Heat-dependent postpollination limitations on maize pollen tube growth and kernel sterility.

Heat stress has a negative impact on pollen development in maize (Zea mays L.) but the postpollination events that determine kernel sterility are less well characterised. The impact of short-term (hours) heat exposure during postpollination was therefore assessed in silks and ovaries. The temperatures inside the kernels housed within the husks was significantly lower than the imposed heat stress. This protected the ovaries and possibly the later phase of pollen tube growth from the adverse effects of heat stress. Failure of maize kernel fertilization was observed within 6 h of heat stress exposure postpollination. This was accompanied by a significant restriction of early pollen tube growth rather than pollen germination. Limitations on early pollen tube growth were therefore a major factor contributing to heat stress-induced kernel sterility. Exposure to heat stress altered the sugar composition of silks, suggesting that hexose supply contributed to the limitations on pollen tube growth. Moreover, the activities of sucrose metabolising enzymes, the expression of sucrose degradation and trehalose biosynthesis genes were decreased following heat stress. Significant increases in reactive oxygen species, abscisic acid and auxin levels accompanied by altered expression of phytohormone-related genes may also be important in the heat-induced suppression of pollen tube growth.

Walking through crossroads-rice responses to heat and biotic stress interactions.

Rice, the most important source of calories for humans is prone to severe yield loss due to changing climate including heat stress. Additionally, rice encounters biotic stresses in conjunction with heat stress, which exacerbates the adverse effects, and exponentially increase such losses. Several investigations have identified biotic and heat stress-related quantitative trait loci (QTLs) that may contribute to improved tolerance to these stresses. However, a significant knowledge gap exists in identifying the genomic regions imparting tolerance against combined biotic and heat stress. Hereby, we are presenting a conceptual meta-analysis identifying genomic regions that may be promising candidates for enhancing combined biotic and heat stress tolerance in rice. Fourteen common genomic regions were identified along chromosomes 1, 2, 3, 4, 6, 10 and 12, which harbored 1265 genes related to heat stress and defense responses in rice. Further, the meta expression analysis revealed 24 differentially expressed genes (DEGs) involved in calcium-mediated stress signaling including transcription factors Myb, bHLH, ROS signaling, molecular chaperones HSP110 and pathogenesis related proteins. Additionally, we also proposed a hypothetical model based on GO and MapMan analysis representing the pathways intersecting heat and biotic stresses. These DEGs can be potential candidate genes for improving tolerance to combined biotic and heat stress in rice. We present a framework highlighting plausible connecting links (QTLs/genes) between rice response to heat stress and different biotic factors associated with yield, that can be extended to other crops.

Is early morning flowering an effective trait to minimize heat stress damage during flowering in rice?

Higher spikelet sterility due to heat stress exposure during flowering in rice is becoming a major threat for sustaining productivity in tropical and sub-tropical regions. Therefore, exploiting and incorporating early morning flowering (EMF) trait into ongoing breeding pipelines could be an effective strategy to minimize the damage. In this study, we have focused on quantifying the time of day of flowering traits such as first spikelet opening time (FSOT) and peak spikelet opening time (PSOT) in a diverse set of cultivars (n = 289) representing major rice growing regions (13 tropical and 20 subtropical countries) over three years (wet season; WS-2012, dry season; DS-2013 and 2014), under field conditions. EMF traits (FSOT and PSOT) and spikelet sterility displayed significant (p < 0.001) variations among cultivars, both within and between seasons (WS and DS). Averaged across two dry seasons, the FSOT ranged between 2.35 h and 5.08 h after dawn compared to 3.05 h and 5.50 h during the WS, while, PSOT varied from 3.32 to 6.27 h in DS and from 3.50 to 7.05 h in WS. On average, PSOT was strongly associated with FSOT both in WS (R2 = 0.78) and DS (R2 = 0.77). A near-isogenic line (IR64 + qEMF3) effectively minimized the spikelet sterility by 71% during dry seasons under field conditions compared to 289 tropical and subtropical cultivars. None of the tropical and subtropical originated cultivars possess EMF trait including the popular IR64, thus indicating the usefulness of incorporating this trait to reduce heat stress damage under hotter climate. Our findings provide evidence for the effectiveness of the EMF trait in overcoming heat stress induced sterility under field conditions. Hence, it sounds logical to introgress EMF trait into currently growing popular rice cultivars for improving their resilience to heat stress episodes coinciding with flowering.

Thermal stress impacts reproductive development and grain yield in rice.

Rice is highly sensitive to temperature stress (cold and heat), particularly during the reproductive and grain-filling stages. In this review, we discuss the effects of low- and high-temperature sensitivity in rice at various reproductive stages (from meiosis to grain development) and propose strategies for improving the tolerance of rice to terminal thermal stress. Cold stress impacts reproductive development through (i) delayed heading, due to its effect on anther respiration, which increases sucrose accumulation, protein denaturation and asparagine levels, and decreases proline accumulation, (ii) pollen sterility owing to tapetal hypertrophy and related nutrient imbalances, (iii) reduced activity of cell wall bound invertase in the tapetum of rice anthers, (iv) impaired fertilization due to inhibited anther dehiscence, stigma receptivity and ability of the pollen tube to germinate through the style towards the ovary, and (v) floret sterility, which increases grain abortion, restricts grain size, and thus reduces grain yield. Heat stress affects grain formation and development through (i) poor anther dehiscence due to restricted closure of the locules, leading to reduced pollen dispersal and fewer pollen on the stigma, (ii) changes in pollen proteins resulting in significant reductions in pollen viability and pollen tube growth, leading to spikelet sterility, (iii) delay in heading, (iv) reduced starch biosynthesis in developing grain, which reduces starch accumulation, (v) increased chalkiness of grain with irregular and round-shaped starch granules, and (vi) a shortened grain-filling period resulting in low grain weight. However, physiological and biotechnological tools, along with integrated management and adaptation options, as well as conventional breeding, can help to develop new rice genotypes possessing better grain yield under thermal stress during reproductive and grain-filling phases.