Physiology and molecular mechanisms involving the regulation of silicon homeostasis in Thai Rice Varieties

Abstract

Silicon (Si) is not an essential element, but it’s beneficial for growth and development of many plant species. Rice is one of the crops that have been shown to respond positively to Si fertilizer in its growth and productivity. Rice yield has been shown to be increased by Si through better vegetative and reproductive growth. The beneficial effects of Si on rice growth are highly influenced by genetic and environmental factors including crop management practices. Of the numerous rice varieties grown in Thailand only limited attention has been paid to investigating the genotype by environment interaction on Si concentration, mechanisms of Si uptake and its effect on the productivity. Understanding how these factors affect Si concentration and its effect on productivity will provide information for manipulating agronomic practices to increase rice productivity and grain quality in the future. Therefore, the objectives of this study were 1) to investigate how variety and environmental factors such as silicon fertilizer application, growing season and the relate macro and micro nutrients application influence on grain yield and Si accumulation in Thai rice varieties and 2) to investigate the factors affecting on the expression of genes involving in Si uptake and transportation in different rice varieties. Genotypic variation of grain yield and Si accumulation were studied among the selected 29 Thailand rice varieties (17 wetland varieties and 12 lowland varieties). All rice varieties were grown in in the pots condition at Chiang Mai University, Thailand during the wet season of 2014, with the KDML 105 as a check variety. Variation of grain yield, straw dry was not distinguishable between the wetland and upland varieties. Grain yield varied among varieties, ranging from 13.9 to 25.0 g plant-1 in the wetland varieties and 14.1 to 22.3 g plant-1 in the upland varieties. The concentration of Si in leaf-sheath, leaves and paddy were significantly differed between the wetland and upland varieties. The Si concentration in leaf-sheath leaves and paddy of the upland varieties were higher than the wetland varieties by 13.8%, 10.1% and 23.0%, respectively. The highest Si concentration was observed in the husk (27.1%) followed by flag leaf (19.4%), leaves (18.6%), leafsheath (16.3%), stem (8.4%), paddy (5.2%) and brown rice (4%), respectively. In addition, there was correlation between grain yield and Si concentration in paddy in the wetland varieties, but not in the upland varieties. In addition to the contribution of rice genetics on grain yield and Si accumulation in different plant parts of rice, the environmental factors also has an impact on grain yield and Si concentration in rice such as Si fertilizer application. The influence of Si fertilizer on grain yield and Si concentration in plant tissues was investigated. The experiment was conducted by growing three rice varieties, SPR1, SMJ and KH CMU in the pot and applying with the two levels of Si at 0 and 150 kg Si/ha in wet season of 2015. Applying Si fertilizer increased grain yield and straw dry weight by 18% and 15% over the Si0 plants with the highest grain yield and straw dry weight was found in SPR1. Applying Si fertilizer had affected on Si concentration in the husk and paddy with variation among the varieties. In SPR1, applying 150 kg Si/ha increased Si concentration in the paddy rice and husk by 59.0% and 54.8%, respectively, but the application had no effect in SMJ and KHC. Applying Si fertilizer significantly increased Si concentration in plant tissues differently by different growth stages. For example, at tillering, the stem of Si150 plants in SPR1 and KHCMU had higher the concentrations by 10.3% and 12.4% respectively, compared with Si0 plants, but it was not affected in SMJ. Applying Si fertilizer also increased leaves Si concentration in SPR1, SMJ and KHCMU varieties by 14.2 to 17.3% at tillering and 13.0 to 36.1% at flowering stage compared to Si0 plants. The effect of Si on spikelet formation, spikelet fertility, grain filling and its impact on grain yield in rice was evaluated in this study. The two modern popular high yielding varieties CNT1 and PTT1 were used in this experiment which were grown in a nutrient solution with and without Si (100 and 0 mg SiO2 L-1) in wet season of 2016. Applying Si increased grain yield by 44% in CNT1 and by 23% in PTT1. In Si deficiency, CNT1 had fewer total spikelets, and the fertilized and filled spikelets responded more strongly to Si than PTT1. Grain yield in both varieties increased with increasing number of spikelets and filled fertilized grains. There were close relationships between Si concentration in the shoots, flag leaf and the husk, which were positively correlated with grain yield, the number of spikelets, and fertilized and filled grains. To evaluate the effects of Si fertilization across 2 cropping seasons on grain yield and Si concentration in Thai rice varieties. A modern high-yield variety, CNT1 and PTT1 were grown in two cropping seasons between dry and wet season with and without silicon application (0 and 150 kg /ha. The responses of grain yield and Si accumulation were in similar range between the 2 varieties. Applying Si increased grain yield by 23% compared with plants without Si, while the highest grain yield occurred in the wet season by 57.3 g/plant compared with dry season. The variation in grain yield was associated with season and Si applications on number of tiller and panicle. The highest Si concentration in stem, husk and flag leaf were found when rice plants were grown in the dry season with Si application, whereas without Si fertilizer, stem, flag leaf and husk in the wet season had higher the concentration than dry season. There was significant correlation between grain yield and Si concentration in leaves, leaf sheath, flag leaf and paddy. The effect of Si fertilizer doses on grain yield and Si accumulation was investigated in the three rice varieties, CNT1, PTT1 and KDML105 which were grown in pot experiment under six levels of Si, 0, 100, 150, 200, 250 and 300 kg Si /ha. The varieties responded to Si fertilizer differently in both their yield and accumulation of Si in plant tissues. Applying Si at 100 kg Si /ha has potentially increased grain yield of PTT1 and KDML105, while applying with 200 kg Si /ha was sufficient to enhance grain yield of CNT1. Although, applying with higher rate (300 kg Si /ha) strongly improved crop yield in rice by increasing the agronomic tails, but the grain yield was statistically similar among 200 and 300 kg Si /ha doses. Rising Si fertilizer application up to 150 kg/ha helped to improve the Si concentration in straw, flag leaf and husk in all varieties, which Si concentration in all of rice tissues was higher under high Si (300 kg Si /ha). The factors affecting on grain yield and Si concentration in rice plants were also evaluated in the molecular level. The Lsi6 is a major transporter involving in Si transfer from roots to panicles for the caryopsis’s development. The effect of Si application on the expression of Lsi6 gene was investigated which the expression of OsLsi6 in CNT1 and PTT1 varieties was detected in the first node below the panicle at the flowering stage of plants grown in pot experiment under 4 levels of Si, 0, 100, 200 and 300 kg Si /ha. The expression of OsLsi6 significantly responded to Si application differently among the varieties. Applying Si fertilizer increased the expression level of OsLsi6 in both CNT1 and PTT1, which the highest was found at 300 kg Si /ha in all varieties. There was significant correlation between relative gene expression of OsLsi6 and Si concentration in flag leaf. The previous studies reported that Si enhanced plant growth and development in rice crop under phosphorus (P) and iron (Fe) deficiency. However, the information on how P and Fe availability affects Si accumulation in rice plant is limits. Therefore, this experiment was conducted to examine the influence of the interactions between Si, Pi, and Fe on rice growth and its accumulation of these elements in rice plant. Two rice (Oryza sativa L.) varieties were used in this study (SPR1, wetland and KH CMU, upland varieties). The seedlings were grown in complete Yoshida media (CT), under Pi (-Pi) and Fe (-Fe) deficiency and simultaneous Pi and Fe (-Pi-Fe) deficiency stresses, with (+Si) and without (−Si) Si application. The results showed that Pi and Fe deficiency affected the varieties differently, with a significant decrease in the shoot biomass in SPR1, while there was no significant decrease in the biomass of KHCMU. The deficiency of Fe caused a reduction of the biomass production in both varieties. However, in Pi and Fe double deficiency stress, the root biomass of KH CMU increased when applied Si fertilizer. The absence of Pi and Fe or both Pi and Fe deficiency had resulted to increase Si concentration in the shoot. Fe deficiency led to the over-accumulation of Pi in the shoot of SPR1 and KH CMU and this effect was further enhanced by a simultaneous Si and Fe deficiency in SPR1 but not found in KH CMU. Pi deficiency increased Fe concentration in the shoots and roots of both rice varieties, while simultaneous Pi and Fe deficiency treatments caused a decrease in Fe concentration in the shoots and roots. Interestingly, under Si deficiency conditions and despite the presence of Fe in the medium, Fe concentration in the shoots and roots of both rice varieties showed a significant decrease. There were no significant in accumulation of OsPHO1;2 in rice root when grown under Si deficiency, Fe deficiency or simultaneous Fe and Si deficiency when comparison to the control treatment. In addition, Fe deficiency and simultaneous Fe and Si deficiency caused an increase in the root-to-shoot transport of Pi in rice. In rice, higher Si accumulation was attributed to high Si uptake ability of plant roots, mediated by Si transporters (Lsi1). The regulation of the Si transporter Lsi1 in rice grown under different Si and Fe regimes were investigated. The three rice varieties used in this experiment were CNT1, PTT1 and KDML105. The roots of a seedling (28 days old) were split into two parts and immersed in nutrient solution with varying Si and Fe supply. This study found that, the responses of Si and Fe were similar among the three rice varieties used in this study. The amount of Lsi1 transcript was mainly altered in response to Sirelated treatments. Split-root experiments showed that the expression of Lsi1 is locally and systemically regulated in response to Si signals. Interestingly, the accumulation of Lsi1 transcripts appeared to be dependent on Fe availability in root growth environment. Results suggest that the expression of Lsi1 depends on a regulatory network that integrates Si and Fe signals. In conclusion, this study has exhibited the variation of Si accumulation in plant parts among Thai rice varieties with high Si accumulation in plant parts and grain yield was found in SPR1, which can be considered as a potential genetic material in breeding program for improving Si concentration and production of rice varieties in the future. However, it is not only different among rice varieties was observed in this study, grain yield and Si concentration can be influenced by environmental factors such as Si fertilizer application and cropping season with different response was observed among the varieties. In addition, the availability of Si and other essential nutrients (P and Fe) was also found to effect on plant growth, Si concentration and the expression of genes involving in Si uptake and transport in rice. This study suggests that applying with appropriate Si fertilizer and rice variety could improve grain yield through the increasing of Si accumulation in plant tissues, which is in parallel with the expression of genes involving in Si uptake and transport. This information is very useful for improving productivity by Si fertilization management in rice crop.