Abstract
Exploring the molecular foundation of the gene-regulatory systems underlying agronomic parameters or/and plant responses to both abiotic and biotic stresses is crucial for crop improvement. Thus, transcription factors, which alone or in combination directly regulated the targeted gene expression levels, are appropriate players for enlightening agronomic parameters through genetic engineering. In this regard, homeodomain leucine zipper (HD-ZIP) genes family concerned with enlightening plant growth and tolerance to environmental stresses are considered key players for crop improvement. This gene family containing HD and LZ domain belongs to the homeobox superfamily. It is further classified into four subfamilies, namely HD-ZIP I, HD-ZIP II, HD-ZIP III, and HD-ZIP IV. The first HD domain-containing gene was discovered in maize cells almost three decades ago. Since then, with advanced technologies, these genes were functionally characterized for their distinct roles in overall plant growth and development under adverse environmental conditions. This review summarized the different functions of HD-ZIP genes in plant growth and physiological-related activities from germination to fruit development. Additionally, the HD-ZIP genes also respond to various abiotic and biotic environmental stimuli by regulating defense response of plants. This review, therefore, highlighted the various significant aspects of this important gene family based on the recent findings. The practical application of HD-ZIP biomolecules in developing bioengineered plants will not only mitigate the negative effects of environmental stresses but also increase the overall production of crop plants.
Highlights
The genes containing the homeobox domain were discovered for the first time in Drosophila
The homeodomain leucine zipper (HD-ZIP) gene family is composed of two functional domains, i.e., HD and leucine zipper (LZ)
The class IV HD-ZIP gene family has been previously characterized in various plants such as Arabidopsis, maize, and rice
Summary
The genes containing the homeobox domain were discovered for the first time in Drosophila. The first HD-containing gene was reported in maize (Zea mays), where a Knotted gene was observed to control the leaf differentiation mechanism. A series of discoveries reported a large set of genes-possessing HD domain and different other additional domains in a single copy of a gene [5] These different homeobox gene families exhibit structure and functional similarities [2]. Most of the HD-ZIP genes in transgenic research showed pronounced effects against drought and salinity [9,10] Apart from that, these genes respond to various other adverse conditions, including heat, heavy metals, and biotic stresses [11,12]. The present review documented several aspects of the homeodomain leucine zipper (HD-ZIP) gene family, such as structural characteristics, interaction with other gene families, and potential in regulating plant growth, development, and responses to environmental cues
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