One kind of biologically active salicylic acid (SA) analogue (acetylsalicylic acid, ASA) and two inactive compounds (4-aminosalicylic acid and 4-aminobenzoic acid), along with SA were chosen to evaluate their role in inducing chilling tolerance of two different chilling-tolerant maize ( Zea mays L.) inbred lines. These compounds were applied as seed treatments or as a hydroponic application. The results showed that four compounds had no significant effect on germination of maize seeds; however, SA or ASA soaking treatments significantly increased the root length, shoot height, and shoot and root dry weights of seedlings grown under chilling stress. Hydroponic applications of SA or ASA significantly alleviated the accumulation of malondialdehyde, hydrogen peroxide, and superoxide radicals in roots and leaves of both lines under chilling stress, and the applications also increased the photosynthetic pigments, including chlorophyll a, chlorophyll b, and carotenoids. However, 4-aminosalicylic acid and 4-aminobenzoic acid applications had no significant effect in ameliorating the growth inhibition of seedlings under chilling stress. This study showed that SA and ASA significantly induced the chilling tolerance of maize; however, 4-aminosalicylic acid and 4-aminobenzoic acid were not effective in inducing tolerance to chilling stress. The results suggest that only SA analogues with biological activity may have the ability to induce chilling tolerance of maize.

Heat shock proteins (HSPs) are a type of conserved molecular chaperone. They exist extensively in plants and greatly contribute to their survival under heat stress. The transcriptional regulation factor heat shock factor (HSF) is thought to regulate the expression of Hsps. In this study, a novel gene designated BcHsfA1 was cloned and characterized from Brassica campestris. Bioinformatic analysis implied that BcHsfA1 belongs to the HsfA gene family and is most closely related to HsfA1 from other plants. Constitutive overexpression of BcHsfA1 significantly improved heat tolerance of tobacco seedlings by affecting physiological and biochemical processes. Moreover, the chlorophyll content of transgenic tobacco plants was significantly increased compared with wild type after heat stress, as were the activities of the important enzymatic antioxidants superoxide dismutase and peroxidase. BcHsfA1 overexpression also resulted in decreased malondialdehyde content and comparative electrical conductivity and increased soluble sugar content in transgenic tobacco plants than wild-type plants exposed to heat stress. Furthermore, we identified 11 candidate heat response genes that were significantly up-regulated in the transgenic lines exposed to heat stress. Together, these results suggested that BcHsfA1 is effective in improving heat tolerance of tobacco seedlings, which may be useful in the development of new heat-resisitant B. campestris strains by genetic engineering.

Salicylic acid (SA) can induce multiple stress tolerance in plants. This study investigated the relationship between SA and antioxidant enzyme activities in maize seedlings under chilling stress. Changes of endogenous SA, antioxidant enzyme activities and malondialdehyde (MDA) concentrations were assessed in two different chilling-tolerant maize inbred lines (Huang C and Mo17) under chilling stress. The results showed that both endogenous free and bound salicylic acid contents increased in roots and leaves of both lines. MDA concentrations also increased significantly in roots and leaves of both lines after chilling stress. In addition, in Huang C, chilling stress increased the activities of four antioxidant enzymes, ascorbate peroxidase (APX), catalase (CAT), glutathione reductase (GR) and peroxidase, while in Mo17, only CAT and APX increased. Furthermore, a regression analysis was conducted between SA and MDA concentrations or antioxidant enzyme activities under chilling stress. The results indicated that MDA concentrations were positively correlated with total SA contents in roots (r = 0.9776, p = 0.0224) and bound SA in leaves (r = 0.9974, p = 0.0458), respectively. Total SA contents had positive correlations with APX activities both in roots (r = 0.9993, p = 0.002) and leaves (r = 0.9630, p = 0.037) and GR in leaves (r = 0.9298, p = 0.0221). Together, these results suggested that chilling stress improved the biosynthesis of endogenous SA, and lipid peroxidation and antioxidant enzyme activities could be indicated by endogenous SA contents of maize seedlings under chilling stress. Furthermore, increased activities of antioxidant enzymes, especially in roots, may contribute to the chilling tolerance of maize seedlings.

To improve our understanding of the mechanism of maize seed germination under deep sowing, transcriptome sequencing and physiological metabolism analyses were performed using B73 embryos separated from ungerminated seeds (UG) or seeds germinated for 2 d at a depth of 2 cm (normal sowing, NS) or 20 cm (deep sowing, DS). Gene ontology (GO) analysis indicated that “response to oxidative stress” and “monolayer-surrounded lipid storage body” were the most significant GO terms in up- and down-regulated differentially expressed genes (DEGs) of DS. Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis suggested that “phenylpropanoid biosynthesis” and “starch and sucrose metabolism” were critical processes in maize seed germination under deep-sowing conditions. Consistent with DEGs, the activities of superoxide dismutase, catalase, peroxidases and α-amylase, as well as the contents of gibberellin 4, indole acetic acid, zeatin and abscisic acid were significantly increased, while the jasmonic-acid level was dramatically reduced under deep-sowing stress. The expressions of six candidate genes were more significantly upregulated in B73 (deep-sowing-tolerant) than in Mo17 (deep-sowing-sensitive) at 20 cm sowing depth. These findings enrich our knowledge of the key biochemical pathways and genes regulating maize seed germination under deep-sowing conditions, which may help in the breeding of varieties tolerant to deep sowing.