This document summarizes the current understanding of fungal hyphal branching. It discusses the two main types of branching (apical and lateral) and some of the internal and external factors that induce branching. Potential mechanisms underlying branch site selection are outlined, including multiple signaling pathways and subcellular structures. Finally, branching in other kingdoms is briefly compared to fungal hyphal branching. However, the molecular basis of hyphal branching remains poorly understood.

1. Branching of fungal hyphae: regula2on,
mechanisms and comparison with other
branching systems
Steven D. Harris1
Department of Plant Pathology and Center for Plant Science Innova<on, University of Nebraska, Lincoln, Nebraska
68588
Mycologia, 100(6), 2008, pp. 823–832. Issued 23 December 2008

2. Abstract:
The ability of rapidly growing hyphae to generate new polarity
axes that result in the forma2on of a branch represents one of
the most important yet least understood aspects of fungal cell
biology. Branching is central to the development of mycelial
colonies and also appears to play a key role in fungal
interac2ons with other organisms.
Mycologia, 100(6), 2008, pp. 823–832. Issued 23 December 2008

3. Abstract:
This review presents a descrip2on of the two major paMerns
of hyphal branching, apical and lateral, and highlights
the roles of internal and external factors in the
induc2on of branch forma2on.
Mycologia, 100(6), 2008, pp. 823–832. Issued 23 December 2008

4. Abstract:
In addi2on, poten2al mechanisms underlying branch site
selec2on are outlined, and the possible roles of mul2ple
signaling pathways (i.e., G protein alpha, Cdc42, NDR kinases)
and subcellular structures (i.e., the Spitzenkorper, sep2ns)
are discussed.
Mycologia, 100(6), 2008, pp. 823–832. Issued 23 December 2008

5. Abstract:
Finally, other forms of branching in the plant and animal
kingdoms are briefly summarized and compared to hyphal
branching.
Key words: apical branch, hyphal morphogenesis,
lateral branch, polarity establishment
Mycologia, 100(6), 2008, pp. 823–832. Issued 23 December 2008

6. INTRODUCTION
The success of fungi in colonizing terrestrial ecosystems can be
largely aMributed to their ability to form hyphae and mycelia
(Rayner et al 1995).
Mycologia, 100(6), 2008, pp. 823–832. Issued 23 December 2008

7. INTRODUCTION
Hyphae are highly polarized cylinders that usually grow by apical
extension at rates that can approach >1 mm/s (Seiler and
Plamann 2003). Fungal hyphae are typically
composed of mul2ple cells demarcated by septa
(Carlile 1995).
Mycologia, 100(6), 2008, pp. 823–832. Issued 23 December 2008

8. INTRODUCTION
This modular paMern of organiza2on contributes to the
differen2a2on of hyphae; apical cells (or hyphal‐2p cells) are
generally engaged in nutrient acquisi2on and sensing of the local
environment, whereas sub‐apical cells generate new hyphae
by lateral branching.
Mycologia, 100(6), 2008, pp. 823–832. Issued 23 December 2008

9. INTRODUCTION
The resul2ng network of hyphae is known as a mycelium.
Mycologia, 100(6), 2008, pp. 823–832. Issued 23 December 2008

10. INTRODUCTION
Hyphal branching appears to serve two general purposes. First, it
increases the surface area of the colony, which presumably
enhances nutrient assimila2on. Second, branches mediate
hyphal fusion events that appear to be important for
exchange of nutrients and signals between different
hyphae in the same colony.
Mycologia, 100(6), 2008, pp. 823–832. Issued 23 December 2008

11. INTRODUCTION
Nevertheless, liMle is known about the molecular basis of hyphal
branching.
Mycologia, 100(6), 2008, pp. 823–832. Issued 23 December 2008

12. INTRODUCTION
Although the molecular processes involved in polarized hyphal
growth would obviously be needed for the forma2on and growth
of a branch, the recent iden2fica2on of mutants with branching‐
specific phenotypes suggests that branch forma2on does not
involve a simple recapitula2on of the polarity establishment
mechanisms that underlie polariza2on of germina2ng spores.
Mycologia, 100(6), 2008, pp. 823–832. Issued 23 December 2008

13. INTRODUCTION
The characteris2c paMern of mycelial organiza2on implies that
individual fungal hyphae exhibit a phenomenon known as apical
dominance, whereby the growing 2p is dominant and suppresses
the forma2on of lateral branches in its vicinity (Schmid and
Harold 1988, Semighini and Harris 2008).
Mycologia, 100(6), 2008, pp. 823–832. Issued 23 December 2008

14. INTRODUCTION
The characteris2c paMern of mycelial organiza2on implies that
individual fungal hyphae exhibit a phenomenon known as apical
dominance, whereby the growing 2p is dominant and suppresses
the forma2on of lateral branches in its vicinity (Schmid and
Harold 1988, Semighini and Harris 2008).
Mycologia, 100(6), 2008, pp. 823–832. Issued 23 December 2008

16. FIG. 2. Branching paMerns in fungal mycelia.
A. Branched hyphae from the leading edge
of a growing Neurospora crassa (strain
FGSC9716) colony. Asterisks mark examples
of apical branching. B. Segment of an
Aspergillus nidulans (strain FGSC28) hypha
stained with Calcofluor White (to show
septa) and Hoechst 33258 (to show nuclei).
Three lateral branches are shown; 1 and 2
emerge from the middle of their respec2ve
compartments, whereas 3 appears to be
associated with a septum.
C. Segment of a Galactomyces candidum
(strain NRRL Y17569) hypha stained with
Calcofluor. Both lateral branches are
associated with septa. D. Hyphal segment
from an A. nidulans sepH1 mutant (strain
AKS71) stained with Calcofluor. Note the
presence of both apical and lateral
branches. Bars, 30 mm (A), 3 mm (B–D).

17. FUTURE QUESTIONS
Mycologists have long recognized the importance of
hyphal branching in the development of the fungal
colony (Carlile 1995, Rayner et al 1995).
Mycologia, 100(6), 2008, pp. 823–832. Issued 23 December 2008

18. FUTURE QUESTIONS
So too have industrial microbiologists, who are well aware that
branching is a cri2cal determinant of colony morphology
that ul2mately affects the yield of fungal fermenta2ons (e.g.
Trinci 1994).
Mycologia, 100(6), 2008, pp. 823–832. Issued 23 December 2008

19. FUTURE QUESTIONS
Accordingly, considerable effort has been expended in an
aMempt to understand the cri2cal temporal and spa2al signals
that trigger branching.
Mycologia, 100(6), 2008, pp. 823–832. Issued 23 December 2008

20. An environmental isolate of Penicillium 1. hypha 2. conidiophore 3. phialide 4. conidia 5. septa