Heat-Induced Cytokinin Transportation and Degradation Are Associated with Reduced Panicle Cytokinin Expression and Fewer Spikelets per Panicle in Rice.

Chao Wu,Shah Fahad,Wencheng Wang,Qiuqian Hu,Jianliang Huang,Qian Li,Pravat K Mohapatra,Shaobing Peng,Lixiao Nie,Kehui Cui

doi:10.3389/fpls.2017.00371

Abstract

Cytokinins (CTKs) regulate panicle size and mediate heat tolerance in crops. To investigate the effect of high temperature on panicle CTK expression and the role of such expression in panicle differentiation in rice, four rice varieties (Nagina22, N22; Huanghuazhan, HHZ; Liangyoupeijiu, LYPJ; and Shanyou63, SY63) were grown under normal conditions and subjected to three high temperature treatments and one control treatment in temperature-controlled greenhouses for 15 days during the early reproductive stage. The high temperature treatments significantly reduced panicle CTK abundance in heat-susceptible LYPJ, HHZ, and N22 varieties, which showed fewer spikelets per panicle in comparison with control plants. Exogenous 6-benzylaminopurine application mitigated the effect of heat injury on the number of spikelets per panicle. The high temperature treatments significantly decreased the xylem sap flow rate and CTK transportation rate, but enhanced cytokinin oxidase/dehydrogenase (CKX) activity in heat-susceptible varieties. In comparison with the heat-susceptible varieties, heat-tolerant variety SY63 showed less reduction in panicle CTK abundance, an enhanced xylem sap flow rate, an improved CTK transport rate, and stable CKX activity under the high temperature treatments. Enzymes involved in CTK synthesis (isopentenyltransferase, LONELY GUY, and cytochrome P450 monooxygenase) were inhibited by the high temperature treatments. Heat-induced changes in CTK transportation from root to shoot through xylem sap flow and panicle CTK degradation via CKX were closely associated with the effects of heat on panicle CTK abundance and panicle size. Heat-tolerant variety SY63 showed stable panicle size under the high temperature treatments because of enhanced transport of root-derived CTKs and stable panicle CKX activity. Our results provide insight into rice heat tolerance that will facilitate the development of rice varieties with tolerance to high temperature.

Highlights

The global mean surface temperature increased rapidly and considerably during the 20th century, and a further increase of 0.3–4.8◦C is predicted by the end of the 21st century (Pachauri et al, 2014)
Our study demonstrates that cytokinin oxidase/dehydrogenase (CKX) activity seems to influence panicle size under high temperature treatments by regulating panicle CTK abundance (Figure 6)
The three high temperature treatments significantly decreased the number of spikelets per panicle in heat-susceptible rice varieties (HHZ, LYPJ, and N22), whereas heat-tolerant variety SY63 showed a relatively stable number of spikelets per panicle

Summary

Introduction

The global mean surface temperature increased rapidly and considerably during the 20th century, and a further increase of 0.3–4.8◦C is predicted by the end of the 21st century (Pachauri et al, 2014). Nighttime temperature has increased more rapidly than has daytime temperature (Peng et al, 2004; Elagib, 2010). Rice plants are highly susceptible to high temperature stress, especially during the reproductive stage (Moldenhauer et al, 2001; Jagadish et al, 2015), during which high temperature may severely reduce grain yield. An increase of 1–4◦C reduced rice grain yield by 0–49%; grain yield decreased 14% for every 1◦C increase in temperature (Singh et al, 2009). Another study showed that an increase of 1◦C in nighttime temperature reduced rice grain yield by 10% (Peng et al, 2004). Heat-induced yield reduction is largely attributed to adverse effects on yield components (Jagadish et al, 2015)

Methods

Results

Discussion

Conclusion