Characterization of Small Heat Shock Proteins Associated with Maize Tolerance to Combined Drought and Heat Stress

To investigate how the mechanisms of small heat shock proteins (sHSPs) in regulating maize leaves respond to the combination of drought and heat stress, leaf protein patterns were monitored using a proteomic approach in maize plants exposed to combined drought and heat stress. Two-dimensional electrophoresis (2-DE) was used to identify combined drought- and heat-responsive protein spots in maize leaves. After Coomassie brilliant blue staining, approximately 450 protein spots were reproducibly detected on each gel, wherein 7 protein spots were expressed only under heat and combined drought and heat stress but were almost undetected under control and drought. Using MALDI-TOF mass spectrometry, a total of seven proteins were identified, including cytochrome b6-f complex iron-sulfur subunit, sHSP17.4, sHSP17.2, sHSP26, guanine nucleotide-binding protein β-subunit-like protein, putative uncharacterized protein, and granule-bound starch synthase IIa. Moreover, the gene expression of three sHSPs was analyzed at the transcriptional level and indicated that all three sHSPs were expressed under several treatments although their expression levels were obviously more enhanced by heat and combined drought and heat stress than by control and drought. In investigations of the effect of abscisic acid (ABA) on the three sHSPs, pretreatment with 100 μM ABA enhanced substantially the expression of the three sHSPs at the protein level, but only slightly at the mRNA level. These results show that transcription levels are not completely concomitant with translation and suggest that ABA induces the post-transcriptional regulation of sHSP17.2, sHSP17.4, and sHSP26 expression, which can lead to a better understanding of the mechanisms of plant response to the combination of drought and heat stress.