Different Chilling Responses of Brassinosteroid-biosynthetic Genes in Arabidopsis and Mung Bean Seedlings

Chilling stress is an inevitable environmental factor that deeply affects plants growth and development. In addition to abscisic acid involved in chilling tolerance, increasing attentions have been focused on brassinosteroid (BR) as a chilling protectant. BR, a newly identified phytohormone that exhibits the highest promotion effect on cell growth has been reported, and the components involved in its biosynthesis and signaling are also identified in Arabidopsis. In mung bean, CYP90A2 is the first clone that shares high homologous (77%) to Arabidopsis CPD/CYP90A1, which is the most important enzyme in BR biosynthesis. There is one copy of CYP90A2 per haploid genome in mung bean. The transcriptions of CYP90A2 are abundantly detected in leaves that increase following seedlings development and exhibit diurnal variation. In the feedback regulation assay, CYP90A2 is down regulated in the existence of 24-epibrassinolide (EBS) and up regulated by Brz220, a BR biosynthetic inhibitor. All of these physiological responses are similar to Arabidopsis CPD gene. Therefore, CYP90A2 is postulated to be involved in BR biosynthesis in mung bean. Comparing the growth of two plant species when exposured to 10℃ chilling stress, Arabidopsis, a chilling-tolerant plant, shows higher growth and survival rate than mung bean, which is regarded as a chilling-sensitive plant. In monitoring the regulations of BR-biosynthetic genes under chilling temperature, we found that the reported genes CYP85A1, CPD/CYP90A1, DWF4/CYP90B1, ROT3/CYP90C1 and CYP90D1, involved in BR biosynthesis in Arabidopsis maintained in a constant level, while mung bean CYP90A2 was strongly down regulated. Thus, brassinosteroid (BR) is considered related to chilling growth. With the prediction that chilling decreases endogenous BR level and leads to a chilling-sensitivity of mung bean, the exogenous spraying of EBS is performed and seedlings are survived from chilling injury. In spite of the deficiency of direct evidences in determining the functional role of CYP90A2 and the uncertainty of endogenous level of BR decreasing under chilling stress, to our knowledge, this was the first study applying molecular approaches to elucidate the correlation between BR and chilling response, with which the mechanisms of chilling tolerance could be further explained.