Changes in physiology, molecular biology, and biochemistry associated with maize seedlings germinated from freeze-damaged seed

Injury to immature maize seed caused by early season frost is of great economic concern to seed producers. Surprisingly, there has been very little research investigating the adverse effects of freezing on immature seed and the resulting biochemical changes in seedlings germinated from freeze damaged seed. In this three part study, we investigated the effects of freezing injury, first as they pertained to the overall viability of the seed, then to how exposure of immature seed to freezing induces biochemical changes in seedlings germinated from these seed. We found that the extent of freezing injury as determined by reduced viability and observed cellular damage was significantly influenced by the physiological maturity of the seed at time of exposure and by the duration of exposure. We found that several changes in gene expression occurred in seedlings germinated from freeze-damaged seed. First, three day old seedlings germinated from freeze-damaged seed had elevated rates of cellular respiration which correlated with depleted levels of maize peroxysomal catalase isozyme I (CAT1). Six day old seedlings also had greater rates of respiration and showed a strong induction of mitochondrial catalase isozyme 3 (CAT3) in response to freezing stress. CAT3 expression was not only increased by freezing, but expression was found in roots where it had previously not been detected in seedlings germinated from seed which had not been exposed to freezing stress. A second change in gene expression in seedlings germinated from freeze damaged seed was the differential expression of dehydrin antibody recognized proteins (DARP\u27s). DARP\u27s were present in ungerminated embryos and in three day old seedlings. In ungerminated embryos, expression of a DARP of approximately 56 kDa was not affected by freeze exposure but did increase with physiological maturity of the seed. In three day old seedlings, a DARP of approximately 56 kDa was differentially expressed in seedlings germinated from seed exposed to freezing while another DARP of approximately 36 kDa was expressed in all seedlings. Expression of the 56 kDa DARP in three day old seedlings was limited to seedlings germinated from seed harvested at 45% and 35% moisture. In seedlings germinated from seed harvested at 45% moisture, expression occurred in seedlings germinated from seed exposed to either 6h or 24h of freezing while in seedlings germinated from seed harvested at 35% moisture, expression was limited to seedlings germinated from seed exposed to 24h of freezing