**2.3 What is the role of lignin in plants?**

Lignin deposition in plant cells: (i) provides rigidity and mechanical strength, allowing the plants to stand; (ii) gives hydrophobicity to the cell walls; (iii) promotes the transportation of water and minerals between cells, (iv) hinders the degradation of polysaccharides; and (v) is a barrier against pathogens and pests, playing an essential role in plant development [19–21].

Lignin has another active role in the plant response to several abiotic and biotic stresses, by reinforcing the cell walls in the neighborhood of infected or wounded tissue (the so-called *defense lignin*, which contrasts with the *developmental lignin* presented before) [22–24]. Plants have suffered different types of abiotic (*e.g.,* drought, mineral deficiencies, and low or high temperatures) and biotic stresses (*e.g.,* insects, bacteria, or fungi attacks) over the years. Lignin is vital for plant growth and its environmental adaptability, that is, the plant can produce different amounts and types of lignins (possessing other physical properties) depending on the growth conditions. Moura and coworkers [25] reported a review explaining the influence of different stresses, such as the increase of cold, drought, or light upon the plants; for example, maize under water deficit reduced the production of ferulic acid but increased *p*-coumaric and caffeic acids in the xylem regions. However, the roots increased their lignin content, stiffening the cell walls and reducing their expansion [25]. More recently, a review was presented focused on the biosynthesis, content, and accumulation of lignin as a plant response to abiotic stresses; during drought, *E. urograndis* increased the lignin content in its roots and leaves and reduced the S/G ratio [26]. In *P. trichocarpa,* the lignin content did not change in young shoots or mature stems, but the S/G ratio decreased significantly in the young shoots. Plants under drought stress may increase the lignin content to reduce water penetration and transpiration from the cell wall, which helps to maintain cell osmotic balance [21].

Plants under a rise in light induced a lignin accumulation, as a way to adapt to the environment [26]. Also, the lignin composition in angiosperm and gymnosperms species under stress revealed a higher amount of condensed linkages (CdC bonds) and more H-units as a response to the environmental conditions, including high nitrogen fertilization, mechanical injuries, or ozone [2, 23, 24].

Therefore, researchers have tried to understand plant adaptability, but it is difficult to generalize the behavior of plants regarding particular stress because each species behaves differently. Also, not only the lignin content or its composition is essential to understand plant behavior, since the production of lignin comprises a complex genetic network with the involvement of multiple enzymes that have different responses to the abiotic and biotic factors [24]. This will not be developed here, but readers are encouraged to consult many of the works on this topic [27–30] and a review about lignin biosynthesis [31].

The diversity of lignins (content and composition) and their recalcitrant behavior to degradation is a challenge for researchers who wish to learn more about the lignin structure and biosynthesis, but mainly for those who work in lignin valorization. Thereby, we discuss below some procedures to isolate the lignin.
