1. Introduction
Gummy stem blight caused by
Stagonosporopsis cucurbitacearum (syn.
Didymella bryoniae) is a very destructive disease in watermelon and other cucurbit species [
1,
2,
3]. The fungus can attack all plant parts, including leaves, stems, fruit and even roots [
1,
4]. The disease can cause very serious losses in watermelon in the field as well as after harvest under favourable conditions [
3,
5,
6,
7].
Control of the disease is difficult for several reasons. Consequently, the disease develops rapidly at high levels of humidity [
5], which is often present in production fields. In addition, no highly resistant cultivars of watermelon are available [
8], although extensive searches for new sources of resistance are ongoing [
9,
10,
11,
12,
13]. Furthermore, the pathogen rapidly develops resistance to fungicides [
2,
14,
15,
16], rendering chemical control difficult.
To reduce the pesticide input, induced resistance has received considerable attention as an environmentally friendly strategy [
17,
18,
19]. Induced resistance denotes the phenomenon that a plant, appropriately stimulated, can defend itself against pathogens [
18,
20]. Thus, treatment of the plants with biotic or abiotic inducers can activate defence responses in the plant against subsequent pathogen attack [
21,
22]. Research on induced resistance in cucurbit plants has, for example, included studies of cucumber against
Gloeosporium orbiculare (syn.
Colletotrichum lagenaria) and
Podosphaera fuliginea (syn.
Sphaerotheca fuliginea) [
23,
24,
25,
26,
27,
28] and of melon against
Sclerotinia sclerotiorum and
Stagonosporopsis cucurbitacearum [
29,
30,
31]. There are apparently only few reports from watermelon implying that protection against pathogens involves induced resistance [
31]. Several chemicals like acibenzolar-S-methyl, methyl jasmonate, K
2HPO
4 and MnCl
2 have been reported to be able to induce resistance in cucumber and melon [
24,
26,
27,
28,
29].
Various defence responses have been inferred as being important in induced resistance in cucurbit plants. Examples include accumulation of phytoalexins [
32,
33]. Likewise, the hypersensitive response as well as reinforcement of plant cell walls appear to be positively correlated to the reduction of pathogen penetration and this effect is mediated by papilla formation and accumulation of callose, lignin, phenolic compounds, H
2O
2 and silicon at the sites of attempted penetration [
30,
31,
33,
34,
35,
36,
37]. Accumulation of PR-proteins like peroxidase, chitinase and β-1,3-glucanase has also been reported as an important mechanism [
29,
31,
37,
38,
39,
40]. A specific peroxidase isoform (45 kDa) was found to accumulate during induction of resistance against
S. cucurbitacearum in watermelon using rhizobacteria [
31] and this isoform was also found to be important when explaining differences in the level of resistance against
S. cucurbitacearum between a susceptible and a moderately resistant accession of watermelon [
41]. Furthermore, activities of important enzymes in the phenyl propanoid pathway have also been increased, such as phenylalanine ammonia lyase, polyphenol oxidase and peroxidase, which are involved in lignification and accumulation of phytoalexins and phenolics [
31,
37,
38,
42,
43]. The hypersensitive reaction has also been mentioned as an important mechanism in phosphate-induced resistance in cucumber [
27] as well as accumulation of reactive oxygen species in cucumber and watermelon, respectively [
27,
31].
Boron is an essential plant micronutrient, usually taken up by the roots, although it can also be applied as seed treatments or foliar sprays [
44,
45] However, in excess, boron causes toxicity to the plants, so an appropriate dosage is critical [
44,
45]. Borax or sodium tetraborate (Na
2B
4O
7) can be used as a fertiliser [
46], but is also known as a pesticide, for example, against insect pests [
47]. It has previously been reported to induce local as well as systemic resistance in rice against
Pyricularia oryzae in Vietnam in greenhouse as well as in field trials [
48,
49]. Boric acid has also been reported to induce local and systemic resistance in cucumber against powdery mildew caused by
Podosphaera fuliginea [
24] and different sources of boron were able to control clubroot in oilseed rape [
50]. Recently, safflower seeds were primed with 5–10 ppm boron or seed dressed with boron to reduce the negative effects of seed-borne pathogens and promote germination and seedling growth [
51].
The present investigation was undertaken to assess the ability of Na2B4O7 to protect watermelon against S. cucurbitacearum and study the mechanisms involved, with the hypothesis being that the compound induces resistance. We show that priming of watermelon seeds with Na2B4O7 can induce resistance against foliar infection caused by S. cucurbitacearum and the mechanisms of disease protection are associated with a higher frequency of hypersensitive cells, increased peroxidase activity, as well as an altered peroxidase isozyme profile.
4. Discussion
Priming of watermelon seeds with Na
2B
4O
7 was able to provide significant levels of protection against
S. cucurbitacearum in the first leaves of both living plants and in detached leaves. Priming with 1 mM Na
2B
4O
7 for 24 h gave the highest level of protection under greenhouse conditions, whereas it was not as pronounced in detached leaves. Previously, boron in different chemical formulations, including Na
2B
4O
7, was shown to protect plants against various diseases [
24,
31,
48,
49,
51]. Na
2B
4O
7 is often considered a biopesticide, chiefly against insect pests [
54,
55]. However, Na
2B
4O
7 has apparently not been used to protect watermelon and other species in Cucurbitaceae against diseases before.
In vitro tests showed that 1 mM Na
2B
4O
7 had no visible effect on
S. cucurbitacearum growth on agar plates. An inhibiting effect on
S. cucurbitacearum was seen only in concentrations higher than 2 mM and therefore 1 mM Na
2B
4O
7 was chosen for further studies. Since 1 mM Na
2B
4O
7 applied to seeds could protect watermelon plants efficiently, without a direct effect on the pathogen, the results could indicate that Na
2B
4O
7 induces systemic resistance against foliar infection by
S. cucurbitacearum. Similarly, Na
2B
4O
7 was found to protect rice systemically from blast at a concentration of 1 mM and this concentration did not inhibit
P. oryzae mycelium growth in in vitro tests [
49]. Interestingly, boron and Na
2B
4O
7 have been found to inhibit other pathogens like
Monilinia laxa strongly in vitro [
56]. Concentration of the compounds plays a pivotal role, but there might also be differences in the sensitivity of different pathogens to the compounds.
In order to verify if induced resistance sensu Kloepper et al. [
20] was involved in the protection, the infection biology of the pathogen was studied to determine at which steps it was arrested. As pointed out earlier [
41,
57], quantitative microscopy of the infection course of the pathogen at consecutive time points provides a unique opportunity for understanding when infection is arrested and link this to defence responses in the host. There was no significant inhibition of spore germination on leaves by 1 mM Na
2B
4O
7, corroborating the in vitro results, whereas the percentage of spores causing penetration was significantly reduced in leaves from Na
2B
4O
7-primed compared to water-primed plants of both the susceptible and the moderately resistant accession.
A comparison of the infection biology of
S. cucurbitacearum in Na
2B
4O
7-induced and water-treated plants showed that there was a higher frequency of single fluorescent epidermal cells in leaves of plants raised from seeds primed with Na
2B
4O
7 than in plants raised from seeds primed with water, although the difference was only significant in accession PI189225 at 48 hai. FEC is an indication that the cells are undergoing the hypersensitive reaction (HR) [
58,
59,
60] and HR has previously been reported as a defence response in the interaction between melon and
S. cucurbitacearum [
30]. HR cells have a major accumulation of reactive oxygen species, phytoalexins and phenolic compounds [
61,
62,
63] and are reported to be an efficient defence response of plants to biotrophic and hemibiotrophic pathogens [
27,
60,
62,
63,
64,
65]. Recently, it was suggested that
S. cucurbitacearum might be considered as a hemibiotrophic pathogen, among others, because it was inhibited by H
2O
2 during penetration and because it sometimes grows in the tissue without causing symptoms [
41]. However, as pointed out previously [
41], further studies should be carried out to study under which conditions a hemibiotrophic behaviour takes place.
A higher frequency of HR-cells has been related to resistance in various plant pathogen interactions. For example, a higher frequency of HR was found to be correlated to the level of resistance in rice infected by
Bipolaris oryzae [
59]. Likewise, a high frequency of HR-cells was also found to be associated with induced resistance in cucumber against
Colletotrichum lagenaria after pre-treatment with the bacteria
Serratia marcescens (isolate 90-166) and
Pseudomonas fluorescens (isolate 89B61) [
25]. We found that HR occurred at a higher frequency after penetration in plants raised from seeds primed with Na
2B
4O
7 than in the control plants. This indicates that HR is a defence response after penetration and that the enhancement of the response following Na
2B
4O
7 treatment could inhibit the pathogen. A comparative observation was reported earlier for induced resistance in barley against the hemibiotrophic pathogen
Pyrenophora teres where inducer treatment enhanced the efficiency of the HR response [
66].
Peroxidases are important enzymes in plant defence [
67,
68]. Thus, they may participate in generation of H
2O
2, which is important for HR and are involved in many defence responses [
65]. Peroxidases also participate in modification of plant cell walls such as lignification, suberisation and in oxidative protein cross-linking in cell walls. Total peroxidase activity was studied in the susceptible accession 232-0125/B and it was found to increase earlier and more rapidly (12 hai) in pathogen-infected plants raised from seeds primed with Na
2B
4O
7 than in pathogen-infected control plants (seeds primed with water). The increase in peroxidase activity at 12 hai correlated with lower penetration frequency in plants treated with Na
2B
4O
7 at 17 hai compared to the control. A similar observation was made in studies of induced resistance of cucumber against
Colletotrichum lagenaria by K
2HPO
4 [
26] and studies of induced resistance of watermelon against
S. cucurbitacearum using
Pseudomonas aeruginosa 23
1-1 as inducer [
31]. Likewise, peroxidases have been implicated in the defence of cucurbit plants against infection by various pathogens, including
S. cucurbitacearum [
28,
30,
41,
69]. The early increase of peroxidase activity in plants treated with Na
2B
4O
7 may therefore be involved in disease protection. However, at the late stages of infection from 48 to 96 hai, peroxidase activity increased rapidly to a very high level in control plants pre-treated with water compared to plants pre-treated with Na
2B
4O
7 and this correlated with larger disease development in control plants. A similar pattern of peroxidase activity has been found in other host-pathogen interactions such as in cucumber, where pre-treatment with K
2HPO
4 can induce resistance against
Colletotrichum lagenaria [
26] and in wheat infected by
Zymoseptoria tritici [
70]. Also, in watermelon infected by
S. cucurbitacearum, a high peroxidase activity was seen in a susceptible compared to a moderately resistant accession at the late stages of infection [
41]. This late increase in peroxidase activity in the compatible interaction could be a defence response, which occurred too late to be effective in stopping the pathogen. Furthermore, as pointed out by Nga et al. [
41], it could also represent a rapid increase of ascorbate peroxidase activity for scavenging the large amounts of H
2O
2, which was produced during the later stages of infection.
Native-PAGE for studying the accumulation of specific peroxidase isozymes was performed in the susceptible accession 232-0125/B and it was found that an acidic (45 kDa) isoform accumulated to a higher extent in leaves from plants raised from seeds primed with Na
2B
4O
7 after inoculation with the pathogen than in the control plants raised from seeds primed with water. The same isoform was found to accumulate to a larger extent in previous studies of watermelon infected by
S. cucurbitacearum. Thus, increased accumulation during infection was observed in the moderately resistant accession (PI189225) and in the susceptible accession 232-0125/B [
41], as well as in watermelon plants protected from infection by the pathogen after protection with the bacterium
Pseudomonas aeruginosa 23
1-1 [
31]. As reviewed earlier [
41], different peroxidase isoforms have been implicated in resistance of cucurbit plants against different pathogens so further characterisation of peroxidase isoforms may contribute to a better understanding of resistance.
A future practical use of Na
2B
4O
7 in crop production will depend on several issues. Boron is an essential element for plant growth [
71,
72] and there is increasing evidence that it may also be essential in human nutrition in minute amounts [
73]. Boron has also shown promise in health-promoting dietary supplements in animals and humans [
74,
75]. However, excess boron poses toxicity issues in humans, animals and plants [
44,
76,
77] so the application to agricultural or horticultural systems should be carefully considered. It could be an advantage to use Na
2B
4O
7 to control gummy stem blight in watermelon in future under field conditions. Thus, it is simple to apply by seed priming and only a small amount of chemical is needed for seed priming compared to foliar application, but it is important to make sure that there will be no negative effects on seed germination and vigour, the environment and human health if used at a large scale. Even if Na
2B
4O
7 will not be possible to use in practical crop production, the potential of this compound to induce defence responses may be useful for further studies on the defence mechanisms operating in watermelon against gummy stem blight and other diseases.