Lin Chen

Iron (Fe) is essential for normal plant growth and development. In rice, Fe deficiency leads to stunted growth, leaf chlorosis, reduced photosynthetic capacity, and ultimately, yield loss. Most studies have focused on investigating the mechanisms of Fe deficiency responses in rice roots; however, the effects of shoot Fe redistribution on Fe deficiency response remain poorly understood. Phloem transport plays a vital role in distributing Fe to new tissues. To investigate the effects of enhanced phloem-mediated Fe transport on rice adaptability to iron deficiency, we subjected transgenic lines with higher phloem Fe efflux rates and wild-type (WT) plants to Fe-deficient conditions. The growth, leaf photosynthetic rate, and Fe content of transgenic and WT seedlings under different Fe concentrations were compared. The results showed that the transgenic lines exhibited elevated shoot length, root length, shoot dry weight, leaf chlorophyll content, and net photosynthetic rates under Fe-deficient conditions. Under both Fe-sufficient and Fe-deficient conditions, the transgenic lines had significantly higher Fe content, Fe accumulation, and phloem Fe efflux rates than the WT. RNA sequencing (RNA-seq) analysis revealed that enhanced Fe transport via phloem resulted in improved Fe availability through the sequestration of Fe ions and vacuolar transport pathways in the shoots. It also upregulated the EARLY LESION LEAF 1 (ELL1) expression and modulated the sucrose synthase activity, thereby promoting chlorophyll synthesis and leaf photosynthesis. Additionally, enhanced Fe transport influenced the gibberellin (GA) catabolism and plant hormone signal transduction in the roots, reducing the GA content and modulating the cytokinin (CTK), jasmonic acid (JA), and ethylene (ETH) signaling to induce Fe deficiency response and promote Fe uptake. These findings demonstrate that phloem-mediated Fe transport participated in Fe deficiency response, and enhancing this improved the adaptability of rice seedlings to low Fe conditions. In specific, rice seedlings with a high capacity for phloem-mediated Fe transport exhibited a strong iron uptake, translocation, and remobilization capacity, thereby maintaining normal growth and development and successfully adapting to the low-Fe environment.

The yield potential of large-panicle rice is often limited by grain-filling barriers caused by the development of inferior spikelets (IS). Photoassimilates, which are the main source of rice grain filling, mainly enter the caryopsis through the dorsal vascular bundle. The distribution of assimilates between superior spikelets (SS) and IS is influenced by auxin-mediated apical dominance; however, the mechanism involved is still unclear. In this study, the effect of auxin signaling on the grain filling of SS and IS was investigated in two large-panicle japonica rice varieties, W1844 and CJ03. Compared to SS, IS displayed delayed initiation of filling and a significantly lower grain weight. Furthermore, the endosperm development in IS remained stagnant at the coenocytic stage. The development of the dorsal vascular bundle in the IS was also slow, and poor sucrose-unloading was observed during the initial grain filling stage. However, the endosperm development in IS immediately started after the improvement of dorsal vascular bundle development. GUS activity staining further revealed that indole-3-acetic (IAA) was localized in the dorsal vascular bundle and surrounding areas, suggesting that the low IAA content observed in the IS during the initial grain filling stage may have delayed the development of the dorsal vascular bundle. Therefore, these results demonstrate that IAA may control sugar transport and unloading by regulating dorsal vascular bundle development, consequently affecting endosperm development in IS.

Poor grain filling in inferior spikelets (IS), which is influenced by the remobilization of nonstructural carbohydrates (NSC) stored in the sheath and internode of rice plants, limits the expected high yield of large‐panicle rice. NSC remobilization from the sheath to the panicle is regulated by the T6P/SnRK1 pathway. However, in large‐panicle rice, it is unclear whether IS grain filling is related to the NSC remobilization mediated by T6P/SnRK1 signaling. In this study, two large‐panicle cultivars—W1844 and CJ03—with distinct differences in IS grain filling were used to explore the physiological mechanism mediating IS development. Compared to W1844, CJ03 IS showed lower expression of the genes related to sucrose uploading, later sucrose peaking, and delayed starch accumulation. In the CJ03, low OsSUTs expression and NSC output, transport rate, and contribution rate were detected in the sheaths and internodes. These results suggest that poor NSC remobilization results in insufficient assimilate supply for the IS, and consequently, poor IS grain filling. Furthermore, poor NSC remobilization coincided with the increased T6P content and decreased SnRK1 activity during grain filling in CJ03 IS. The expression levels of genes related to T6P metabolism and those encoding the catalytic subunit of SnRK1 were consistent with the observed T6P content and SnRK1 activity in the sheaths and internodes. Therefore, IS grain filling is potentially affected by T6P/SnRK1 signaling‐mediated NSC remobilization in large‐panicle rice.

Sucrose non-fermenting-1-related protein kinase 1 (SnRK1) and AMP-activated protein kinase (AMPK) are highly conserved. Compound 991 is an AMPK activator in mammals. However, whether 991 also activates SnRK1 remains unknown. The addition of 991 significantly increased SnRK1 activity in desalted extracts from germinating rice seeds in vitro. To determine whether 991 has biological activity, rice seeds were treated with different concentrations of 991. Germination was promoted at low concentrations but inhibited at high concentrations. The effects of 991 on germination were similar to those of OsSnRK1a overexpression. To explore whether 991 affects germination by specifically affecting SnRK1, germination of an snrk1a mutant and wild type under 1 μM 991 treatment was compared. The snrk1a mutant was insensitive to 991. Phosphoproteomic analysis showed that the differential phosphopeptides induced by 991 and OsSnRK1a overexpression largely overlapped. Furthermore, SnRK1 might regulate rice germination in a dosage-dependent manner by regulating the phosphorylation of S285-PIP2;4, S1013-SOS1, and S110-ABI5. These results indicate that 991 is a specific SnRK1 activator in rice. The promotion and inhibition of germination by 991 also occurred in wheat seeds. Thus, 991 is useful for exploring SnRK1 function and the chemical regulation of growth and development in crops.

The remobilization of non-structural carbohydrates (NSCs) in the stem is essential for rice grain filling so as to improve grain yield. We conducted a two-year field experiment to deeply investigate their relationship. Two large-panicle rice varieties with similar spikelet size, CJ03 and W1844, were used to conduct two treatments (removing-spikelet group and control group). Compared to CJ03, W1844 had higher 1000-grain weight, especially for the grain growth of inferior spikelets (IS) after removing the spikelet. These results were mainly ascribed to the stronger sink strength of W1844 than that of CJ03 contrasting in the same group. The remobilization efficiency of NSC in the stem decreased significantly after removing the spikelet for both CJ03 and W1844, and the level of sugar signaling in the T6P-SnRK1 pathway was also significantly changed. However, W1844 outperformed CJ03 in terms of the efficiency of carbon reserve remobilization under the same treatments. More precisely, there was a significant difference during the early grain-filling stage in terms of the conversion of sucrose and starch. Interestingly, the sugar signaling of the T6P and SnRK1 pathways also represented an obvious change. Hence, sugar signaling may be promoted by sink strength to remobilize the NSCs of the rice stem during grain filling to further advance crop yield.