Research progress on the impacts of low light intensity on rice growth and development

The mechanism of carbon and nitrogen metabolism under low temperature and low light stress after heading in late indica rice

Double-season late indica rice frequently experiences low temperature accompanying with low light stress during the grain filling stage in southern China, which alters the carbon and nitrogen metabolism of the rice grains, thereby impacting both grain yield and quality. However, the physiological mechanism is still unclear. A pot experiment using two late indica rice cultivars (high-quality and common-quality rice cultivars) was conducted under control (CK), low temperature (LT) and low temperature and light (LT + LL) to investigate the grain filling, photosynthetic characteristics, carbon and nitrogen metabolic enzymes and related gene expression. The results indicated that both LT and LT + LL treatments primarily reduced grain weight in the two cultivars compared to CK. This effect was particularly pronounced for LT + LL, which showed a significant difference in both superior and inferior grains of the common-quality rice cultivar. This reduction in grain weight was attributed to an average decrease in the photosynthetic rate of rice leaves by 23.5%, a decrease in Fv/Fm by 6.2%, and a decrease in the original grain filling rate by 10.3%. LT and LT + LL treatments decreased the activities of soluble starch synthase (SSS), granules bound starch synthetase (GBSS) and starch branch enzyme (SBE) in the early grain filling stage, and its related gene expression including OsAGPL2, OsAGPS2b, OsGBSSI and OsSSIIIa, and the influence degree was intensified by LT + LL, compared with LT. For nitrogen metabolism, the activity of glutamic oxalic aminotransferase (GOT) also significantly decreased by LT and LT + LL during grain filling in the cultivars, which was relatively lower under LT + LL, and only decreased the glutamic pyruvic transaminase (GPT) in the early grain filling stage but increased in the later period. However, the related gene expressions of OsGOT1B and OsGS1;3 were enhanced significantly by LT and LT + LL treatments, and reached the highest level under LT + LL treatment, implying the disordered process of nitrogen metabolism. The results suggest that low temperature decreased photosynthetic traits to hinder grain filling in the cultivars, mainly derived from the worsened carbon and nitrogen metabolism, and LT + LL combined stress aggravated the damage degree.

Physiological and Transcriptome Responses of Sweet Potato [Ipomoea batatas (L.) Lam] to Weak-Light Stress

In the relay intercropping system of maize/sweet potato, the growth of the sweet potatoes is seriously limited by weak light stress in the early stage due to shade from maize plants. However, it is not clear how the weak light affects sweet potatoes and causes tuberous root loss. By setting two light intensity levels (weak light = 30% transmittance of normal light), this study evaluated the responses of two sweet potato cultivars with different tolerances to weak light in a field-based experiment and examined the divergence of gene expression related to light and photosynthesis in a pot-based experiment. The results showed that under weak light, the anatomic structure of functional leaves changed, and the leaf thickness decreased by 39.98% and 17.32% for Yuhongxinshu-4 and Wanshu-7, respectively. The ratio of S/R increased, and root length, root superficial area, and root volume all decreased. The photosynthetic enzyme rubisco was weakened, and the net photosynthetic rate (Pn) declined as well. The level of gene expression in Wanshu-7 was higher than that of Yuhongxinshu-4. The KEGG analysis showed that differentially expressed genes from the two cultivars under weak-light stress used the same enrichment pathway, mainly via glutathione metabolism and flavonoid biosynthesis. After full light levels were restored, the differentially expressed genes were all enriched in pathways such as photosynthesis, photosynthetic pigment synthesis, and carbon metabolism. These findings indicated that weak light changed the plant morphology, photosynthetic physiology and gene expression levels of sweet potatoes, which eventually caused losses in the tuberous root yield. The more light-sensitive cultivar (Wanshu-7) had stronger reactions to weak light. This study provides a theoretical basis and strategy for breeding low-light-tolerant varieties and improving relay intercropping production in sweet potatoes.

Lanthanum Supplementation Alleviates Tomato Root Growth Suppression under Low Light Stress

Supplementation with rare earth elements (REEs) such as lanthanum and cerium has been shown to promote plant elongation and/or increase crop yields. On the other hand, there are reports that REE supplementation of plants has no such effect. The appropriate modes for REE utilization and the underlying mechanism are not fully understood. In this study, we investigated how REE supplementation of plants under low light stress affects plant growth and gene expression. Under low light stress conditions, tomato root elongation was observed to be reduced by about half. This suppression of root elongation was found to be considerably alleviated by 20 mM lanthanum ion supplementation. This effect was plant-species-dependent and nutrient-condition-dependent. Under low light stress, the expression of the genes for phytochrome-interacting factor, which induces auxin synthesis, and several auxin-synthesis-related proteins were markedly upregulated by lanthanum ion supplementation. Thus, we speculate that REE supplementation of plants results in auxin-induced cell elongation and alleviates growth suppression under stress conditions.

Comparative transcriptome profiling of low light tolerant and sensitive rice varieties induced by low light stress at active tillering stage

Low light intensity is a great limitation for grain yield and quality in rice. However, yield is not significantly reduced in low light tolerant rice varieties. The work therefore planned for comparative transcriptome profiling under low light stress to decipher the genes involved and molecular mechanism of low light tolerance in rice. At active tillering stage, 50% low light exposure for 1 day, 3 days and 5 days were given to Swarnaprabha (low light tolerant) and IR8 (low light sensitive) rice varieties. Illumina (HiSeq) platform was used for transcriptome sequencing. A total of 6,652 and 12,042 genes were differentially expressed due to low light intensity in Swarnaprabha and IR8, respectively as compared to control. CAB, LRP, SBPase, MT15, TF PCL1 and Photosystem I & II complex related gene expressions were mostly increased in Swarnaprabha upon longer duration of low light exposure which was not found in IR8 as compared to control. Their expressions were validated by qRT-PCR. Overall study suggested that the maintenance of grain yield in the tolerant variety under low light might be results of accelerated expression of the genes which enable the plant to keep the photosynthetic processes moving at the same pace even under low light.

Response of Potato Tuber Number and Spatial Distribution to Plant Density in Different Growing Seasons in Southwest China

The aim of this study was to explore the effects of different density treatments on potato spatial distribution and yield in spring and fall. Plant density influenced yield and composition, horizontal, and vertical distribution distances between potato tubers, and spatial distribution position of tuber weights. The results indicated that: (1) Spring potato yield had a convex quadratic curve relationship with density, and the highest value was observed at 15.75 × 10(4) tubers per hectare. However, the yield of fall potatoes showed a linear relationship with plant density, and the highest value was observed at 18 × 10(4) tubers per hectare; (2) Density had a greater influence on the tuber weight of spring potatoes and fruit number of single fall potatoes; (3) The number of potato tubers in the longitudinal concentration exhibited a negative linear relationship with density, whereas the average vertical distribution distance of tubers exhibited a positive incremental hyperbolic relationship. For spring and fall potato tubers, the maximum distances were 8.4152 and 6.3316 cm, and the minimum distances 8.7666 and 6.9366 cm, respectively; and (4) Based on the artificial neural network model of the spatial distribution of tuber weight, density mainly affected the number and spatial distribution of tubers over 80 g. Tubers over 80 g were mainly distributed longitudinally (6-10 cm) and transversely (12-20 cm) within the high density treatment, and the transverse distribution scope and number of tubers over 80 g were reduced significantly. Spring potato tubers over 80 g grown at the lowest density were mainly distributed between 12 and 20 cm, whereas those at the highest density were primarily distributed between 10 and 15 cm.