Auxin-Mediated Regulation of Dorsal Vascular Cell Development May Be Responsible for Sucrose Phloem Unloading in Large Panicle Rice

Abstract

Large panicle rice cultivars often fail to fulfill their high-yield potential due to the poor grain filling of inferior spikelets (IS), which appears as initially stagnant development and low final seed weight. Understanding the mechanism of the initial stagnancy is important to improve IS grain filling. In this study, superior spikelets (SS) were removed from two homozygous japonica rice varieties (W1844 and CJ03) with the same sink capacity in an attempt to force photosynthate transport to the IS. The results showed that SS removal increased the grain weight, sucrose content, starch accumulation, and endogenous IAA levels of IS during the initial grain-filling stage. SS removal also improved the patterns of vascular cells in the dorsal pericarp and the expression levels of genes involved in sucrose transport (OsSUTs and OsSWEETs) and IAA metabolism (OsYUCs and OsPINs). Exogenous IAA application advanced the initiation of grain filling by increasing the sucrose content and the gene expression levels of sucrose transporters. These results indicate that auxin may act like a signal substance and play a vital role in initial grain filling by regulating dorsal vascular cell development and sucrose phloem unloading into caryopsis.

Keywords: auxin; grain filling; inferior spikelets; rice; sucrose phloem unloading; vascular cell development.

FIGURE 1

Grain weight (A,B) of SS and IS of CJ03 and W1844 at the early grain-filling stage. CK, control group; R1, upper 1/3 part of the spikelets were removed; R2, upper 2/3 part of the spikelets were removed. CK-SS, superior spikelets of control panicle, CK-IS, inferior spikelets of control panicle, R1-IS, inferior spikelets of upper 1/3 removed panicle, R2-IS, inferior spikelets of upper 2/3 removed panicle. Vertical bars represent the mean values ± SE (n = 3).

FIGURE 2

Fructose (A,B), glucose (C,D), and sucrose (E,F) contents of SS and IS of CJ03 and W1844 at the early grain filling stage. CK-SS, superior spikelets of control panicle, CK-IS, inferior spikelets of control panicle, R2-IS, inferior spikelets of upper 2/3 removed panicle. Vertical bars represent the mean values ± SE (n = 3).

FIGURE 3

Ultrastructure of the dorsal phloem of the vascular bundle in CJ03 and W1844 caryopsis at the early grain-filling stage. A, G, Ultrastructure of the dorsal vascular bundle at 4 days post-anthesis in superior spikelets; B, H, Ultrastructure of the dorsal vascular bundle at 10 days post-anthesis in superior spikelets; C, I, Ultrastructure of the dorsal vascular bundle at 4 days post-anthesis in inferior spikelets; D, J, Ultrastructure of the dorsal vascular bundle at 10 days post-anthesis in inferior spikelets; E, K, Ultrastructure of the dorsal vascular bundle at 4 days post-anthesis in inferior spikelets under spikelet removal; F, L, Ultrastructure of the dorsal vascular bundle at 10 days post-anthesis in inferior spikelets under spikelet removal. SE, sieve element; iSE, immature sieve element; CC, companion cell; PC, parenchyma cells; P, plasmodesmata; M, mitochondrion; N, Nucleus. Bar = 2 μm.

FIGURE 4

Relative expression levels of OsSUT1 (A,B), OsSUT3 (C,D), and OsSUT4 (E,F) in SS and IS at the early grain-filling stage of CJ03 and W1844. CK-SS, superior spikelets of control panicle, CK-IS, inferior spikelets of control panicle, R2-IS, inferior spikelets of upper 2/3 removed panicle. Vertical bars represent the mean values ± SE (n = 3). Significant differences at each time point are indicated by different letters (P < 0.05) as determined by Duncan’s test.

FIGURE 5

Relative expression levels of OsSWEET4 (A,B), OsSWEET11 (C,D), and OsSWEET15 (E,F) in SS and IS at the early grain-filling stage of CJ03 and W1844. CK-SS, superior spikelets of control panicle, CK-IS, inferior spikelets of control panicle, R2-IS, inferior spikelets of upper 2/3 removed panicle. Vertical bars represent the mean values ± SE (n = 3). Significant differences at each time point are indicated by different letters (P < 0.05) as determined by Duncan’s test.

FIGURE 6

Starch (A,B) and IAA (C,D) contents of SS and IS of rice at the early grain-filling stage of CJ03 and W1844. CK-SS, superior spikelets of control panicle, CK-IS, inferior spikelets of control panicle, R2-IS, inferior spikelets of upper 2/3 removed panicle. Vertical bars represent the mean values ± SE (n = 3).

FIGURE 7

Relative expression levels of OsYUC9 (A,B), OsYUC11 (C,D), and OsTAR1 (E,F) in the SS and IS of CJ03 and W1844 at the early grain-filling stage. CK-SS, superior spikelets of control panicle, CK-IS, inferior spikelets of control panicle, R2-IS, inferior spikelets of upper 2/3 removed panicle. Vertical bars represent the mean values ± SE (n = 3). Significant differences at each time point are indicated by different letters (P < 0.05) as determined by Duncan’s test.

FIGURE 8

Relative expression levels of OsPIN1b (A,B), OsPIN5b (C,D), and OsPIN10a (E,F) in the SS and IS of CJ03 and W1844 at the early grain-filling stage. CK-SS, superior spikelets of control panicle, CK-IS, inferior spikelets of control panicle, R2-IS, inferior spikelets of upper 2/3 removed panicle. Vertical bars represent the mean values ± SE (n = 3). Significant differences at each time point are indicated by different letters (P < 0.05) as determined by Duncan’s test.

FIGURE 9

Grain weights (A,B) and IAA (C,D) contents of the SS and IS of CJ03 and W1844 under IAA application at the early grain-filling stage. CK, control group; IAA, IAA sprayed group. Vertical bars represent the mean values ± SE (n = 3).

FIGURE 10

Fructose (A,B), glucose (C,D), and sucrose (E,F) contents of the SS and IS of CJ03 and W1844 under IAA application at the early grain-filling stage. CK, control group; IAA, IAA sprayed group. Vertical bars represent the mean values ± SE (n = 3).

FIGURE 11

Relative expression levels of OsSUTs (A,B) and OsSWEETs (C,D) in the SS and IS of rice under IAA application at 6 DAP. CK, control group; IAA, IAA sprayed group. Vertical bars represent the mean values ± SE (n = 3). Significant differences at each time point are indicated by different letters (P < 0.05) as determined by Duncan’s test.