An improved system to establish highly embryogenic haploid cell and protoplast cultures from pollen calluses of maize (Zea mays L.) - He, Guangyuan; Zhang, Jinrui; Li, Kexiu; Xiong, Zhiyong; Chen, Mingjie; Chang, Junli; Wang, Yuesheng; Yang, Guangxiao; Barnabás, Beáta