In plants, bZIP (basic leucine zipper) transcription factors regulate diverse processes such as development and stress responses. However, few of these transcription factors have been functionally characterized in maize (Zea mays). In this study, we characterized the bZIP transcription factor gene ZmbZIP4 from maize. ZmbZIP4 was differentially expressed in various organs of maize and was induced by high salinity, drought, heat, cold, and abscisic acid treatment in seedlings. A transactivation assay in yeast demonstrated that ZmbZIP4 functioned as a transcriptional activator. A genome-wide screen for ZmbZIP4 targets by immunoprecipitation sequencing revealed that ZmbZIP4 could positively regulate a number of stress response genes, such as ZmLEA2, ZmRD20, ZmRD21, ZmRab18, ZmNHX3, ZmGEA6, and ZmERD, and some abscisic acid synthesis-related genes, including NCED, ABA1, AAO3, and LOS5. In addition, ZmbZIP4 targets some root development-related genes, including ZmLRP1, ZmSCR, ZmIAA8, ZmIAA14, ZmARF2, and ZmARF3, and overexpression of ZmbZIP4 resulted in an increased number of lateral roots, longer primary roots, and an improved root system. Increased abscisic acid synthesis by overexpression of ZmbZIP4 also can increase the plant’s ability to resist abiotic stress. Thus, ZmbZIP4 is a positive regulator of plant abiotic stress responses and is involved in root development in maize.

Summary: Glycine betaine plays an important role in some plants, including maize, in conditions of abiotic stress, but different maize varieties vary in their capacity to accumulate glycine betaine. An elite maize inbred line DH4866 was transformed with the betA gene from Escherichia coli encoding choline dehydrogenase (EC 1.1.99.1), a key enzyme in the biosynthesis of glycine betaine from choline. The transgenic maize plants accumulated higher levels of glycine betaine and were more tolerant to drought stress than wild‐type plants (non‐transgenic) at germination and the young seedling stage. Most importantly, the grain yield of transgenic plants was significantly higher than that of wild‐type plants after drought treatment. The enhanced glycine betaine accumulation in transgenic maize provides greater protection of the integrity of the cell membrane and greater activity of enzymes compared with wild‐type plants in conditions of drought stress.

Summary: Maize is a globally important food, feed crop and raw material for the food and energy industry. Plant architecture optimization plays important roles in maize yield improvement. PIN‐FORMED (PIN) proteins are important for regulating auxin spatiotemporal asymmetric distribution in multiple plant developmental processes. In this study, ZmPIN1a overexpression in maize increased the number of lateral roots and inhibited their elongation, forming a developed root system with longer seminal roots and denser lateral roots. ZmPIN1a overexpression reduced plant height, internode length and ear height. This modification of the maize phenotype increased the yield under high‐density cultivation conditions, and the developed root system improved plant resistance to drought, lodging and a low‐phosphate environment. IAA concentration, transport capacity determination and application of external IAA indicated that ZmPIN1a overexpression led to increased IAA transport from shoot to root. The increase in auxin in the root enabled the plant to allocate more carbohydrates to the roots, enhanced the growth of the root and improved plant resistance to environmental stress. These findings demonstrate that maize plant architecture can be improved by root breeding to create an ideal phenotype for further yield increases.

Salinity is one of the major environmental factors limiting plant growth and productivity. An H⁺-PPase gene, TsVP from Thellungiella halophila, was transferred into cotton (Gossypium hirsutum) in sense and antisense orientations under control of the cauliflower mosaic virus (CaMV) 35S promoter. Southern and Northern blotting analysis showed that the sense or antisense TsVP were integrated into the cotton genome and expressed. Transgenic plants overexpressing the vacuolar H⁺-PPase were much more resistant to 150 and 250 mM NaCl than the isogenic wild-type plants. In contrast, the plants from the antisense line (L–2), with lower H⁺-PPase activity, were more sensitive to salinity than the wild-type plants. Overexpressing TsVP in cotton improved shoot and root growth and photosynthetic performance. These transgenic plants accumulated more Na⁺, K⁺, Ca²⁺, Cl⁻ and soluble sugars in their root and leaf tissues under salinity conditions compared with the wild-type plants. The lower membrane ion leakage and malondialdehyde (MDA) level in these transgenic plants suggest that overexpression of H⁺-PPase causes the accumulation of Na⁺ and Cl⁻ in vacuoles instead of in the cytoplasm, thus reducing their toxic effects. On the other hand, the increased accumulation of ions and sugars decreases the solute potential in cells, and facilitates water uptake under salinity, which is an important mechanism for the increased salt tolerance in TsVP-overexpressing cotton.

Drought stress is the most important environmental stress limiting maize production. ZmPTF1, a phosphate starvation-induced basic helix-loop-helix (bHLH) transcription factor, contributes to root development and low-phosphate tolerance in maize. Here, ZmPTF1 expression, drought tolerance, and the underlying mechanisms were studied by using maize ZmPTF1 overexpression lines and mutants. ZmPTF1 was found to be a positive regulator of root development, ABA synthesis, signalling pathways, and drought tolerance. ZmPTF1 was also found to bind to the G-box element within the promoter of 9-cis-epoxycarotenoid dioxygenase (NCED), C-repeat-binding factor (CBF4), ATAF2/NAC081, NAC30, and other transcription factors, and to act as a positive regulator of the expression of those genes. The dramatically upregulated NCEDs led to increased abscisic acid (ABA) synthesis and activation of the ABA signalling pathway. The up-regulated transcription factors hierarchically regulate the expression of genes involved in root development, stress responses, and modifications of transcriptional regulation. The improved root system, increased ABA content, and activated ABA-, CBF4-, ATAF2-, and NAC30-mediated stress responses increased the drought tolerance of the ZmPTF1 overexpression lines, while the mutants showed opposite trends. This study describes a useful gene for transgenic breeding and helps us understand the role of a bHLH protein in plant root development and stress responses.