## 4 The Impacts of Microalgae/Cyanobacteria and Their Extracts on Crops' Production
### Crops' Protection
#### 4.2.1 Protection against Biotic Factors  
Biotic factors that can negatively impact crops' productivities include the presence of organisms, such as insects, nematodes, bacteria and fungi [7, 36, 119]. Protection against these pathogenic organisms is commonly addressed by polysaccharides, which recognise the signalling molecules present in the pathogenic organism cell wall, and induce several defence responses [36]. Common defence responses include the regulation of signalling pathways, gene expression and induction of specific biosynthetic pathways, which typically result in the production secondary metabolites with antioxidant, antimicrobial and fungicidal properties (e.g., phenolic compounds, terpenoids and other compounds) [7, 36, 40, 96, 120]. When applied in agricultural crops, microalgae and cyanobacteria may promote these defence mechanisms, as these microorganisms are a great source of polysaccharides [36, 121]. In fact, induction of plant defence mechanisms in the presence of microalgae and cyanobacteria has already been described in the literature. For exam  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Mode of Action** & **Algae/Algal Extracts** & **Target Crop** & **Observed Improvements** & **Ref. ** \\ \hline \hline \multirow{6}{*}{_Anabauma lava_ and _Calothrix elenkini_} & \multicolumn{3}{c}{Increase in the activity of \(\beta\)-1,3 endoglucanase in shoots} \\  & & & and roots \\  & & & Increase in fungicidal activity \\  & & & Increase in germination rates \\  & & & Increase in shoot and root length \\  & & & Increase in plant dry weight \\ \hline \multirow{6}{*}{_Anabauma tornulas_, _Anabauma lava_, _Anabauma acalleras_} & \multicolumn{3}{c}{Increase in the activity of hydrolytic and defence enzymes} \\  & & & (endoglucanase, peroxidase, polyphenol oxidase and phenylalanine ammonia) lyase & [109] \\  & & & Increase in fresh and dry weight of plant \\  & & & Increase in the activity of hydrolytic and defence enzymes \\  & & & Increase in the activity of hydrodynamic and defence enzymes \\  & & & (peroxidase, polyphenol oxidase and phenylalanine ammonia lyase) \\  & & & Ammonia lyase) \\  & & & Increase in nitrogenase activity and indole acetic acid production \\  & & & Increase in plant growth \\ \hline Aqueous extract from _Cystostius_ & \multicolumn{3}{c}{Tomato} & Antibacterial activity against _Agrobacteriumium tam  
ple, in the study performed by [PERSON], et al. [109], inoculation of spice crops' seeds with the cyanobacteria _Anabaena laxa_ and _Calothrix elenkiii_ significantly improved the activity of the enzyme \(\beta\)-1,3-endoglucanase in shoots and roots, which is responsible for the breakdown of the cell wall components of pathogens. Moreover, the authors reported an increase in the fungicidal activity, plant dry weight and shoot and root length. Similarly, [PERSON], et al. [111] demonstrated an increase in the activity of plant defence enzymes, namely peroxidase, polyphenol oxidase and phenylalanine ammonia lyase, in shoots and roots of rice plants treated with C. _elenkiii_. Increased activity of these enzymes is associated with the up-regulation of metabolic pathways that induce the production of thousands of metabolites, such as phenolic compounds, that have a toxic effect against pathogenic organisms. For example, phenylalanine ammonia lyase results in the up-regulation of the phenylpropanol pathway, which is responsible for the production of phytoalexins, phenolic substances known for their antimicrobial and antioxidant properties [36]. More recently, [PERSON], et al. [40] demonstrated that elicitation of plant defence mechanisms may be a result of algal polysaccharides' activity.