**1. Introduction**

Phenylalanine ammonia-lyase (PAL) produces precursors of various secondary metabolites, including lignin, phytoalexin, and phenolic compounds. This gene family is also associated with the production of the first enzyme of the phenylpropanoid pathway [1–3]. *PAL* genes have a molecular mass in the range of 270–330 kilodalton (kDa) and are present in higher plants, yeast, some bacteria, and fungi. However, these genes are not found in animals because they have another histidine ammonia lyase (HAL) [4]. The PAL family encodes a variety of protective compounds such as components of the cell wall, flavonoids, phytoalexins, and furanocoumarin [5,6]. The conversion of L-phenylalanine to cinnamic acid, linking primary metabolism with secondary metabolism catalyzed by the PAL enzymes, also plays an essential role in phenylpropanol biosynthesis, a speed-limiting step in

**Citation:** Rasool, F.; Uzair, M.; Naeem, M.K.; Rehman, N.; Afroz, A.; Shah, H.; Khan, M.R. Phenylalanine Ammonia-Lyase (PAL) Genes Family in Wheat (*Triticum aestivum* L.): Genome-Wide Characterization and Expression Profiling. *Agronomy* **2021**, *11*, 2511. https://doi.org/10.3390/ agronomy11122511

Academic Editors: Channapatna S. Prakash, Ali Raza, Xiling Zou and Daojie Wang

Received: 18 October 2021 Accepted: 20 November 2021 Published: 10 December 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

phenylpropanol metabolism [1].This metabolic pathway is involved in the production of various natural products (phytoalexin, hydroxycinnamic acids, flavonoids, etc.), and is also reported as a role player in phenolic glycoside and benzene compound synthesis, which are part of several enzyme-regulated reactions [1,2,7–10]. Thus, phenylpropanoids play a critical role for the growth, development, and survival of vascular plants [1]. *PAL* activity is induced dramatically in reply to various stimuli such as tissue wounding, pathogenic attack, light, low temperature, and hormonal triggers [5,11].

The first plant PAL was found in *Petroselinum crispum* in crystal forms [12]. The *PAL* encoding genes are typically discovered as small gene families comprising one to five members [13,14]. During the evolution of higher plants, PAL diversified into different functions. Both HAL and PAL have different primary protein sequences, but they perform similar functions *in vivo*. It was thought that PAL is formed from HAL when the fungi and plants separated from other kingdoms [15,16]. There are two (the first is horizontal gene transfer (HGT) and the second is gene duplication) methods of evolution are reported. Studies showed that gene duplication is the major method of evolution and gymnosperms are thought to be the ancestors of angiosperms [16,17]. For instance, four *PAL* gene family members in *Arabidopsis thaliana* [18,19], five in *Populus trichocarpa* [20], three in *Scutellaria baicalensis* [21], and three in *Coffeac anephora* [22] have been recognized and functionally described. Nevertheless, some studies have indicated more than five *PAL* genes in certain plants. Moreover, five separate *PAL* genes were recognized in *Pinus taeda* [23]. Furthermore, as many as thirteen *PAL* genes were discovered in *Cucumis sativus* [24], twelve in *Citrullus lanatus* [24], thirteen in *Cucumis melo,* and sixteens in *Vitis vinifera* [25].

Wheat (*Triticum aestivum*) is an important source of starch, protein, and minerals in the diet for more than 35% of the world's inhabitants. It is grown on a variety of soil and in a range of environmental conditions [26]. To prevent environmental stresses, the wheat plant has evolved multiple plant protection systems [27]. Previous studies showed the involvement of the *PAL* gene family in coping with the environmental stresses by activating the transcriptional processes. The *PAL* gene family is responsible for the adaptation and resistance of plants to unfavorable biotic and abiotic environmental conditions. It also controls the expression and inhibition of genes to amend different biochemical pathways. Our research explored, a theoretical way. the functional characterization, and differential expression analysis of the *PAL* gene family engaged in the root development of six different wheat genotypes. This study carries immense importance in understanding the stress tolerance mechanisms in wheat and the role of the *PAL* gene family in the same.
