Il-Ryong Choi

Water and phosphorus (P) fertilizer are two of the most critical inputs in rice cultivation. Irrigation and chemical fertilizers are becoming limiting factors under climate change and urbanization, which is leading to significant losses in yield. The Pup1 quantitative trait locus (QTL) confers tolerance to P starvation through enhanced early-stage root vigor and P uptake in indica rice grown in the tropics. Whether the QTL works in temperate rice genetic backgrounds grown in temperate regions remains to be elucidated. To address this question, we introgressed the Pup1 QTL into three temperate rice varieties—MS11, TR22183, and Dasanbyeo—using marker-assisted backcrossing and high-density genotyping. The selected lines all harbored the full Pup1 QTL with recurrent parent genome recovery rates ranging from 66.5% to 99.8%. Under the rainfed and P non-supplied conditions, Pup1 introgression lines did not show clear advantages over the recurrent parents in terms of vegetative growth and grain yield per plant, but exhibited enhanced yield responses to P application, except in Dasanbyeo, which a temperate rice that is genetically similar to indica. Our results suggest that Pup1 confers enhanced P uptake in temperate rice and that the efficacy of Pup1 might depend on the subspecific genomic background of the rice, whether it is japonica or indica.

Temperate japonica rice is mainly cultivated in temperate regions. Many temperate japonica varieties have a superior grain quality that is preferred in Northeast Asian countries such as Japan, Korea, and China. The changes in consumers’ preferences in Southeast Asia and Western countries has contributed to increasing the demand for temperate japonica. Most temperate japonica varieties developed in temperate regions typically exhibit extra-early flowering under the short-day conditions in the tropics, which usually results in severely reduced yields. Since 1992, we have been developing temperate japonica varieties that can adapt to tropical environments to meet the increasing demand for temperate japonica rice, having released six varieties in the Philippines. Especially, the yield of one of the temperate japonica varieties, Japonica 7, was comparable to the yields of leading indica varieties in the Philippines. Here, we discuss the current breeding initiatives and future plans for the development of tropical-region-bred temperate japonica rice.

Rice orange leaf phytoplasma (ROLP) causes clear orange to yellowish leaf discoloration and severe stunting in rice seedlings. The ecological and biological characteristics of ROLP are largely unknown because the disease has not widely caused serious problems in rice cultivated areas, thereby leading to the low accumulation of research data. However, in the past decade, the disease became a threat to rice production, particularly in South China and India; it has also been recognised in other Asian countries, such as Vietnam, Thailand and the Philippines. Here, we observed the occurrence of ROLP in paddies of the Southeast Asian counties (Cambodia, Vietnam and the Philippines) and found that the isolates in the Philippines and Vietnam were monophyletic, while those in India, Thailand and Cambodia were more diverse, suggesting their potential origins. In Cambodia, it was revealed that following polymerase chain reaction (PCR) detection, the known ROLP-insect vectors, N. virescens Distant and Recilia dorsalis Motchulsky, were ROLP-positive, indicating their roles in pathogen dispersal. Moreover, fluorescent and scanning electron microscopy revealed the intensive accumulation of the phytoplasma in phloem tissues and massive accumulation of storage starch in vascular bundle sheath and parenchyma. Altogether, this study illustrated the genetic variability of global ROLP isolates and the pathogen’s biological impact on rice tissue.

Rice orange leaf phytoplasma (ROLP) belongs to the “Candidatus Phytoplasma asteris” 16SrI-B subgroup, which is solely transmitted by the zigzag-striped leafhopper (Recilia dorsalis Motchulsky) and the green leafhopper (Nephotettix cincticeps Uhler) (Hemiptera: Cicadellidae). Recently, rice plants showing orange leaf discoloration have become ubiquitous in several paddies of two provinces in the Philippines. In total of 98 symptomatic rice plants, 82% (Laguna) and 95% (Mindanao) were ROLP-positive by nested PCR detection. These plants showed more varying symptoms than previously reported. The vector insect R. dorsalis was scarcely present but green paddy leafhopper, N. virescens Distant (Hemiptera: Cicadellidae), was commonly observed in the paddies, thus the ability of N. virescens to transmit ROLP was thoroughly investigated. Newly emerged adult N. virescens, which fed on ROLD-source rice plants, were used to inoculate a susceptible rice seedling and was serially transferred into a new healthy seedling. Resultant positive transmission rates varied from 5.1% to 17.8%. The transmission ability of the insects was generally decreased over time. These findings suggest that N. virescens is an alternative vector of ROLP in the Philippines. Altogether, this study highlighted the increasing importance of ROLD-reemergence in Southeast and East Asia and proved the need for careful management of this alternative vector insect.

Editorial: Viruses threatening stable production of cereal crops

Rice tungro disease is a complex disease caused by the interaction between Rice tungro bacilliform virus and Rice tungro spherical virus (RTSV). RTSV alone does not cause recognizable symptoms in most Asian rice (Oryza sativa) plants, whereas some African rice (O. glaberrima) plants were found to become stunted by RTSV. Stunting of rice plants by virus infections usually accompanies the suppression of various cell wall-related genes. The expression of cell wall-related genes was examined in O. glaberrima and O. sativa infected with RTSV to see the relationship between the severity of stunting and the suppression of cell wall-related genes by RTSV. The heights of four accessions of O. glaberrima were found to decline by 14 to 34% at 28 days post-inoculation (dpi) with RTSV, whereas the height reduction of O. sativa plants by RTSV was not significant. RTSV accumulated more in O. glaberrima plants than in O. sativa plants, but the level of RTSV accumulation was not correlated with the degree of height reduction among the four accessions of O. glaberrima. Examination for expression of genes for cellulose synthase A5 (CESA5) and A6 (CESA6), cellulose synthase-like A9 (CSLA9) and C7, and -expansin 1 (expansin 1) and 15 precursors in O. glaberrima and O. sativa plants between 7 and 28 dpi with RTSV showed that the genes such as those for CESA5, CESA6, CSLA9, and expansin 1were more significantly suppressed in stunted plants of O. glaberrima at 14 dpi with RTSV than in O. sativa, suggesting that stunting of O. glaberrima might be associated with these cell wall-related genes suppressed by RTSV. Examination for expression of these genes in O. sativa plants infected with other rice viruses in previous studies indicated that the suppression of the expansin 1 gene is likely to be a signature response commonly associated with virus-induced stunting of Oryza species. These results suggest that stunting of O. glaberrima by RTSV infection might be associated with the suppression o