Tomato is an agronomically valuable crop in many countries, either grown in fields or greenhouses and therefore has been bred and genetically improved for centuries. Nonetheless, it remains vulnerable to diseases, consequently requiring the use of chemical pesticides, mainly in greenhouses. On the other hand, it is vital to diminish the usage of environmental-unfriendly phytochemicals and prefer less harmful tools to fight pathogens.
Plants respond to pathogens by means of elaborated mechanisms. In many cases, these responses can result in resistance. Besides constitutive defenses, plant tissues can counteract pathogens assault thru inducible responses (Frost et al. 2008). These require the specific recognition of the attacker, which in turn triggers a multileveled defense (Dangl et al. 2013) that can result in resistance. If the plant is unable to recognize the pathogen it will not activate the resistance machinery. Different actors are involved in this phenomenon specifically phytohormones, such as salicylic acid (SA) and jasmonic acid (JA) (Dangl et al. 2013), small signaling molecules, proteins, enzymes and protective molecules (for example: reactive oxygen species, the cell wall component callose and pathogenesis-associated proteins, PR; Borges and Sandalio 2015; Dangl et al. 2013). The latter forms a heterogeneous group of proteins (17 members) with specific capabilities and systems (Sels et al. 2008).
Our recent investigation described the effect of extracts obtained from plants widespread and easy to find, also known for their antimicrobial effects, on the activation of plant defenses against pathogens. Aqueous extracts of hot pepper ‘Rocoto’, wild rue and ethanolic extracts of clove powder were sprayed on tomato plants and their ability to trigger defenses against pathogens was evaluated through several experiments.
mRNA quantification (PCR real time) revealed that the expression level of different defense genes (PR proteins) was increased for several hours upon treatment with the plant extracts, with clove and wild rue being the strongest stimulating solutions. This would suggest that by treating with the plant extracts one can activate the molecular defense system even if the plant has not recognized the pathogen. Interestingly, this stimulating effect could also be reproduced, slightly amplified, in tomato plants after a second treatment, 15 days after the first thus suggesting that repeated treatments could enhance plant defenses and keep plant alerted against new attacks.
Further analyses suggested that our plant extracts not only activate front-line defenses such as PR proteins (working directly against the pathogens) but also main regulators of plant resistance such as those belonging to the MAPK group (MAPK2 and MAPK7; Stulemeijer et al. 2007; Kong et al. 2012) involved in the coordinate regulation of resistance mechanisms. Both MAPK-encoding genes appeared to be activated upon treatment with pepper, rue and clove extracts. Additional analyses showed that the plant extracts used are capable to trigger both SA-dependent and -independent signaling pathways thus confirming that these plant extracts act as defense priming agents.
To assess if the three plant extracts are capable of setting off a hypersensitive reaction (a typical necrotic reaction seen in pathogen-resistant plants), tomato and tobacco (model plant for infiltration experiments) leaves were infiltrated with pepper, wild rue and clove extracts using a syringe. Leaf phenotype changes were checked 1 or 2 days later. In tobacco, necrotic regions associated with the hypersensitive reaction appeared 24 and 48hours after every treatment. The more potent necrotic signs were induced with clove extract, in each time point. The whole infiltration region seemed necrotic and translucent with concomitant cell death. Pepper and wild rue extracts did cause necrotic lesions, however at a lesser magnitude and particularly at 48h. No effects were ever seen in negative control plants, infiltrated with water.
These simple extracts are derived from plants quite widespread, with low expenses and are widely recognized for antimicrobial action. Therefore these extracts combine numerous capabilities (antimicrobial action, resistance induction, inexistent toxicity for the environment) as shown by several studies. They might consequently be employed as phytoprotective molecules in the battle against pathogens to protect useful crops.
Furthermore, these extracts do not pose a danger to the environment or the farmer and thanks to their low-price can reduce farming expenses, specifically in developing countries.
These findings are described in the recently published article entitled Natural extracts from pepper, wild rue and clove can activate defenses against pathogens in tomato plants, in the journal European Journal of Plant Pathology. This work was led by Ines Malo, Morena De Bastiani, Pablo Arevalo & Giovanni Bernacchia from Universidad Politécnica Salesiana and Università  di Ferrara.