Prospective companion cells differentiate into abnormal sieve elements in colchicine-treated roots ofTriticum aestivum

Abstract

Companion cells (CCs) of seminal roots of wheat (Triticum aestivum L.) flank protophloem sieve elements (PSEs) on their inner side. In roots growing in the presence of 2 mM colchicine prospective CCs are induced to differentiate into abnormal sieve elements. The most convincing evidence that they deviate from their programmed developmental sequence is the deposition of dense crystalloid inclusions within their plastids, a feature characterizing sieve elements of gramineous species. These plastids, however, are ultrastructurally different from those of normal and abnormal PSEs in that they contain only one kind of crystalloid inclusion, exceptionally dense stroma, swollen vesicular structures and plastoglobuli. Mitochondria accumulate in large groups and appear unusually swollen and deformed. Walls develop abnormal thickenings which are deposited irrespective of the position of plasmodesmata. Nuclei become polyploid and appear highly lobed, but degenerate similarly to those in normal sieve elements. Aggregates of paracrystalline material occur in the cytoplasm between the nucleus and cell wall. The end walls display features characterizing developing sieve plates, callose is deposited around plasmodesmata, but sieve pores are not formed. Aberrant CCs may reach “maturity” and co-exist with colchicine-affected PSEs. Their arrangement may be similar to or differ from that of normal phloem poles. Not all cells in CC files are induced to deviate into abnormal sieve elements, nor does their development occur in sequence. Because developmental changes also occur in polyploid cells, which result from colchicine blocked cell divisions, it is concluded that cell division is not a prerequisite for cell differentiation.