Purpose To elucidate the regulation mechanism of maize chlorophyll metabolism under heat stress, providing a scientific basis for heat resistance research in maize.
Methods The heat-tolerant maize inbred line HZ4 and the heat-sensitive maize inbred line L9K were treated under heat stress from the trumpet stage to the flowering stage, with sampling every five days for a total of six times. The contents of chlorophyll and its synthetic precursors were measured, and the relative expression levels of genes related to chlorophyll metabolism were detected by RT-qPCR.
Results The contents of chlorophyll a, chlorophyll b, total chlorophyll, and synthesis precursors protoporphyrin Ⅸ (Proto Ⅸ), Mg-protoporphyrin Ⅸ (MgP Ⅸ), and protochlorophyllide (Pchlide) in maize leaves decreased significantly under heat stress, while δ-aminolevulinic acid (ALA) and porphobilinoge (PBG) contents increased significantly, with the variation amplitudes of these components being smaller in HZ4 than in L9K. The expression levels of six chlorophyll synthesis-related genes (PBGD, UROD, MgPMT, POR, CHLG, CAO) were significantly down-regulated, with HZ4 showing significantly higher levels than L9K. Meanwhile, four chlorophyll degradation-related genes (CBR, CLH, PPH, PAO) were significantly up-regulated, with expression levels in HZ4 significantly lower than in L9K.
Conclusion Heat stress results in the decrease of chlorophyll content in maize due to the down-regulated expression of synthesis-related genes and up-regulated expression of degradation-related genes. The sites of inhibition in maize chlorophyll biosynthesis under heat stress are located at the PBG to Proto Ⅸ transition stage, mainly due to down-regulation of PBGD and UROD gene expression. The differential expression of chlorophyll metabolism-related genes is the molecular mechanism behind the differences in chlorophyll content between heat-tolerant and heat-sensitive maize.
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