demonstrated that priming had a significant effect on the number of tillers, number of fertile panicles, and consequently grain yield of new NERICA rice varieties. Promising yield improvement has also been reported for maize [85, 98], onion [99], okra [100], and sugar beet [73]. Beside its impact on quantitative parameters, priming may also improve the quality of harvested plants, as recently reported by Janecho et al. [101] for the vitamin content and nutritional value of legumes.

#### **4.5. Stress resistance**

Most of the studies performed on the seedlings issued from primed seeds demonstrated a clear improvement of resistance to environmental constraints. **Table 1** is providing a nonexhaustive list of recent publications dealing with stress resistance improvement on cultivated plant species. Frequently, such improvement is obvious just after emergence at the seedling level, but progressively disappears at the adult stage. For example, some young plants issued from priming treatments displayed improvement of resistance to chilling [84], low temperature [75], salinity [43, 102], high temperature [80], drought [24, 65, 103], and UV exposure [82]. Some interesting studies also demonstrated that priming may afford resistance to biotic stresses such as *Fusarium oxysporum* in tomato [104], viral disease in *Brassica rapa* [105], and downy mildew in pearl millet [106]. Such a large set of data suggests that seed priming may elicit numerous pathways contributing to stress resistance. The molecular basis involved in this stress resist‐ ance remains intact during the dehydration phase following priming and may contribute to stress resistance during the final germination step. Moreover, some data suggest that a single priming treatment may induce resistance to various stresses.

Numerous data are also available concerning the priming effect on the plant behavior exposed to salinity. It is frequently reported that priming-induced stress resistance may be a conse‐

10 New Challenges in Seed Biology - Basic and Translational Research Driving Seed Technology

of this improvement still needs to be identified, especially in terms of regulation of monovalent

A huge amount of studies is devoted to the impact of seed priming on the seed germination phase and early seedling growth. Most of those studies are conducted under controlled environmental conditions in plant growth chambers or greenhouses. Data reporting a real improvement under field conditions remain rare. Yield effect may be linked to a faster plant establishment allowing a longer growth period. Khan et al. [93] reported that plant issued from primed seed benefits from a longer period of assimilates accumulation in sugar beet. Con‐ versely, in some cases, phenological evolution of cultivated plants may be modified by priming: in chickpea, plants issued from priming encountered an earlier seed maturity allowing them to escape disease or heat terminal stress in the season [94]. Yield increase may also result from a higher plant density observed as a consequence of priming-induced increase

Since less than a decade, several data started to be available for priming-induced yield improvement in rice. Shah et al. [96] demonstrated that priming had a positive effect on the weight of 1000 grains in this species. Boron priming induced an obvious decrease in panicle sterility and consequently improved the number of grains per inflorescence [91]. Binang et al. [97] also demonstrated that priming had a significant effect on the number of tillers, number of fertile panicles, and consequently grain yield of new NERICA rice varieties. Promising yield improvement has also been reported for maize [85, 98], onion [99], okra [100], and sugar beet [73]. Beside its impact on quantitative parameters, priming may also improve the quality of harvested plants, as recently reported by Janecho et al. [101] for the vitamin content and

Most of the studies performed on the seedlings issued from primed seeds demonstrated a clear improvement of resistance to environmental constraints. **Table 1** is providing a nonexhaustive list of recent publications dealing with stress resistance improvement on cultivated plant species. Frequently, such improvement is obvious just after emergence at the seedling level, but progressively disappears at the adult stage. For example, some young plants issued from priming treatments displayed improvement of resistance to chilling [84], low temperature [75], salinity [43, 102], high temperature [80], drought [24, 65, 103], and UV exposure [82]. Some interesting studies also demonstrated that priming may afford resistance to biotic stresses such as *Fusarium oxysporum* in tomato [104], viral disease in *Brassica rapa* [105], and downy mildew in pearl millet [106]. Such a large set of data suggests that seed priming may elicit numerous pathways contributing to stress resistance. The molecular basis involved in this stress resist‐

over Na+

nutrition. Both osmo- and hydropriming

selectivity of seedlings, but the underlying molecular basis

quence of an improved discrimination for K+

were efficiently used to influence K+

**4.4. Yield-related parameters**

in germination percentage [95].

nutritional value of legumes.

**4.5. Stress resistance**

cation transporters.



**Table 1.** Nonexhaustive list of recent studies devotes to priming-induced increase in the stress resistance of cultivated plant species.

The priming procedure itself implies frequently the use of stressing agent, as it is the case for PEG and salt. In some cases, priming may be performed at low temperature to reduce the kinetics of seed hydration. A slow hydration may be considered as a stressing process since the water content is too low to allow radicle elongation (see point 5). It may thus induce defense responses within embryos. This is especially the case for biochemical processes involved in protection against reactive oxygen species (point 8). Several components of the ROS-mediated signaling pathways are activated during the first hydration phase of the priming process. The ultimate stress resistance in the seedlings may then be linked to the persistence of the antiox‐ idative defenses after final germination. Since management of oxidative stress is an important component of resistance to a wide range of stress, this observation may, at least partly, explain the cross-resistance phenomena.

**Environmental constraint Plant species Priming treatment Reference**

Chilling and low temperatures *Vigna radiata* Hydropriming/proline [25]

12 New Challenges in Seed Biology - Basic and Translational Research Driving Seed Technology

High temperatures *Lactuca sativa* Hydropriming [76]

Heavy metals *Trifolium repens* PEG [47, 179]

*Phythium ultimum Zea mays* Biopriming [53] *Verticillium Brassica napus* Biopriming [56] *Downy mildew Pennisetim glaucum* Biopriming [59]

Yellow mosaic virus *Vigna radiata* On farm priming [75] *Fusarium oxysporum Solanum lycopersicum* Methyl jasmonate [104] Double-stranded DNA virus *Brassica rapa* Biopriming [105]

**Table 1.** Nonexhaustive list of recent studies devotes to priming-induced increase in the stress resistance of cultivated

The priming procedure itself implies frequently the use of stressing agent, as it is the case for PEG and salt. In some cases, priming may be performed at low temperature to reduce the kinetics of seed hydration. A slow hydration may be considered as a stressing process since the water content is too low to allow radicle elongation (see point 5). It may thus induce defense responses within embryos. This is especially the case for biochemical processes involved in protection against reactive oxygen species (point 8). Several components of the ROS-mediated signaling pathways are activated during the first hydration phase of the priming process. The ultimate stress resistance in the seedlings may then be linked to the persistence of the antiox‐

Biotic stresses

plant species.

*Cicer arietinum* Osmo/hydropriming [170]

*Zea mays chitosan* [62]

*Glycine max* Osmopriming [79] *Oryza sativa* Salicylic acid [84] *Beta vulgaris* Osmopriming [93] *Spinacea oleracea* Osmopriming [107] *Nicotiana tabacum* Putrescine [166]

*Daucus carota* PEG [80]

*Poa pratensis* PEG, Gibberellins [179]

Nutrient priming [71]

BABA [106]

The dehydration step that follows the partial hydration phase is also a major stressing phase. Numerous studies focus on late embryogenesis abundant proteins (LEA) normally involved in the acquisition of desiccation tolerance. Chen et al. [107] showed that the major dehydrins disappear during osmopriming while Maia et al. [108] conversely suggested that PEG may induce LEA synthesis. Water deprivation associated with the dehydration phase may also trigger accumulation of transcription factors, some of them being specifically involved in stress resistance [109, 110]. Molecular chaperones such as heat shock proteins (HSP) also explain a priming-induced improvement of stress resistance [78]. It may also be hypothesized that priming-induced modification of the seed hormonal status, mainly an increase in ABA, may somewhat influence the seed and young seedling response to environmental constraints in relation to a faster activation of ABA-responsive genes involved in stress acclimation [108, 109].

The beneficial impact of priming treatments relies on numerous properties as indicated in **Figure 2**.

**Figure 2.** General overview of biochemical and physiological basis of priming effects. Priming modifies seed ultra‐ structure, reserve mobilization, regulation of oxidative status and cell cycle, and seed water content. The obtained seedlings may be improved for growth, mineral nutrition, and stress resistance. The components of priming effect may be revealed through an integrated convergent proteomic, transcriptomic, and metabolomics holistic approach.
