Mitogen-activated protein kinase (MAPK/MPK) cascades are pivotal and highly conserved signaling modules widely distributed in eukaryotes; they play essential roles in plant growth and development, as well as biotic and abiotic stress responses. With the development of sequencing technology, the complete genome assembly of rice without gaps, T2T (Telomere-to-Telomere)—NIP (version AGIS-1.0), has recently been released. In this study, we used bioinformatic approaches to identify and analyze the rice MPK kinases (MKKs) based on the complete genome. A total of seven OsMKKs were identified, and their physical and chemical properties, chromosome localization, gene structure, subcellular localization, phylogeny, family evolution, and cis-acting elements were evaluated. OsMKKs can be divided into four subgroups based on phylogenetic relationships, and the family members located in the same evolutionary branch have relatively similar gene structures and conserved domains. Quantitative real-time PCR (qRT-PCR) revealed that all OsMKKs were highly expressed in rice seedling leaves. The expression levels of all OsMKKs were more or less altered under exogenous hormone and abiotic stress treatments, with OsMKK1, OsMKK6, and OsMKK3 being induced under almost all treatments, while the expression of OsMKK4 and OsMKK10-2 was repressed under salt and drought treatments and IAA treatment, respectively. In this study, we also summarized the recent progress in rice MPK cascades, highlighted their diverse functions, and outlined the potential MPK signaling network, facilitating further studies on OsMKK genes and rice MPK cascades.

Leaf angle (LA) is a major agronomic trait of rice plant architecture, which is determined by the development of the leaf lamina joint (LJ) and closely related to plant yield. The LJ is formed by the leaf collar belt (ligule band), leaf tongue (ligule) and leaf ear (auricle). Parenchyma cells (PCs), sclerenchyma cells (SCs), vascular bundles (VBs), and arenchyma tissue (Ac) are present in the cross sections of LJ. The genetic and molecular regulation of rice leaf angle and LJ development has been well investigated in recent years. However, the underlying complex regulatory network still requires further elucidation and comprehensive discussion. In this review, we briefly describe the cellular characteristics of four typical stages of LJ development, and systematically summarize the genes regulating rice leaf inclination based on their roles in PC proliferation and elongation, as well as SC proliferation and differentiation. We also introduce the emerging regulatory pathways of phytohormones and transcription factors (TFs), environmental cues that are involved in rice LJ development, discussing the relevant intricate signal network that is relevant to provide further insights into the genetic improvement of leaf angle traits in rice breeding.