The Inferior Grain Filling Initiation Promotes the Source Strength of Rice Leaves

Abstract

Poor grain-filling initiation in inferior spikelets severely impedes rice yield improvement, while photo-assimilates from source leaves can greatly stimulate the initiation of inferior grain-filling (sink). To investigate the underlying mechanism of source-sink interaction, a two-year field experiment was conducted in 2019 and 2020 using two large-panicle rice cultivars (CJ03 and W1844). The treatments included intact panicles and partial spikelet removal. These two cultivars showed no significant difference in the number of spikelets per panicle. However, after removing spikelet, W1844 showed higher promotion on 1000-grain weight and seed-setting rate than CJ03, particularly for inferior spikelets. The reason was that the better sink activity of W1844 led to a more effective initiation of inferior grain-filling compared to CJ03. The inferior grain weight of CJ03 and W1844 did not show a significant increase until 8 days poster anthesis (DPA), which follows a similar pattern to the accumulation of photo-assimilates in leaves. After removing spikelets, the source leaves of W1844 exhibited lower photosynthetic inhibition compared to CJ03, as well as stronger metabolism and transport of photo-assimilates. Although T6P levels remained constant in both cultivars under same conditions, the source leaves of W1844 showed notable downregulation of SnRK1 activity and upregulation of phytohormones (such as abscisic acid, cytokinins, and auxin) after removing spikelets. Hence, the high sink strength of inferior spikelets plays a role in triggering the enhancement of source strength in rice leaves, thereby fulfilling grain-filling initiation demands.

Keywords: Phytohormone; Rice; Sink; Source; Sugar.

Fig. 1

Changes in grain growth during grain filling stage in 2019 and 2020. A schematic diagram of rice panicle in CJ03 and W1844 at maturity; B heat maps of grain weight in different position at maturity; C heat maps of seed setting rate in different position at maturity; D dynamic of grain weight in CJ03 and W1844 during early grain filling stage; T0, control group with no removing-spikelets; T1, removing top 2/3 of the spikelets in panicle; SS, superior spikelets; IS, inferior spikelets; Significant differences at each time point with same color are indicated by different letters (P < 0.05) as determined by Duncan’s test; The data are the means of three replications ± SD (n = 3)

Fig. 2

The accumulation of carbohydrates in source leaves of CJ03 and W1844 in 2019 and 2020. A Dry weight of total source leaves in CJ03 and W1844 at 8 DPA; B photosynthetic accumulation of top three leaves in CJ03 and W1844 at 8 DPA; C, CJ03; W, W1844; T0, control group with no removing-spikelets; T1, removing top 2/3 of the spikelets in panicle; DPA, days post anthesis; Significant differences are indicated by different letters with same color (P < 0.05) as determined by Duncan’s test. Bars mean SD (n = 9)

Fig. 3

Relative expression levels of sucrose transporters in the top three leaves at 8 DPA in 2020. Significant differences are indicated by different letters (P < 0.05) as determined by Duncan’s test; The data are the means of three biological replications ± SD, consisting of 3 technical replications in each biological replication

Fig. 4

Activities of key enzymes and relative gene expression on carbon metabolism in the top three leaves at early grain filling stage of 2020. A The activity of SPS, AGPase, and α-Amylase in the top three leaves at 8 DPA; B the level of gene expression relating carbon metabolism in the top three leaves at 8 DPA; C working model of carbon metabolism in source leaves of rice during daytime; C, CJ03; W, W1844; T0, control group with no removing-spikelets; T1, removing top 2/3 of the spikelets in panicle; Four types of genes expression level were measured: relating sucrose synthesis (OsSPS1), relating starch synthesis (OsSUS3, OsSUS4, and OsAGPL1), and relating starch degradation (OsAmy3); The data are the means of three biological replications ± SD, consisting of 3 technical replications in each biological replication

Fig. 5

Antagonistic regulation of compound levels and gene expression levels relating to the hormone-sugar pathway in the top three leaves at 8 DPA of 2020. C, CJ03; W, W1844; T0, control group with no removing-spikelets; T1, removing top 2/3 of the spikelets in panicle; Blue color correspond to sugar signaling pathway; Yellow color correspond to hormone metabolism; Different letters indicate significant differences among treatments (P < 0.05); The data are the means of three biological replications ± SD, consisting of 3 technical replications in each biological replication

Fig. 6

A working model of rice leaves in response to altered sink–source relations at early grain filling stage