Scheme 2

The two-dimensional model of MT configuration in the sphere-shaped uni-nucleate microspores induced to embryogenesis. Most representative unique directions of visible MT are represented: cortical MT near the cell surface (CMT, dark green) and endoplasmic MT (EMT, light green) in the cytoplasm including the vicinity of the nucleus. Freshly isolated microspores (FM) were subjected to stress treatments with 0.7 M mannitol for five days at 25°C (MAN) or with mannitol and additionally with 0.2% n-butanol for 4 h (MANB) to induce embryogenesis and were subsequently cultured in MS3M medium for 4 or 8 hours for recovering (MAN-4R, MAN-8R, MANB-4R). In FM, long CMT bundles were randomly oriented while long EMT determined a net-like structure wrapped around the nucleus. After treatments, the uni-nucleate microspores showed characteristic pattern of MT orientation and fragmentation. MAN affects the MT length and orientation. While CMT were fragmented and disorganized, short EMT bundles formed a protective nest-like structure around the nucleus. After MAN recovering (MAN-4R), microspores displayed randomly oriented, fine, and longer CMT bundles as compared with MAN and long EMT determined a net-like structure around the nucleus. In MAN-R8, thin and long CMT bundles were oriented, and thickened EMT bundles were found in the vicinity of the nucleus. The additional treatment with n-butanol (MANB) resulted in the fragmentation of CMT and EMT bundles. As a result, thick bundles of subcortical MT and wavy aggregates of EMT in the midplane of the nucleus were observed. This MT configuration resembled a PPB-like structure at the initial phase of formation. Recovering of MANB (MANB-4R) involved a lengthening of both, the CMT and the EMT. A band of subcortical MT encircled the midplane of the nucleus to be like a PPB-like structure. Moreover, few thick EMT bundles surrounded the nucleus