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

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.

Cytokinin action in response to abiotic and biotic stresses in plants

“…For example, exogenous application of CK and enhanced endogenous CK concentrations resulted in a decrease of the inhibitory effect of heat stress on photosynthesis and chloroplast development (Caers, Rudelsheim, & Van Onckelen, ; X. Liu & Huang, ) and caused a higher activity of the antioxidant system and up‐regulation of heat shock proteins in tobacco and creeping bentgrass (Harding & Smigocki, ; Veerasamy, He, & Yuang, ; Xu, Tian, Gianfagna, & Huang, ; Xu, Gianfagna, & Huang, ). Noteworthy, CK treatment increased thermotolerance of reproductive tissue in maize, rice, and passion fruit (Cheikh & Jones, ; Sobol et al, ; Wu et al, , ) suggesting priming of heat stress defence by CK (see below).…”

“…It has probably been best documented for protection of reproductive tissues from the detrimental consequences of heat treatment causing flower or kernel abortion. CK treatment during the reproductive phase increased thermotolerance of maize, rice, and passion fruit and thus increased yield stability (Cheikh & Jones, ; Sobol et al, ; Wu et al, , ). It is conceivable that in this case, the protective effect of CK was at least partly due to an improved sink capacity of the endangered tissue.…”

Plants under Stress: Involvement of Auxin and Cytokinin

“…However, the stress-induced increment of total CKX activity is specific to the susceptible varieties. Moreover, reduction of cytokinin transport from root to shoot via xylem sap and decreased LOG and IPT activities seem to be responsible for the lower number of spikelets per panicle obtained under high temperature treatments in the susceptible lines [ 75 ]. Similarly, increased CKX activity was observed under high temperature stress in tobacco plants [ 56 ].…”

Endogenous Hormones Inhibit Differentiation of Young Ears in Maize (Zea mays L.) Under Heat Stress

“…This gives rise to a complex regulatory network of hormones that collectively regulate life processes such as differentiation and grain development in young ears (Wolters and Jürgens, 2009). For example, cytokinins can regulate the ear size and thermotolerance of rice (Wu et al, 2017). A decrease in cytokinin content, induced by heat stress, has been shown to lead to inhibited ear differentiation, floret fertility, pollen viability, and anther cracking, as well as a smaller grain size, none of which are conducive to yield formation (Sobol et al, 2014;Wu et al, 2016).…”