CHARACTERIZATION AND MANIPULATION OF EMBRYOGENIC RESPONSE FROM IN-VITRO-CULTURED IMMATURE INFLORESCENCES OF RICE (ORYZA-SATIVA L)

Characterization and optimization of the embryogenic response from in- vitro-cultured immature inflorescences of rice (Oryza sativa L. sub-sp ecies indica and japonica) are described. Histological and morphologic al analyses revealed that the parenchymatous ground tissue present in the region between the second whorl of sterile bracts and the base of the fertile bracts, the embryogenically competent region (ECR), was in volved in the embryogenic response. Initial cell divisions within the ECR occurred in the vicinity of the pro-vascular regions of the spikel et. Continued cell divisions resulted in groups of proliferating units and each single proliferating unit was the product of a coordinated b ehavior of neighboring cells functioning as a morphogenic group. Furth er proliferation of this embryogenic tissue was due to the development of cambium-like tissue(s) often forming an embryogenic stratum which under optimal culture conditions produced plants at a high frequency. The morphogenic pathways governing plant regeneration from spikelets o f the immature rice inflorescence were dependent upon the growth-regul ator composition of the culture medium. Three different modes of plant regeneration were observed: (i) direct plant regeneration, (ii) plant regeneration with an intervening callus phase (prolific non-embryogen ic growth associated with unorganized, loose and mucilaginous tissue), and (iii) plant regeneration without an intervening callus phase (com pact embryogenic tissue with highly organized growth). The efficiency of plant regeneration, via somatic embryogenesis without an intervenin g callus phase, was increased by optimizing the culture conditions. In a two-step procedure, immature inflorescences of rice were first cult ured on a conditioning medium supplemented with 2.0 mg . l(-1) 2,4-dic hlorophenoxyacetic acid + 1.5 mg . l(-1) kinetin + 0.75 mg . l(-1) alp ha-naphthaleneacetic acid for a period of two weeks. The conditioning medium, with the appropriate culture conditions, allowed redirection o f partially differentiated cells of the ECR into embryogenically compe tent pro-embryogenic groups. Maturation of these pro-embryogenic group s was achieved by transferring them to an embryo proliferation medium, and plants could then be regenerated at a high frequency upon their t ransfer to the regeneration medium.