Pre‐prophase band of microtubules, absent from tip‐growing moss filaments, arises in leafy shoots during transition to intercalary growth

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Abstract
In the presence of cytokinin, undetermined side branch initials of the moss, Physcomitrella patens, arc induced to form buds and then leafy shoots rather than to develop as tip‐growing filaments. This represents a transition between the two modes of plant cell expansion–tip growth and uniform intercalary growth. The organization of microtubules in filaments is different from that in leafy shoots and can be traced back to the influence of phytohormones on side branch initials. Microtubules either focus at a particular region (as in tip‐growing cells) or in the presence of high levels of cytokinin form swollen bud initials in which microtubules are more diffusely organized. Higher levels of cytokinin are capable of destabilizing tip microtubules in caulonemal filaments. Although caulonemata are not normally target cells, this implies that cytokinin may exert its morphogenetic effects by altering microtubule organization.In tip‐growing filaments, interphase microtubules trace a meandering course through the cytoplasm towards the tip and are not for the main part associated with the plasma membrane as are cortical arrays. There is no pre‐prophase band of microtubules to indicate the future division plane, even though the oblique division plane is known to be precisely controlled relative to environmental factors. This microtubule cycle contrasts with cells of the leafy shoots that develop from buds: in these, the interphase array is cortical, consisting of flat‐pitched microtubular helices that do not focus upon a growing tip. It is now shown that pre‐prophase hands occur at this stage.The absence of bands does not readily correlate with imprecise control of the division plane. Instead, it is proposed that the ability to form pre‐prophase bands depends upon the arrangement of microtubules in the preceding interphase array. Ways in which bands might be formed are discussed and the generality of these ideas is tested by observations on higher plant cells.

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