Date Published: January 30, 2019
Publisher: Public Library of Science
Author(s): Lei Tian, Xiyong Zhao, Haohao Liu, Lixia Ku, Shunxi Wang, Zanping Han, Liancheng Wu, Yong Shi, Xiaoheng Song, Yanhui Chen, Keqiang Wu.
The circadian clock regulates numerous biological processes in plants, especially development and stress responses. CIRCADIAN CLOCK-ASSOCIATED 1 (CCA1) is one of the core components of the day–night rhythm response and is reportedly associated with ambient temperature in Arabidopsis thaliana. However, it remains unknown if alternative splicing of ZmCCA1 is modulated by external stress in maize, such as drought stress and photoperiod. Here, we identified three ZmCCA1 splice variants in the tropical maize line CML288, which are predicted to encode three different protein isoforms, i.e., ZmCCA1.1, ZmCCA1.2, and ZmCCA1.3, which all retain the MYB domain. In maize, the expression levels of ZmCCA1 splice variants were influenced by photoperiod, tissue type, and drought stress. In transgenic A. thaliana, ZmCCA1.1 may be more effective than ZmCCA1.3 in increasing drought tolerance while ZmCCA1.2 may have only a small effect on tolerance to drought stress. Additionally, although CCA1 genes have been found in many plant species, alternative CCA1 splicing events are known to occur in species-specific ways. Our study provides new sight to explore the function of ZmCCA1 splice variants’ response to abiotic stress, and clarify the linkage between circadian clock and environmental stress in maize.
Subtropical and tropical maize lines have been used as germplasm to improve maize quality and yield compared with temperate maize lines due to their abundant genetic variations. However, photoperiod sensitivity in maize restricts the utilization of subtropical and tropical germplasms under long day (LD) conditions because of a delayed floral transition [1, 2]. The circadian clock is involved in photoperiod-mediated flowering and accurately perceives external input signals to generate endogenous rhythmic outputs during an approximate 24 h cycle, which can be synchronized with the environment by regulating key basic metabolic processes including photosynthesis, hypocotyl elongation, and floral transition . Moreover, the clock can also regulate the stomatal aperture, rhythmic leaf movement, and roots and stem circumnutating in plants [4, 5].
The maize inbred line CML288 used in this study was collected from the International Maize and Wheat Improvement Center (CIMMYT) in Mexico. All A. thaliana were Columbia-0 (Col-0) unless otherwise specified.
Alternative splicing (AS) is a widespread phenomenon in higher eukaryotes. In Arabidopsis, at least 61% of intron-containing genes are AS . In O. sativa, 33% of all rice genes undergo AS . Approximatively, 55.3% of maize genes may be subjected to AS . In this study, we confirmed by homologous cloning and the 3′RACE technique that ZmCCA1 undergoes extensive AS, which occurred at alternative 5′ splice site, alternative poly (A), or by a combination of two or more AS types, encoding three protein isoforms, i.e., ZmCCA1.1, ZmCCA1.2, and ZmCCA1.3. AS isoforms were found to be conserved between one maize homolog and its sorghum ortholog in our study, but absent from the second maize homolog. These results may support the published predication that AS isoforms may have been lost after the maize whole genome duplication event .