According to previous research, antagonistic yeast may replace chemical fungicides, prevent the postharvest diseases of fruits and vegetables, and significantly maintain the good qualities of fruits and vegetables. In this study (Fig. 1), the application of A. pullulans S-2 significantly reduced the incidence of tomato postharvest diseases, also maintained fruit moisture, firmness, TA, ascorbic acid and lycopene. Our results were highly comparable with the previous study .
The influence of A. pullulans S-2 on the change of microbial community structure on tomato surface is the key point of our study. The high-throughput sequencing technology was used to identify the microbiome of tomato surface during storage. From the analysis of ACE index (Fig. 2), we found that the application of A. pullulans S-2 had a more significant impact on fungal diversity. The rapid colonization of A. pullulans S-2 and inhibition of other pathogenic fungi in the early stage of storage resulted in the decrease of fungal diversity, so the diversity of the treatment group was less than the control group. At the later stage of storage, the tomato fruit began to rot, the abundance of pathogenic microorganisms and the microbial diversity of the treatment group was increased. The previous study has shown that rotting could significantly reduce fungal diversity , so the microbial diversity of the control group decreased due to the severe rot (Fig. 2A).
Then, we analyzed the effect of A. pullulans S-2 on the microbial community structure of tomato surface from the perspective of colony composition. From the perspective of fungi (Fig. 4B), after A. pullulans S-2 treatment, the five genera namely Aureobasidium, Cladosporium, Mycosphaerella, Alternaria, and Penicillium were changed significantly in the control and treatment groups. In the treatment group, only Aureobasidium was significantly higher than the control group, and the other four genera were lower compared to the control group (Fig. 5). Aureobasidium is a ubiquitous strain on the surface of tomatoes, and it also occupies a certain proportion in the naturally grown group (CK).
Cladosporium, Mycosphaerella, Alternaria, Penicillium are several common pathogens. Studies have shown that Cladosporium mainly exists in the leaves of various plants and can cause tomato leaf mold disease [29, 30]. Mycosphaerella is the primary source of melon and banana leaf spot [31, 32], but it doesn't seem to cause postharvest diseases. Alternaria and Penicillium can infect various fruits and vegetables, leading to black spot and blue mold of postharvest tomato [33–35].
It was observed that A. pullulans S-2, showed an outstanding ability to survive and colonize, and compete with pathogenic fungi such as Cladosporium, Mycosphaerella, Alternaria, Penicillium, for nutrition and space, thereby inhibiting the growth of pathogenic fungi and achieving the effect of disease control. Our results are consistent with the previous report that A. pullulans can compete for nutrition and space to prevent pathogenic fungi .
Stating from the bacterial genus (Fig. 8), the main genera found in our study was reported to be beneficial to plants on the surface. Among them, Pantoea is quite special. It is widespread in plants, and most of them are screened out from diseased plants. Therefore, it is generally believed that bacteria belonging to this genus were pathogenic in the early stage. Among them, the notorious pathogens, including Pantoea stewartia subsp. Stewartii and Pantoea ananatis, were reported to infect corn and rice . However, with the deepening of research, it was found that some species in the genus have nitrogen fixation and are considered to plant growth-promoting bacteria. They play a role in controlling Botrytis cinerea, which is very complex and fascinating [38–40].
Most species in the genus Brevibacterium have been proven to promote plant growth, among which Brevibacterium casei MH8a can enhance the ability of white mustard seeds to absorb metals, and Brevibacterium linens in Compost teas have a specific control effect on tomato diseases [41, 42]. In order to enhance the beneficial effects of Brevibacterium strains, it has been used in combination with other strains. For example, the synthetic community of Brevibacterium frigoritolerans HRS1 and the other three bacteria has a stronger immune effect against tomato bacterial wilt . Brevibacterium halotolerans and Trichoderma harzianum have a synergistic interaction in improving the growth and yield of peppermint . There are reports that the bacteria in the genus Brachybacterium also can promote plant growth .
Studies proved that Serratia, Glutamicibacter, and Pseudomonas had been recognized as plant growth-promoting bacteria. Serratia proteamaculans can effectively control tomato early blight and promote plant growth . A salt-tolerant PGPR strain Glutamicibacter sp. YD01 can stimulate plant growth and development and alleviate the adverse effects on plants under salt stress conditions . Pseudomonas was often isolated from the rhizosphere of plants, and some of them have been reported as rhizosphere bacteria that promote plant growth [48, 49].
The effect of A. pullulans S-2 on bacteria is ‘simple and clear’ on the fungal community and mainly affects the growth and decay rate of many bacterial species. The proportion of Pantoea, Brevibacterium, and Brachybacterium in the treatment group was significantly lower than that of the control group, and the three genera Serratia, Glutamicibacter, and Pseudomonas were significantly higher compared to the control group. It can be understood as the ‘replacement’ of the dominant species.