Drought is a natural disaster that has a profound impact on global agricultural production, significantly reduces crop yields and thereby poses a severe threat to worldwide food security. Addressing the challenge of effectively improving crop drought resistance (DR) to mitigate yield loss under drought conditions is a global issue. An optimal root system architecture (RSA) plays a pivotal role in enhancing crops' capacity to efficiently uptake water and nutrients, which consequently strengthens their resilience against environmental stresses. In this review, we discuss the compositions and roles of crop RSA and summarize the most recent developments in augmenting drought tolerance in crops by manipulating RSA-related genes. Based on current research, we propose the potential optimal RSA configuration that could be helpful in enhancing crop DR. Lastly, we discussed the existing challenges and future directions for breeding crops with enhanced DR capabilities through genetic improvements targeting RSA.

Drought is one of the most important factors contributing to crop yield loss. To develop drought‐tolerant maize (Zea mays L.) varieties, it is important to explore the genetic mechanism and genes involved. In this study, we identified 69 quantitative trait loci (QTLs) for plant height, ear height, anthesis‐silking interval, ear weight, cob weight, 100‐kernel weight, and ear length in two F_2:3 populations in both drought‐stressed and unstressed conditions. These QTLs explained 4.0 to 17.2% of phenotypic variation in a single watering condition. Approximately 52 of the 69 QTLs were identified under water‐stressed conditions. Moreover, 21 stable QTLs were validated in one or two F_2:3 populations under multiple water‐stressed conditions. Remarkably, bin 4.09 (umc2287–umc2011) had two stable QTLs for ear height and anthesis‐silking interval; bin 1.07_1.08 (bnlg1025–mmc0041) identified three stable QTLs for ear, cob, and 100‐kernel weights; bin 4.08_4.09 (umc2041–umc2287) validated four stable QTLs for ear weight, cob weight, 100‐kernel weight, and ear length; and bin 9.04_9.06 (umc1120–umc2134) mapped three stable QTLs for ear weight, cob weight, and ear length that were consistent with phenotypic correlations among traits, supporting pleiotropy of QTLs and playing important roles in conferring growth and yield advantages under drought stress. Additionally, we identified 36 meta‐QTLs across 26 populations under 52 well‐watered and 38 water‐stressed conditions using a meta‐analysis, and we predicted 39 candidate genes in the corresponding meta‐QTL intervals. These results provide valuable information for further mapping quantitative traits and revealing the genetic basis of drought tolerance.

The growth and development of maize roots are closely related to drought tolerance. In order to clarify the molecular mechanisms of drought tolerance between different maize (Zea mays L.) varieties at the protein level, the isobaric tags for relative and absolute quantitation (iTRAQ) quantitative proteomics were used for the comparative analysis of protein expression in the seedling roots of the drought-tolerant Chang 7-2 and drought-sensitive TS141 maize varieties under 20% polyethylene glycol 6000 (PEG 6000)-simulated drought stress. We identified a total of 7723 differentially expressed proteins (DEPs), 1243 were significantly differentially expressed in Chang 7-2 following drought stress, 572 of which were up-regulated and 671 were down-regulated; 419 DEPs were identified in TS141, 172 of which were up-regulated and 247 were down-regulated. In Chang 7-2, the DEPs were associated with ribosome pathway, glycolysis/gluconeogenesis pathway, and amino sugar and nucleotide sugar metabolism. In TS141, the DEPs were associated with metabolic pathway, phenylpropanoid biosynthesis pathway, and starch and sucrose metabolism. Compared with TS141, the higher drought tolerance of Chang 7-2 root system was attributed to a stronger water retention capacity; the synergistic effect of antioxidant enzymes; the strengthen cell wall; the osmotic stabilization of plasma membrane proteins; the effectiveness of recycling amino acid; and an improvement in the degree of lignification. The common mechanisms of the drought stress response between the two varieties included: The promotion of enzymes in the glycolysis/gluconeogenesis pathway; cross-protection against the toxicity of aldehydes and ammonia; maintenance of the cell membrane stability. Based on the proteome sequencing information, the coding region sequences of eight DEP-related genes were analyzed at the mRNA level by quantitative real-time PCR (qRT-PCR). The findings of this study can inform the future breeding of drought-tolerant maize varieties.

Salt stress is a common abiotic stress that limits the growth, development and yield of maize (Zea mays L.). To better understand the response of maize to salt stress and the mechanism by which exogenous glycine betaine (GB) alleviates the damaging effects of salt stress, the morphology, physiological and biochemical indexes, and root transcriptome expression profiles of seedlings of salt-sensitive inbred line P138 and salt-tolerant inbred line 8723 were compared under salt stress and GB-alleviated salt stress conditions. The results showed that under salt stress the growth of P138 was significantly inhibited and the vivo ion balance was disrupted, whereas 8723 could prevent salt injury by maintaining a high ratio of K⁺ to Na⁺. The addition of a suitable concentration of GB could effectively alleviate the damage caused by salt stress, and the mitigating effect on salt-sensitive inbred line P138 was more obvious than that on 8723. Transcriptome analysis revealed that 219 differentially expressed genes (DEGs) were up-regulated and 153 DEGs were down-regulated in both P138 and 8723 under NaCl treatment, and that 487 DEGs were up-regulated and 942 DEGs were down-regulated in both P138 and 8723 under salt plus exogenous GB treatment. In 8723 the response to salt stress is mainly achieved through stabilizing ion homeostasis, strong signal transduction activation, increasing reactive oxygen scavenging. GB alleviates salt stress in maize mainly by inducing gene expression changes to enhance the ion balance, secondary metabolic level, reactive oxygen scavenging mechanism, signal transduction activation. In addition, the transcription factors involved in the regulation of salt stress response and exogenous GB mitigation mainly belong to the MYB, MYB-related, AP2-EREBP, bHLH, and NAC families. We verified 10 selected up-regulated DEGs by quantitative real-time polymerase chain reaction (qRT-PCR), and the expression results were basically consistent with the transcriptome expression profiles. Our results from this study may provide the theoretical basis for determining maize salt tolerance mechanisms and the mechanism by which GB regulates salt tolerance.

Salt stress is one of the key abiotic stresses that causes great loss of yield and serious decrease in quality in maize (Zea mays L.). Therefore, it is very important to reveal the molecular mechanism of salt tolerance in maize. To acknowledge the molecular mechanisms underlying maize salt tolerance, two maize inbred lines, including salt-tolerant 8723 and salt-sensitive P138, were used in this study. Comparative proteomics of seedling roots from two maize inbred lines under 180 mM salt stress for 10 days were performed by the isobaric tags for relative and absolute quantitation (iTRAQ) approach. A total of 1056 differentially expressed proteins (DEPs) were identified. In total, 626 DEPs were identified in line 8723 under salt stress, among them, 378 up-regulated and 248 down-regulated. There were 473 DEPs identified in P138, of which 212 were up-regulated and 261 were down-regulated. Venn diagram analysis showed that 17 DEPs were up-regulated and 12 DEPs were down-regulated in the two inbred lines. In addition, 8 DEPs were up-regulated in line 8723 but down-regulated in P138, 6 DEPs were down-regulated in line 8723 but up-regulated in P138. In salt-stressed 8723, the DEPs were primarily associated with phenylpropanoid biosynthesis, starch and sucrose metabolism, and the mitogen-activated protein kinase (MAPK) signaling pathway. Intriguingly, the DEPs were only associated with the nitrogen metabolism pathway in P138. Compared to P138, the root response to salt stress in 8723 could maintain stronger water retention capacity, osmotic regulation ability, synergistic effects of antioxidant enzymes, energy supply capacity, signal transduction, ammonia detoxification ability, lipid metabolism, and nucleic acid synthesis. Based on the proteome sequencing information, changes of 8 DEPs abundance were related to the corresponding mRNA levels by quantitative real-time PCR (qRT-PCR). Our results from this study may elucidate some details of salt tolerance mechanisms and salt tolerance breeding of maize.