Ultrastructure of soft rot decay in bamboo cell walls

The penetration of soft rot hyphae and cavity formation was greatly influenced by the microfibrillar orientation of bamboo cell walls. Penetration hyphae normally changed direction when they encountered a different micro-fibrillar orientation of the cell wall, particularly associated with a narrow lamella. The microfibrillar structure of the bamboo cell walls led to the development of 3 types of soft rot branching; Typical 'T' branching, 'tangentially' orientated branching, and 'radially' orientated branching. Penetrating hyphae that grew tangentially followed the microfibrillar angle of the narrow lamellae and the cavity was confined to just outside the transitional region between the broad and narrow lamellae. Radially orientated hyphae normally traversed across the cell wall either via pits or by direct penetration. Soft rot hyphae were able to exert a strong mechanical force to penetrate through the cell wall. Considerable distortion of the microfibrillar structure of fibre and parenchyma cell walls was commonly observed resulting from penetration hypha and during hyphal enlargement accompanying penetration. Lignin played an important part in decay produced by soft rot fungi. High lignin content was demonstrated to act as a barrier for the development of soft rot as shown in the decay resistance of the compound middle lamella.