The Effect of Heat Stress at Molecular Level During the Tricellular Stage of Pollen Development in Maize (Zea Mays)
Egesa, Andrew Ogolla
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Heat stress is one of the major limiting factors for crop productivity, resulting in significant yield losses annually. Reproductive development is extremely sensitive to increased temperatures. With the increase in the frequency and intensity of short waves of high temperatures, a detrimental effect on plant development is expected to rise especially on the reproductive development. Within cereals, maize is a fundamental crop from a global food security perspective. Therefore, understanding how heat stress affects male gametophyte development in maize is critical to improving crop resilience to adverse conditions. During maize pollen development, MADS-box transcription factors among other genes have been found to be strictly regulated, implying their role in ensuring optimal pollen development and quality. However, a gap in knowledge exists on the genetic pathways that regulate gene expression during heat stress, which remains unfilled. As such, this research investigated the effect of heat stress during tricellular stage of pollen development. Maize plants at early tricellular stage of pollen development were subjected to moderate heat stress regime (35o C /25o C, day/night) for forty-eight hours, while a parallel set of control plants were maintained at (25o C /21o C) control conditions. Maturation of anthers contribute to accumulation of biomolecules important for pollen tolerance to stress and proper germination. Heat stress hastened development of maize anthers resulting in improper maturation and reduction in pollen dehiscence. Furthermore, pollen germination was strongly decreased (P value < 0.001 at 95% level of significance) in heat stressed plants (25.82% 3.4 55%) in comparison to control plants (81.05% 2.84%2.84%). Using quantitative real time PCR (qRT-PCR), the transcriptional responses caused by heat stress on pollen specific genes were further evaluated. The late pollen gene (zmMADS2), pollen specific protein C13 and Tigro1570 proteins were down regulated (a negative fold change ≥ 0.5X) in heat stressed plants, while, those genes coding for CBL-interacting protein kinase 20, Profilins and plastocyanin proteins were upregulated (a positive fold change ≥ 1X). Maize heat shock factor fourteen (zmHSF14) had the highest expression among the five maize heat shock transcription factors (zmHSFs), activated by moderate heat stress in pollen. Overall, these findings provide a great opportunity to screen for more heat tolerance during reproductive development in other maize genotypes. The results of this work are important in breeding for abiotic stress tolerance in maize and respond to the needs of keeping agriculture competitive while ending world hunger, through producing crops that are resilient to climate change.