Calcium-dependent protein kinases (CPKs/CDPKs) are Ca²⁺-sensors that decode Ca²⁺ signals into specific physiological responses. Research has reported that CDPKs constitute a large multigene family in various plant species, and play diverse roles in plant growth, development, and stress responses. Although numerous CDPKs have been exhaustively studied, and many of them have been found to be involved in plant hormone biosynthesis and response mechanisms, a comprehensive overview of the manner in which CDPKs participate in phytohormone signaling pathways, regulating nearly all aspects of plant growth, has not yet been undertaken. In this article, we reviewed the structure of CDPKs and the mechanism of their subcellular localization. Some CDPKs were elucidated to influence the intracellular localization of their substrates. Since little work has been done on the interaction between CDPKs and cytokinin signaling pathways, or on newly defined phytohormones such as brassinosteroids, strigolactones and salicylic acid, this paper mainly focused on discussing the integral associations between CDPKs and five plant hormones: auxins, gibberellins, ethylene, jasmonates, and abscisic acid. A perspective on future work is provided at the end.

As a sessile organism, rice often faces various kinds of abiotic stresses, such as drought stress. Drought stress seriously harms plant growth and damages crop yield every year. Therefore, it is urgent to elucidate the mechanisms of drought resistance in rice. In this study, we identified a glycine-rich RNA-binding protein, OsGRP3, in rice. Evolutionary analysis showed that it was closely related to OsGR-RBP4, which was involved in various abiotic stresses. The expression of OsGRP3 was shown to be induced by several abiotic stress treatments and phytohormone treatments. Then, the drought tolerance tests of transgenic plants confirmed that OsGRP3 enhanced drought resistance in rice. Meanwhile, the yeast two-hybrid assay, bimolecular luminescence complementation assay and bimolecular fluorescence complementation assay demonstrated that OsGRP3 bound with itself may affect the RNA chaperone function. Subsequently, the RNA-seq analysis, physiological experiments and histochemical staining showed that OsGRP3 influenced the phenylpropanoid biosynthesis pathway and further modulated lignin accumulation. Herein, our findings suggested that OsGRP3 enhanced drought resistance in rice by altering the phenylpropanoid biosynthesis pathway and further increasing lignin accumulation.

Nitrogen (N) is an essential nutrient for plant growth and development. The root system architecture is a highly regulated morphological system, which is sensitive to the availability of nutrients, such as N. Phenotypic characterization of roots from LY9348 (a rice variety with high nitrogen use efficiency (NUE)) treated with 0.725 mM NH₄NO₃ (1/4N) was remarkable, especially primary root (PR) elongation, which was the highest. A comprehensive analysis was performed for transcriptome and proteome profiling of LY9348 roots between 1/4N and 2.9 mM NH₄NO₃ (1N) treatments. The results indicated 3908 differential expression genes (DEGs; 2569 upregulated and 1339 downregulated) and 411 differential abundance proteins (DAPs; 192 upregulated and 219 downregulated). Among all DAPs in the proteome, glutamine synthetase (GS2), a chloroplastic ammonium assimilation protein, was the most upregulated protein identified. The unexpected concentration of GS2 from the shoot to the root in the 1/4N treatment indicated that the presence of an alternative pathway of N assimilation regulated by GS2 in LY9348 corresponded to the low N signal, which was supported by GS enzyme activity and glutamine/glutamate (Gln/Glu) contents analysis. In addition, N transporters (NRT2.1, NRT2.2, NRT2.3, NRT2.4, NAR2.1, AMT1.3, AMT1.2, and putative AMT3.3) and N assimilators (NR2, GS1;1, GS1;2, GS1;3, NADH-GOGAT2, and AS2) were significantly induced during the long-term N-deficiency response at the transcription level (14 days). Moreover, the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis demonstrated that phenylpropanoid biosynthesis and glutathione metabolism were significantly modulated by N deficiency. Notably, many transcription factors and plant hormones were found to participate in root morphological adaptation. In conclusion, our study provides valuable information to further understand the response of rice roots to N-deficiency stress.

The cytoplasmic male sterility (CMS) and nuclear-controlled fertility restoration system is a favorable tool for the utilization of heterosis in plant hybrid breeding. Many restorer-of-fertility (Rf) genes have been characterized in various species over the decades, but more detailed work is needed to investigate the fertility restoration mechanism. Here, we identified an alpha subunit of mitochondrial processing peptidase (MPPA) that is involved in the fertility restoration process in Honglian-CMS rice. MPPA is a mitochondrial localized protein and interacted with the RF6 protein encoded by the Rf6. MPPA indirectly interacted with hexokinase 6, namely another partner of RF6, to form a protein complex with the same molecular weight as the mitochondrial F₁F₀-ATP synthase in processing the CMS transcript. Loss-of-function of MPPA resulted in a defect in pollen fertility, the mppa^+/− heterozygotes showed semi-sterility phenotype and the accumulation of CMS-associated protein ORFH79, showing restrained processing of the CMS-associated atp6-OrfH79 in the mutant plant. Taken together, these results threw new light on the process of fertility restoration by investigating the RF6 fertility restoration complex. They also reveal the connections between signal peptide cleavage and the fertility restoration process in Honglian-CMS rice.

As a special class of rice germplasm, wide-compatibility varieties (WCVs) guarantee the fertility of hybrids when there is cross-fertilization between two subspecies. In this study, Chenghui9348 was identified as a new member of the WCV family that improves pollen fertility in an inter-subspecific hybrid. Cytological analysis showed that the abnormal mitosis of microspores resulted in the sterility of pollens at the early bicellular stage in the inter-subspecific hybrid. Furthermore, the new F12 locus, corresponding to improvements in fertility of the indica-japonica hybrid, was found to co-segregate with the RM1047 marker and associated with a region of approximately 630 kb flanked by the D1101 and D1164 markers on chromosome 12. In this region, two putative genes were predicted as the candidates for wide-compatibility genes (WCGs). Sequence analysis revealed that, compared with indica/japonica alleles, deletion/insertion occurred within exons of both putative genes. Together, the present study identified another new WC locus, F12, and offers more opportunities for further exploitation of inter-subspecific hybrids in rice.