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In maize (Zea mays L.), lateral roots are formed in the differentiation zone of all root types in a multi-step process. The maize mutant lateral rootless 1 (lrt1) is defective in lateral root formation in primary and seminal roots but not in shoot-borne roots. We cloned the lrt1 gene by mapping in combination with BSA-seq and subsequent validation via CRISPR/Cas9. The lrt1 gene encodes a 209 kDa homolog of the DDB1-CUL4-ASSOCIATED FACTOR (DCAF) subunit of the CUL4-based E3 ubiquitin ligase (CRL4) complex localized in the nucleus. DDB1-CUL4-ASSOCIATED FACTOR proteins are encoded by an evolutionary old gene family already present in nonseed plants. They are adaptors that bind substrate proteins and promote their ubiquitylation, thus typically marking them for subsequent degradation in the 26S proteasome. Gene expression studies demonstrated that lrt1 transcripts are expressed preferentially in the meristematic zone of all root types of maize. Downregulation of the rum1 gene in lrt1 mutants suggests that lrt1 acts upstream of the lateral root regulator rum1. Our results demonstrate that DCAF proteins play a key role in root-type-specific lateral root formation in maize. Together with its role in nitrogen acquisition in nitrogen-poor soil, lrt1 could be a promising target for maize improvement., (© 2022 The Authors. New Phytologist © 2022 New Phytologist Foundation.)

Stalk lodging in maize ( Zea mays ) causes significant yield losses due to breaking of stalk tissue below the ear node before harvest. Here, we identified the maize brittle stalk4 ( bk4 ) mutant in a Mutator F2 population. This mutant was characterized by highly brittle aerial parts that broke easily from mechanical disturbance or in high-wind conditions. The bk4 plants displayed a reduction in average stalk diameter and mechanical strength, dwarf stature, senescence at leaf tips, and semisterility of pollen. Histological studies demonstrated a reduction in lignin staining of cells in the bk4 mutant leaves and stalk, and deformation of vascular bundles in the stalk resulting in the loss of xylem and phloem tissues. Biochemical characterization showed a significant reduction in p -coumaric acid, Glc, Man, and cellulose contents. The candidate gene responsible for bk4 phenotype is Chitinase-like1 protein ( Ctl1 ), which is expressed at its highest levels in elongated internodes. Expression levels of secondary cell wall cellulose synthase genes ( CesA ) in the bk4 single mutant, and phenotypic observations in double mutants combining bk4 with bk2 or null alleles for two CesA genes, confirmed interaction of ZmCtl1 with CesA genes. Overexpression of ZmCtl1 enhanced mechanical stalk strength without affecting plant stature, senescence, or fertility. Biochemical characterization of ZmCtl1 overexpressing lines supported a role for ZmCtl1 in tensile strength enhancement. Conserved identity of CTL1 peptides across plant species and analysis of Arabidopsis ( Arabidopsis thaliana ) ctl1-1 ctl2-1 double mutants indicated that Ctl1 might have a conserved role in plants., (© 2019 American Society of Plant Biologists. All Rights Reserved.)

Cytokinins are hormones that play an essential role in plant growth and development. The irreversible degradation of cytokinins, catalyzed by cytokinin oxidase, is an important mechanism by which plants modulate their cytokinin levels. Cytokinin oxidase has been well characterized biochemically, but its regulation at the molecular level is not well understood. We isolated a cytokinin oxidase open reading frame from maize (Zea mays), called Ckx1, and we used it as a probe in northern and in situ hybridization experiments. We found that the gene is expressed in a developmental manner in the kernel, which correlates with cytokinin levels and cytokinin oxidase activity. In situ hybridization with Ckx1 and transgenic expression of a transcriptional fusion of the Ckx1 promoter to the Escherichia coli beta-glucuronidase reporter gene revealed that the gene is expressed in the vascular bundles of kernels, seedling roots, and coleoptiles. We show that Ckx1 gene expression is inducible in various organs by synthetic and natural cytokinins. Ckx1 is also induced by abscisic acid, which may control cytokinin oxidase expression in the kernel under abiotic stress. We hypothesize that under non-stress conditions, cytokinin oxidase in maize plays a role in controlling growth and development via regulation of cytokinin levels transiting in the xylem. In addition, we suggest that under environmental stress conditions, cytokinin oxidase gene induction by abscisic acid results in aberrant degradation of cytokinins therefore impairing normal development.