**1. Introduction**

Weeping is a result of drooping branch development. The buds first develop upward, then as the tree grows, the apex of the branches shifts to growing downward. The reasons for the formation of the drooping branches are relatively convoluted, nevertheless. The development of drooping branches may be influenced by genetic make-up, hormones, and secondary growth, but it may also be intimately linked to certain external conditions, such as gravity, light, and supporting forces [1]. The weep gene mutation that led to the gravity anomaly in *Prunus persica* is what causes the weeping characteristic [2]. The weeping characteristics of *Salix babylonica* were brought on by a lack of mechanical support, brought on by an excessive trunk elongation [3]. For the weeping of *P. mume*, the secondary cell walls of branches in the pendant extension zone are thinner than those of upright branches [4]. To modulate the weeping phenotype, IAA and GA3 biosyntheticpathway-related genes were discovered in *P. mume* [5,6]. Recently, Zheng et al. found that genes related to cell division, cell development, and plant hormones played an important role in the tortuous-branch phenotype of *P. mume* [7]. Overexpression of a secondarywall-associated cellulose synthase gene (*PtdCesA8*) was shown to result in a weeping phenotype in *Populus tremuloides* [8]. Ornamental plants often attract attention because

**Citation:** Gu, C.; Shang, L.; Zhang, G.; Wang, Q.; Ma, Q.; Hong, S.; Zhao, Y.; Yang, L. Identification and Expression Analysis of NAC Gene Family in Weeping Trait of *Lagerstroemia indica*. *Plants* **2022**, *11*, 2168. https://doi.org/10.3390/ plants11162168

Academic Editors: Aiping Song and Yu Chen

Received: 31 July 2022 Accepted: 15 August 2022 Published: 21 August 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 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/).

of their specific traits. A certain plant architecture, known as weeping, has significant aesthetic appeal. Therefore, exploring the causes of plant drooping is potentially valuable for cultivating ornamental plants.

The ability to synthesize proteins via biological activities is controlled by transcription factors and some cis-acting elements, which also govern the spatiotemporal expression of downstream genes and eventually have an impact on the growth and development of organisms [9]. The genes NAM (no apical meristematic tissue) from *Petunia hybrida* [10], *ATAF1/2* (*Arabidopsis* transcription activation factor) found in *Arabidopsis thaliana* [11], and *CUC2* (cup2 shaped cotyledon) from *A. thaliana* are the sources of the name *NAC* [11]. At the N-terminal end of the protein, 150 amino acids make up its conserved structural domain, including five substructural components (A, B, C, D, and E) [12]. According to multiple studies, *NAC* has a wide spectrum of regulatory effects on plant growth and development [13–17]. Therefore, it is potentially possible to probe the weeping properties of plants from *NAC*.

Many studies show that the *NAC* gene family affects the weeping of plants by controlling the synthesis of secondary cell walls [15,18]. Inhibiting the expression of *SND1* and *NST1* in *Arabidopsis* at the same time can limit the synthetic gene expression in the three components of secondary cell walls (cellulose, xylem, and lignin), causing the flower stem cells to droop and fail to create secondary walls [15]. Studies on *SND4* and *SND5* revealed similar functions [19]. While the secondary cell wall of the stem of the overexpressed *PtrWND1B-s* plants greatly thickened and allowed them to grow upright, the secondary cell wall of the overexpressed *PtrWND1B-l* plants was unable to do so and displayed a phenotype similar to that of *PtrWND1B-RNAi* [18]. *NAC15* was significantly expressed in the poplar xylem and is most likely the primary factor for transgenic tobacco to develop tall stems [20]. Lignin is an important component in secondary cell walls and *EjNAC1* is associated with fruit lignification by activating genes involved in lignin biosynthesis [21]. The main regulatory mechanism in melatonin in enhancing flower stem strength was also to promote lignin accumulation and changed S/G lignin ratio [22]. Co-expression analysis of *Pm024213* showed that most of the related genes were involved in auxin and lignin biosynthesis [23].

Because of its lovely blossoms and exquisite branches, the crape myrtle (*Lagerstroemia indica* L.), a deciduous shrub and small tree, has considerable ornamental value [24]. *L. indica* branches were classified into flat, upright, and weeping branches by Tian [25]. The study of the molecular mechanisms of *L. indica's* weeping trait will benefit from an understanding of the *NAC* gene family function. However, nothing is known about the *NAC* gene studies in *L. indica*. In this study, the *NAC* genes in *L. indica* were discovered using a variety of bioinformatics techniques. Additionally, the physiological and biochemical properties of the LiNAC proteins were examined and a phylogenetic tree of the LiNAC proteins in *L. indica* and *A. thaliana* was created. To clarify whether *LiNAC* genes are involved in regulating the weeping trait of *L. indica*, we analyzed the relative expression levels of branches at different growth stages. Overall, the findings might support a theoretical hypothesis of *NAC* gene family in *L. indica*.
