**1. Introduction**

*O. longistaminata* (*OL*) is a perennial wild rice with well-developed rhizomes. Rhizome is the key organ for vegetative reproduction and perenniality of *OL* [1–5]. The rhizome of *OL* is a horizontally growing underground stem that can grow out of the soil and become a new plant (ramet). Axillary buds on the nodes of the rhizome can produce secondary rhizomes or new ramets (new plants genetically identical to the parent). In addition, the rhizome is also an important storage organ of water and nutrients, such as starch [6], which is of great significance for *OL* to survive through harsh environments.

The development of rhizome in *OL* is a complex trait, maybe controlled by single, dual or multiple genes. A single dominant allele *Rhz* on chromosome 4 was firstly proposed for the rhizomatous growth habit in *OL*, and modifying genes have also been suggested to be involved for the various *Rhz* phenotypes among F<sup>2</sup> plants [7]. A pair of dominant complementary genes, *Rhz2* and *Rhz3,* have subsequently been identified for the regulation of rhizomes in *OL* [4]. However, only *Rhz2* and *Rhz3* did not guarantee the presence of rhizomes in the hybrid progeny between cultivated rice and *OL* (Hu, 2015, presentation entitled "Progress in Perennial Rice Breeding and Genetics", accessible from http://pwheat. anr.msu.edu/2015/02/ (accessed on 25 September 2022)). Thirteen QTLs (quantitative trait

**Citation:** Fan, Z.; Huang, G.; Fan, Y.; Yang, J. Sucrose Facilitates Rhizome Development of Perennial Rice (*Oryza longistaminata*). *Int. J. Mol. Sci.* **2022**, *23*, 13396. https://doi.org/ 10.3390/ijms232113396

Academic Editor: Jianhua Zhu

Received: 27 September 2022 Accepted: 27 October 2022 Published: 2 November 2022

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loci) regulating rhizome development have recently been mapped by Fan, et al. [8]. Further study from Hu's group identified 13 major-effect loci that jointly control rhizomatousness in *OL*, and as many as 51 QTLs for the rhizome abundance [9]. On the whole, the development of rhizome is regulated by multiple genes [8–10].

In addition to the genetic factor, environmental factors, such as sugars, also play an important role in rhizome development [6,11]. Sucrose can promote secondary rhizome development from the axillary buds of isolated rhizomes [6,11]. The rhizome usually grows horizontally below the ground for a certain distance, and then grows upward due to the negative gravitropism, and finally grows out of the soil and becomes a new ramet. It has been found that sucrose can retard the upward growth of rhizomes [6,11], causing speculation that sucrose is the key stimulus inducing the development and transformation of rhizomes [6]. However, the molecular mechanism by which sucrose affects rhizome development in *OL* remains unclear. In the previous studies, the isolated rhizomes were all cultured with sucrose in vitro [6,11], and the valid experiment about the effect of sucrose in rhizome development should be carried out in vivo. In this study, *OL* seedling was cultured in solid medium with different concentrations of sucrose to observe the effects of sucrose on the formation and development of rhizomes. In addition, the regulatory network of rhizome development in *OL* was probed by comparative transcriptome analysis.

#### **2. Results**
