Disruptions of sugar utilization and carbohydrate metabolism in rice developing anthers aggravated heat stress-induced pollen abortion.

High temperature (HT) stress at reproductive stage is one of most important environment negatively affecting spikelet fertility and rice yield. In this study, the effect of HT exposure on the sugar composition and carbohydrate metabolism in developing anthers and its relation to floret fertility and pollen viability were investigated by different temperature regimes under well-controlled climatic condition. Result showed that HT exposure during microspore development significantly reduced the starch deposition in developing anther and evidently disrupted the spatial distribution of sugar and starch concentrations in different compartments of rice anther, with the higher ratio of sucrose to hexose concentrations in HT-stressed anthers relative to the control ones. Under HT exposure, the amount of starch deposition in the fraction of sporophytic tissues dropped evidently, while the concentrations of sucrose and starch in anther wall tissue enhanced significantly, suggesting that HT exposure impaired the translocation of sucrose from the anther wall tissue to the sporophytic tissues inside rice anther. Furthermore, we presented possible contribution of various genes and key enzymes involving in sugar conversion and carbohydrate metabolism in developing anther to the formation of HT-induced pollen abortion by disrupting the sugar utilization in HT-stressed anther. HT exposure suppressed the activities of cell wall and vacuolar invertase, hexokinase, and ADP-glucose pyrophosphorylase in developing anther, while it was opposite for the effect of HT exposure on sucrose synthase and fructokinase. HT-induced suppression of OsCWIN3 in the anther walls might be strongly responsible for the HT-induced impairments of sugar utilization in HT-stressed anthers.

ABA-triggered ROS burst in rice developing anthers is critical for tapetal programmed cell death induction and heat stress-induced pollen abortion.

High temperatures (HT) cause pollen abortion and poor floret fertility in rice, which is closely associated with excessive accumulation of reactive oxygen species (ROS) in the developing anthers. However, the relationships between accumulation of abscisic acid (ABA) and ROS, and their effects on tapetum-specific programmed cell death (PCD) in HT-stressed anthers are poorly characterised. Here, we determined the spatiotemporal changes in ABA and ROS levels, and their relationships with tapetal PCD under HT exposure. Mutants lacking ABA-activated protein kinase 2 (SAPK2) functions and exogenous ABA treatments were used to explore the effects of ABA signalling on the induction of PCD and ROS accumulation during pollen development. HT-induced pollen abortion was tightly associated with ABA accumulation and oxidative stress. The higher ABA level in HT-stressed anthers resulted in the earlier initiation of PCD induction and subsequently abnormal tapetum degeneration by activating ROS accumulation in developing anthers. Interactions between SAPK2 and DEAD-box ATP-dependent RNA helicase elF4A-1 (RH4) were required for ABA-induced ROS generation in developing anthers. The OsSAPK2 knockout mutants showed the impaired PCD responses in the absence of HT. However, the deficiency of SAPK2 functions did not suppress the ABA-mediated ROS generation in HT-stressed anthers.

Different Phosphorus Supplies Altered the Accumulations and Quantitative Distributions of Phytic Acid, Zinc, and Iron in Rice (Oryza sativa L.) Grains.

Development of rice cultivars with low phytic acid (lpa) is considered as a primary strategy for biofortification of zinc (Zn) and iron (Fe). Here, two rice genotypes (XS110 and its lpa mutant) were used to investigate the effect of P supplies on accumulations and distributions of PA, Zn, and Fe in rice grains by using hydroponics and detached panicle culture system. Results showed that higher P level increased grain PA concentration on dry matter basis (g/kg), but it markedly decreased PA accumulation on per grain basis (mg/grain). Meanwhile, more P supply reduced the amounts and bioavailabilities of Zn and Fe both in milled grains and in brown grains. Comparatively, lpa mutant was more susceptive to exogenous P supply than its wild type. Hence, the appropriate P fertilizer application should be highlighted in order to increase grain microelement (Zn and Fe) contents and improve nutritional quality in rice grains.

Involvement of CAT in the detoxification of HT-induced ROS burst in rice anther and its relation to pollen fertility.

KEY MESSAGE: HT-induced ROS burst in developing anther is closely related to the lowered CAT activity as the result of the markedly suppressed OsCATB transcript, thereby causing severe fertility injury for rice plants exposed to HT at meiosis stage. The reproductive stage of rice plants is highly sensitive to heat stress. In this paper, different rice cultivars were used to investigate the relationship of HT-induced floret sterility with reactive oxygen species (ROS) detoxification in rice anthers under well-controlled climatic conditions. Results showed that high temperature (HT) exposure significantly enhanced the ROS level and malondialdehyde (MDA) content in developing anther, and the increase in ROS amount in rice anther under HT exposure was closely associated with HT-induced decline in the activities of several antioxidant enzymes. For various antioxidant enzymes, SOD and CAT were more susceptible to the ROS burst in rice anther induced by HT exposure than APX and POD, in which SOD and CAT activity in developing anther decreased significantly by HT exposure, whereas APX activity was relatively stable among different temperature regimes. HT-induced decrease in CAT activity was attributable to the suppressed transcript of OsCATB. This occurrence was strongly responsible for HT-induced increase in ROS level and oxidative-damage in rice anther, thereby it finally caused significant reduction in pollen viability and floret fertility for the rice plants exposed to HT during meiosis. Exogenous application of 1000 µM salicylic acid (SA) may alleviate HT-induced reduction in pollen viability and floret fertility, concomitantly with the increased CAT activity and reduced ROS level in rice anther.