Root architecture development, an agronomic trait that influences crop yield, is regulated by multiple plant hormones. Abscisic acid (ABA) is a stress hormone that responds to multiple stresses, including salt, drought, and cold stress, and modulates various aspects of plant growth and development. In recent years, it has been found that ABA synthesized under mild stress or well-watered conditions can support plant growth and stress resistance by positively regulating root architecture development. In this review, we summarize the molecular, cellular, and organismal basis of ABA homeostasis in the root and how ABA signaling affects root architecture development both as an inhibitor and as an activator. We discuss the implications of these studies and the potential for exploiting the components of ABA signaling in designing crop plants with improved root system development and stress resistance.

Rice grain yield and quality are negatively impacted by high temperature stress. Irrigation water temperature significantly affects rice growth and development, thus influencing yield and quality. The role of cooler irrigation water in counteracting high temperature induced damages in rice grain yield and quality are not explored. Hence, in the present study two rice hybrids, Liangyoupeijiu (LYPJ) and IIyou 602 (IIY602) were exposed to heat stress and irrigated with water having different temperatures in a split-split plot experimental design. The stress was imposed starting from heading until maturity under field-based heat tents, over two consecutive years. The maximum day temperature inside the heat tents was set at 38 ℃. For the irrigation treatments, two different water sources were used including belowground water with cooler water temperature and pond water with relatively higher water temperature. Daytime mean temperatures in the heat tents were increased by 1.2–2.0 ℃ across two years, while night-time temperature remained similar at both within and outside the heat tents. Cooler belowground water irrigation did have little effect on air temperature at the canopy level but decreased soil temperature (0.2–1.4 ℃) especially under control. Heat stress significantly reduced grain yield (33% to 43%), panicles m⁻² (9% to 10%), spikelets m⁻² (15% to 22%), grain-filling percentage (13% to 26%) and 1000-grain weight (3% to 5%). Heat stress significantly increased chalkiness and protein content and decreased grain length and amylose content. Grain yield was negatively related to air temperature at the canopy level and soil temperature. Whereas grain quality parameters like chalkiness recorded a significantly positive association with both air and soil temperatures. Irrigating with cooler belowground water reduced the negative effect of heat stress on grain yield by 8.8% in LYPJ, while the same effect was not seen in IIY602, indicating cultivar differences in their response to irrigation water temperature. Our findings reveal that irrigating with cooler belowground water would not significantly mitigate yield loss or improve grain quality under realistic field condition. The outcome of this study adds to the scientific knowledge in understanding the interaction between heat stress and irrigation as a mitigation tool. Irrigation water temperature regulation at the rhizosphere was unable to counteract heat stress damages in rice and hence a more integrated management and genetic options at canopy levels should be explored in the future.

The grain filling of inferior spikelets is much less complete than that of superior spikelets in rice cultivars with large panicles and numerous spikelets and is promoted by moderate soil drying (MD) post-anthesis. A growing body of evidence has shown that microRNAs function in regulating grain development. However, little is known about the mechanism of microRNA control of grain filling of inferior spikelets in response to MD. In this study, grain filling of inferior spikelets was promoted by MD treatment in Nipponbare. Small-RNA profiling at the most active grain-filling stage was conducted in inferior spikelets under control (CK) and MD treatment. Of 521 known and 128 novel miRNAs, 38 known and 9 novel miRNAs were differentially expressed between the CK and MD treatments. Target genes of differentially expressed miRNAs were involved in multiple developmental and signaling pathways associated with catalytic activity, carbohydrate metabolism, and other functions. Both miR1861 and miR397 were upregulated by MD, leading to a decrease in OsSBDCP1 and OsLAC, two negative regulators of SSIIIa activity and BR signaling, respectively. In contrast, miR1432 abundance was reduced by MD, resulting in upregulation of OsACOT and thus an elevated content of both ABA and IAA. These results suggest that both starch synthesis and phytohormone biosynthesis are regulated by differentially expressed miRNAs in inferior spikelets in response to MD treatment. Our results suggest the molecular mechanisms by which miRNAs regulate grain filling in inferior spikelets of rice under moderate soil drying, providing potential application in agriculture to increase rice yields by genetic approaches.

Coleorhiza hairs are hairlike structures in seeds of the grass family (Poaceae). The molecular mechanisms underlying its formation are largely unknown, study on this topic will expand our understanding of the effects of water status on germination during rice (Oryza sativa L.) direct seeding. Seeds of Nipponbare were treated under two water conditions: in one, half of the seed surface was immersed in water and the other half was embryo side in air (EIA), and in the other, the whole seed was covered by water (CBW). Coleorhiza hairs formed only in EIA samples. Transcriptomics was used to identify the gene regulation during coleorhiza hair formation in EIA (vs. CBW) embryos and endosperm. Embryos displayed more transcriptome modulation even though smaller in size than the endosperm. Differentially expressed genes (DEGs) were enriched in both primary and secondary metabolism and showed changes in abscisic acid, auxin, jasmonic acid, and salicylic acid signatures. Metabolites enrichment data were positively correlated with gene expression changes in the affected metabolic functional pathways. The presence of shorter coleorhiza hairs in an OsRHL1 (Os06g0184000, a coleorhiza hair formation regulation candidate gene) knockout mutant suggested that root hair-associated DEGs share molecular regulators that control the formation of coleorhiza hairs.

Alternate wetting and drying (AWD) irrigation has been widely used as an efficient rice production method to obtain better yield without continuous flooding (CF) of the paddy field. However, how this practice affects gene expression to regulate rice physiology and morphology is largely unknown. In this study, we used two rice varieties, Nipponbare, a lowland rice cultivar, and Gaoshan 1, an upland cultivar, and found that root dry weight (RDW) and root oxidation activity (ROA) in both cultivars substantially increased in response to AWD. We then analyzed the differences in transcriptome profiles of their roots irrigated in AWD vs. CF conditions. AWD responsive genes are mainly involved in lignin biosynthetic pathway and phytohormone signal transduction pathway and belong mainly to bHLH, bZIP, NAC, WRKY, and HSF transcription factor families. We discussed how these differentially expressed genes may contribute to the morphological adaptations observed in roots exposed to AWD. This analysis also provides useful information to explain the similarities and differences in adaptation to AWD irrigation between the two rice ecotypes.