Major QTLs, qARO1 and qARO9, Additively Regulate Adaxial Leaf Rolling in Rice

Moderate leaf rolling is considered optimal for the ideal plant type in rice ($\textit{Oryza sativa}$ L.), as it improves photosynthetic efficiency and, consequently, grain yield. Determining the genetic basis of leaf rolling $\textit{via}$ the identification of quantitative trait loci (QTLs) could facilitate the development of high-yielding varieties. In this study, we identified three stable rice QTLs, $\textit{qARO1}$, $\textit{qARO5}$, and $\textit{qARO9}$, which control adaxial leaf rolling in a recombinant inbred line (RIL) population derived from a cross between Tong 88-7 (T887) and Milyang 23 (M23), using high-density SNP markers. These QTLs controlled the rolling phenotype of both the flag leaf (FL) and secondary leaf (SL), and different allelic combinations of these QTLs led to a wide variation in the degree of leaf rolling. Additive gene actions of $\textit{qARO1}$ and $\textit{qARO9}$ on leaf rolling were observed in a backcross population. In addition, $\textit{qARO1}$ (markers: 01id4854718 and 01asp4916781) and $\textit{qARO9}$ (markers: 09id19650402 and 09id19740436) were successfully fine-mapped to approximately 60- and 90-kb intervals on chromosomes 1 and 9, respectively. Histological analysis of near-isogenic lines (NILs) revealed that $\textit{qARO1}$ influences leaf thickness across the small vein, and $\textit{qARO9}$ affects leaf thickness in the entire leaf and bulliform cell area, thus leading to adaxial leaf rolling. The results of this study advance our understanding of the genetic and molecular bases of adaxial leaf rolling, and this information can be used for the development of rice varieties with the ideal plant type.