Brassinosteroids function in spikelet differentiation and degeneration in rice

Abstract

Brassinosteroids (BRs) play crucial roles in many aspects of plant development. However, their function in spikelet differentiation and degeneration in rice (Oryza sativa L.) remains unclear. Here, we investigated the roles of these phytohormones in spikelet development in field‐grown rice subjected to five different nitrogen (N) fertilization treatments during panicle differentiation. BR levels and expression of genes involved in BR biosynthesis and signal transduction were measured in spikelets. Pollen fertility and the number of differentiated spikelets were closely associated with 24‐epicastasterone (24‐epiCS) and 28‐homobrassinolide (28‐homoBL) levels in spikelets. Enhanced BR biosynthesis and signal transduction, in response to N treatment, enhanced spikelet differentiation, reduced spikelet degeneration, and increased grain yield. Increases in proton‐pumping ATPase activity, ATP concentration, energy charge, and antioxidant system (AOS) levels were consistent with 24‐epiCS and 28‐homoBL concentrations. Exogenous application of 24‐epiCS or 28‐homoBL on young panicles induced a marked increase in endogenous 24‐epiCS or 28‐homoBL levels, energy charge, AOS levels, spikelet differentiation, and panicle weight. The opposite effects were observed following treatment with a BR biosynthesis inhibitor. Our findings indicate that, in rice, BRs mediate the effects of N fertilization on spikelet development and play a role in promoting spikelet development through increasing AOS levels and energy charge during panicle development.

Response of brassinosteroids to nitrogen rates and their regulation on rice spikelet degeneration during meiosis

“…For all N treatments, N as urea was also applied at pretransplanting (one day before transplanting), early tillering (seven days after transplanting), and panicle initiation at 60, 30, and 60 kg/ha N, respectively. The developmental stages were observed by frequent inspection of the meristems using a microscope and leaf remainder (LR) through peeling and examining leaves as described previously (Zhang, Sheng, et al, 2019; Zhang, Zhu, et al, 2019). The panicle initiation was defined as the first appearance of a differentiated apex (LR: 4.0–3.5) and the SD as the appearance of glumous flower primordia at the tips of elongating primary rachis branches (the length of the young panicles was approximately 1.0–1.5 mm, LR: 2.0–1.6).…”

“…Sampling and measurements were mainly made at PMCm when the ligule of the flag leaf was 0 cm below that of the penultimate leaf because rice spikelet degeneration mainly occurs at this stage (Wang, Zhang, & Yang, 2018; Zhang, Sheng, et al, 2019; Zhang, Zhu, et al, 2019). One hundred main stems were tagged in each plot during tillering, and young panicles were sampled from the tagged stems at PMCm for the measurement of BRs contents, cytochrome C oxidase (CCO) and succinate dehydrogenase (SDH) activities, levels of ATP and energy charge (EC), T‐AOC, H 2 O 2 content, and expression levels of the VPE genes ( OsVPE2 and OsVPE3 ).…”

“…Spikelet degeneration, or spikelet abortion, is very common in cereals such as rice ( Oryza sativa L.) that is a serious physiological defect and constraint to grain production (Yoshida, Ohmori, Kitano, Taguchi‐Shiobara, & Hirano, 2012; Zhang, Sheng, et al, 2019). Reduction or elimination of spikelet degeneration to increase spikelet number is critical for increasing rice grain yield, and great effort has been made to decrease spikelet degeneration in rice using agronomic, genetic, and molecular approaches (Heng et al, 2018; Tang, Sun, Xu, & Yu, 2011; Zhang, Sheng, et al, 2019; Zhang, Zhu, et al, 2019). However, the mechanism underlying spikelet degeneration remains unclear.…”

“…Among the nutritional factors, nitrogen (N) is one of the most yield‐limiting nutrients in crop production, and its proper management is essential to increase spikelet number and consequently increase rice grain yield (Ding et al, 2016; Zhang, Zhu, et al, 2019; Zhang et al, 2013). Our previous work showed that BRs in young panicles mediate the effects of N top‐dressing times during panicle development on rice spikelet differentiation and degeneration (Zhang, Zhu, et al, 2019). However, little is known about how BRs respond to treatment with different N rates at spikelet differentiation (SD) or how they regulate spikelet degeneration at pollen mother cell meiosis metaphase (PMCm) in rice.…”

“…The enhancement of membrane integrity is associated with the maintenance of a higher energy status, such as adenosine triphosphate (ATP) level (Jin et al, 2013; Liu et al, 2016). Our previous work showed that an appropriate N top‐dressing time during rice panicle development could significantly increase BRs levels, which effectively increase spikelet differentiation and decreased spikelet degeneration by stimulating antioxidase activities and energy status in young panicles (Zhang, Zhu, et al, 2019). How BRs mediate effects of various N application rates at SD on rice spikelet degeneration is poorly understood, and the objective of this study was to elucidate their mechanism.…”