**1. Introduction**

As an endangered species endemic to Yunnan Province, Southwestern China, *Luculia yunnanensis* is an evergreen shrub or small tree that belongs to the genus *Luculia*, of the family Rubiaceae. This species mainly grows on limestone mountains, secondary shrubby woodland, and open slopes at altitudes of between 1200 and 3200 m. It possesses high ornamental value due to its attractive plant shape, long flowering period, striking pink flowers, sweet fragrance, and the fact that it is evergreen. There is thus great value in its development and utilization.

**Citation:** Zhang, Y.; Liu, X.; Li, Y.; Liu, X.; Ma, H.; Qu, S.; Li, Z. Basic Characteristics of Flower Transcriptome Data and Derived Novel EST-SSR Markers of *Luculia yunnanensis*, an Endangered Species Endemic to Yunnan, Southwestern China. *Plants* **2022**, *11*, 1204. https:// doi.org/10.3390/plants11091204

Academic Editor: Hye Ryun Woo

Received: 25 March 2022 Accepted: 28 April 2022 Published: 29 April 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/).

Our previous field investigations of *L. yunnanensis* and its recorded distribution showed an obvious shrinking trend in its distribution range, which currently only comprises Lushui, Fugong, and Gongshan counties in Nujiang Prefecture, Yunnan Province. The distribution range has been reduced from seven counties in five prefectures or cities to three counties in one prefecture, and the administrative area of the distribution area is 10,465.48 km2 (only 20% of its historical range). According to the IUCN threat level criteria, *L. yunnanensis* belongs to the vulnerable species category [1]; thus, there is an urgent need for relevant biological conservation research.

The assessment of genetic diversity is one of the most significant parts of the research on the conservation of rare and endangered plants [2]. Considered to be an important component that could be seen as a determination of the viability of a species, it may affect species' adaptive capacity and evolutionary potential, and is often regarded as an indirect warning of the potential extinction of endangered species [3,4]. A decrease in genetic diversity in a species may lead to the degradation of its adaptability to environmental changes, leading to a decline in its evolutionary ability [5,6]. Thus, only by clarifying the genetic diversity of threatened species and taking various corresponding factors into consideration can we propose feasible strategies regarding their protection and utilization.

DNA molecular markers are usually used to estimate the genetic diversity of plants, and a sufficient number of polymorphic primers is the prerequisite for molecular genetic diversity analysis. As one of the most commonly used molecular marker techniques, EST-SSR has the advantages of simple operation, good stability, high accuracy, rich variability, good repeatability, and interspecific transmissibility [7–10]. Compared with genomic SSR markers, EST-SSR markers have low development cost and can be applied to species without reference genomes, and the application of EST-SSR markers has greatly promoted the use of molecular marker techniques in plant research [10–12], as well as being widely used in fields such as plant genetics and breeding, conservation, and the development of germplasm [13–16]. To date, the development of EST-SSR primers for *L. yunnanensis* has not been reported. Zhou [17] made use of a modified biotin–streptavidin capture method to develop 13 pairs of SSR primers suitable for analyzing the genetic diversity of *Luculia*. These 13 pairs of primers can effectively distinguish between *Luculia pinceana* and *L. yunnanensis*. With the same method, Ma [18] developed 24 pairs of SSR primers, and 11 pairs of them were polymorphic in two populations of *L. yunnanensis*. However, this number of markers is far from meeting the needs of follow-up studies on *L. yunnanensis*. Therefore, in this study we designed a series of EST-SSR primers based on the flower transcriptome data of *L. yunnanensis*, and developed a set of polymorphic EST-SSR primers that can be used for subsequent population genetic diversity analysis and molecular-markerassisted breeding, which are of great significance for formulating resource conservation and utilization strategies for *L. yunnanensis*.
